java critical thinking questions

The complete guide to Java interview questions and interview prep

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While an older language, Java remains one of the most popular high-level programming languages today. It entered the landscape as a high-level language designed for simplicity and platform independence . Java is running on most of our mobile devices and laptops, and Java developers remain in high demand.

Whether you’re preparing for a role in game development or Android development, mastering Java might take you closer to finding a developer job you love .

Today, we present you with this complete guide to preparing for Java interviews.

We’ll cover :

Common Java interview questions

Core Java interview questions for beginners

Is Java an object-oriented programming language?

Does java support multiple inheritances for classes, what is jdk, method overloading: which method will be invoked, what is a default constructor, resources for common java interview questions, advanced java interview questions and answers for experienced developers, can enum implement an interface in java, what are garbage collection (gc) roots, what are the three phases of garbage collection, rest api questions, resources for advanced java interview questions, java coding interviews, java algorithm interview questions, java data structure interview questions, java collections interview questions and answers, what are collections, what is the collection interface, what are collection interface bulk operations.

• What are the collection types in the Java Collections Framework?

Java multithreading interview questions and answers

What is multithreading, what are the benefits of multithreading, java 8 interview questions and answers, what is function composition, what is a functional interface, stream api questions, java ee interview questions and answers, spring framework interview questions, list the hibernate framework’s core interfaces., are singleton beans thread-safe, how to prepare for java interviews, study the fundamentals, java system design interviews, prepare for specific roles and companies, wrapping up and next steps, answer any java interview problem by learning the patterns behind common questions..

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Review these 50 questions to crack your Java programming interview

by javinpaul

A list of frequently asked Java questions from programming job interviews.

Hello, everybody! Over the past few years, I have been sharing a lot of Java Interview questions and discussion individually. Many of my readers have requested that I bring them together so that they can have them in the same spot. This post is the result of that.

This article contains more than 50 Java Interview questions covering all important topics like Core Java fundamentals, Java Collection Framework , Java Multithreading and Concurrency , Java IO , JDBC , JVM Internals , Coding Problems , Object-Oriented programming , etc.

The questions are also picked up from various interviews and they are, by no means, very difficult. You might have seen them already in your phone or face-to-face round of interviews.

The questions are also very useful to review important topics like multithreading and collections. I have also shared some useful resources for further learning and improvement like The Complete Java MasterClass to brush up and fill gaps in your Java skills.

So what are we waiting for? Here is the list of some of the most frequently asked Java questions in interviews for both beginner and experienced Java developers.

50+ Java Interview Questions for 2 to 3 years Experienced Programmers

So, without wasting any more of your time, here is my list of some of the frequently asked Core Java Interview Question s for beginner programmers. This list focuses on beginners and less experienced devs, like someone with 2 to 3 years of experience in Java.

1) How does Java achieve platform independence? ( answer ) hint: bytecode and Java Virtual Machine

2) What is ClassLoader in Java? ( answer ) hint: part of JVM that loads bytecodes for classes. You can write your own.

3) Write a Java program to check if a number is Even or Odd? ( answer ) hint: you can use bitwise operator, like bitwise AND, remember, even the number has zero at the end in binary format and an odd number has 1 in the end.

4) Difference between ArrayList and HashSet in Java? ( answer ) hint: all differences between List and Set are applicable here, e.g. ordering, duplicates, random search, etc. See Java Fundamentals: Collections by Richard Warburton to learn more about ArrayList, HashSet and other important Collections in Java.

5) What is double checked locking in Singleton? ( answer ) hint: two-time check whether instances is initialized or not, first without locking and second with locking.

6) How do you create thread-safe Singleton in Java? ( answer ) hint: many ways, like using Enum or by using double-checked locking pattern or using a nested static class.

7) When to use the volatile variable in Java? ( answer ) hint: when you need to instruct the JVM that a variable can be modified by multiple threads and give hint to JVM that does not cache its value.

8) When to use a transient variable in Java? ( answer ) hint: when you want to make a variable non-serializable in a class, which implements the Serializable interface. In other words, you can use it for a variable whose value you don’t want to save. See The Complete Java MasterClass to learn about transient variables in Java.

9) Difference between the transient and volatile variable in Java? ( answer ) hint: totally different, one used in the context of serialization while the other is used in concurrency.

10) Difference between Serializable and Externalizable in Java? ( answer ) hint: Externalizable gives you more control over the Serialization process.

11) Can we override the private method in Java? ( answer ) hint: No, because it’s not visible in the subclass, a primary requirement for overriding a method in Java.

12) Difference between Hashtable and HashMap in Java? ( answer ) hint: several but most important is Hashtable , which is synchronized, while HashMap is not. It's also legacy and slow as compared to HashMap .

13) Difference between List and Set in Java? ( answer ) hint: List is ordered and allows duplicate. Set is unordered and doesn't allow duplicate elements.

14) Difference between ArrayList and Vector in Java ( answer ) hint: Many, but most important is that ArrayList is non-synchronized and fast while Vector is synchronized and slow. It's also legacy class like Hashtable .

15) Difference between Hashtable and ConcurrentHashMap in Java? ( answer ) hint: more scalable. See Java Fundamentals: Collections by Richard Warburton to learn more.

16) How does ConcurrentHashMap achieve scalability? ( answer ) hint: by dividing the map into segments and only locking during the write operation.

17) Which two methods you will override for an Object to be used as Key in HashMap ? ( answer ) hint: equals and hashcode

18) Difference between wait and sleep in Java? ( answer ) hint: The wait() method releases the lock or monitor, while sleep doesn't.

19) Difference between notify and notifyAll in Java? ( answer ) hint: notify notifies one random thread is waiting for that lock while notifyAll inform to all threads waiting for a monitor. If you are certain that only one thread is waiting then use notify , or else notifyAll is better. See Threading Essentials Mini-Course by Java Champion Heinz Kabutz to learn more about threading basics.

20) Why you override hashcode, along with equals() in Java? ( answer ) hint: to be compliant with equals and hashcode contract, which is required if you are planning to store your object into collection classes, e.g. HashMap or ArrayList .

21) What is the load factor of HashMap means? ( answer ) hint: The threshold that triggers the re-sizing of HashMap is generally 0.75, which means HashMap resize itself if it's 75 percent full.

22) Difference between ArrayList and LinkedList in Java? ( answer ) hint: same as an array and linked list, one allows random search while other doesn't. Insertion and deletion easy on the linked list but a search is easy on an array. See Java Fundamentals: Collections , Richard Warburton’s course on Pluralsight, to learn more about essential Collection data structure in Java.

23) Difference between CountDownLatch and CyclicBarrier in Java? ( answer ) hint: You can reuse CyclicBarrier after the barrier is broken but you cannot reuse CountDownLatch after the count reaches to zero.

24) When do you use Runnable vs Thread in Java? ( answer ) hint: always

25) What is the meaning of Enum being type-safe in Java? ( answer ) hint: It means you cannot assign an instance of different Enum type to an Enum variable. e.g. if you have a variable like DayOfWeek day then you cannot assign it value from DayOfMonth enum.

26) How does Autoboxing of Integer work in Java? ( answer ) hint: By using the valueOf() method in Java.

27) Difference between PATH and Classpath in Java? ( answer ) hint: PATH is used by the operating system while Classpath is used by JVM to locate Java binary, e.g. JAR files or Class files. See Java Fundamentals: The Core Platform to learn more about PATH , Classpath , and other Java environment variable.

28) Difference between method overloading and overriding in Java? ( answer ) hint: Overriding happens at subclass while overloading happens in the same class. Also, overriding is a runtime activity while overloading is resolved at compile time.

29) How do you prevent a class from being sub-classed in Java? ( answer ) hint: just make its constructor private

30) How do you restrict your class from being used by your client? ( answer ) hint: make the constructor private or throw an exception from the constructor

31) Difference between StringBuilder and StringBuffer in Java? ( answer ) hint: StringBuilder is not synchronized while StringBuffer is synchronized.

32) Difference between Polymorphism and Inheritance in Java? ( answer ) hint: Inheritance allows code reuse and builds the relationship between class, which is required by Polymorphism, which provides dynamic behavior. See Java Fundamentals: Object-Oriented Design to learn more about OOP features.

33) Can we override static method in Java? ( answer ) hint: No, because overriding resolves at runtime while static method call is resolved at compile time.

34) Can we access the private method in Java? ( answer ) hint: yes, in the same class but not outside the class

35) Difference between interface and abstract class in Java? ( answer ) hint: from Java 8 , the difference is blurred. However, a Java class can still implement multiple interfaces but can only extend one class.

36) Difference between DOM and SAX parser in Java? ( answer ) hint: DOM loads whole XML File in memory while SAX doesn’t. It is an event-based parser and can be used to parse a large file, but DOM is fast and should be preferred for small files.

37) Difference between throw and throws keyword in Java? ( answer ) hint: throws declare what exception a method can throw in case of error but throw keyword actually throws an exception. See Java Fundamentals: Exception Handling to learn more about Exception handling in Java.

38) Difference between fail-safe and fail-fast iterators in Java? ( answer ) hint: fail-safe doesn’t throw ConcurrentModificationException while fail-fast does whenever they detect an outside change on the underlying collection while iterating over it.

39) Difference between Iterator and Enumeration in Java? ( answer ) hint: Iterator also gives you the ability to remove an element while iterating while Enumeration doesn’t allow that.

40) What is IdentityHashMap in Java? ( answer ) hint: A Map , which uses the == equality operator to check equality instead of the equals() method.

41) What is the String pool in Java? ( answer ) hint: A pool of String literals. Remember it's moved to heap from perm gen space in JDK 7.

42) Can a Serializable class contains a non-serializable field in Java? ( answer ) hint: Yes, but you need to make it either static or transient.

43) Difference between this and super in Java? ( answer ) hint: this refers to the current instance while super refers to an instance of the superclass.

44) Difference between Comparator and Comparable in Java? ( answer ) hint: Comparator defines custom ordering while Comparable defines the natural order of objects, e.g. the alphabetic order for String . See The Complete Java MasterClass to learn more about sorting in Java.

45) Difference between java.util.Date and java.sql.Date in Java? ( answer ) hint: former contains both date and time while later contains only date part.

46) Why wait and notify method are declared in Object class in Java? ( answer ) hint: because they require lock which is only available to an object.

47) Why Java doesn’t support multiple inheritances? ( answer ) hint: It doesn’t support because of a bad experience with C++, but with Java 8, it does in some sense — only multiple inheritances of Type are not supported in Java now.

48) Difference between checked and unchecked Exception in Java? ( answer ) hint: In case of checked, you must handle exception using catch block, while in case of unchecked, it’s up to you; compile will not bother you.

49) Difference between Error and Exception in Java? ( answer ) hint: I am tired of typing please check the answer

50) Difference between Race condition and Deadlock in Java? ( answer ) hint: both are errors that occur in a concurrent application, one occurs because of thread scheduling while others occur because of poor coding. See Multithreading and Parallel Computing in Java to learn more about deadlock, Race Conditions, and other multithreading issues.

Closing Notes

Thanks, You made it to the end of the article … Good luck with your programming interview! It’s certainly not going to be easy, but by following this roadmap and guide, you are one step closer to becoming a DevOps engineer .

If you like this article, then please share with your friends and colleagues, and don’t forget to follow javinpaul on Twitter!

Additional Resources

• Java Interview Guide: 200+ Interview Questions and Answers
• Spring Framework Interview Guide — 200+ Questions & Answers
• Preparing For a Job Interview By John Sonmez
• Java Programming Interview Exposed by Markham
• Cracking the Coding Interview — 189 Questions and Answers
• Data Structure and Algorithms Analysis for Job Interviews
• 130+ Java Interview Questions of Last 5 Years

P.S. — If you need some FREE resources to learn Java, you can check out this list of free Java courses to start your preparation. P. S. S. — I have not provided the answer to the interview questions shared in the image “ How many String objects are created in the code?” can you guess and explain?

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Java Interview Questions for 2023

Crack your next tech interview with confidence!

So, let’s dive deep into the surplus of useful interview questions on Java.

Java Interview Questions and Answers

𝑸𝒖𝒆𝒔𝒕𝒊𝒐𝒏𝒔 𝑳𝒊𝒔𝒕.

• Java Interview Questions
• What is java
• List the features of Java Interview Questions.
• What do you understand by Java virtual machine?
• What is the difference between JDK, JRE, and JVM?
• What is the platform?
• What are the main differences between the Java platform and other platforms?
• How many types of memory areas are allocated by JVM?
• Why is Java a platform independent language?
• What do you understand by an instance variable and a local variable?
• What gives Java its 'write once and run anywhere' nature?
• What are the various access specifiers in Java?
• What is the purpose of static methods and variables?
• What is an object?
• What is the constructor?
• How many types of constructors are used in Java?
• Does constructor return any value?
• What are the advantages of Packages in Java?
• What do you understand by copy constructor in Java?
• What are the differences between the constructors and methods?
• What is the static variable?
• What are the restrictions that are applied to the Java static methods?
• What is the difference between static (class) method and instance method?
• Can we make constructors static?
• Explain the concept of constructor overloading
• What is the static block?
• Can we execute a program without main() method?
• Explain method overloading and overriding by citing relevant examples.
• Can single try block and multiple catch blocks can co-exist in a Java Program. Explain.
• Explain the use of final keyword in variable, method and class.
• Explain public static void main(String args[]) in Java.
• What are wrapper classes in Java?
• What is the difference between Array list and vector in Java?
• What is the difference between this() and super() in Java?
• Why Java Strings are immutable in nature?
• What is singleton class in Java and how can we make a class singleton?
• Can you call a constructor of a class inside another constructor?
• What happens when the main() isn't declared as static?
• In Java, how do you decide whether to use a string literal or a string object?
• Can we override private methods in Java?
• What is the difference between StringBuffer and String?

OOPs Concepts

• What is OOPS (Object Oriented Programming) and what are the four pillars of oops?
• What is Inheritance?
• What do you mean by data encapsulation?
• What is Polymorphism?
• What is runtime polymorphism or dynamic method dispatch?
• What do you mean by an interface in Java?
• What are the different types of inheritance in Java?
• What is multiple inheritance? Is it supported by Java?
• What is an association?
• What do you mean by aggregation?
• What is the difference between abstract classes and interfaces?
• Why we can’t create the object of abstract class in java?
• If a child class inherits base class then are the constructor of the base class also inherited by the child class?
• What happens if we make the constructor final?
• Are there any limitations of Inheritance?
• What is a superclass?
• What is a subclass?
• What is enum in java?
• Pointers are used in C/ C++. Why does Java not make use of pointers?
• Can you tell the difference between equals() method and equality operator (==) in Java?
• What are the differences between HashMap and HashTable in Java?
• Java works as “pass by value” or “pass by reference” phenomenon?
• What is operator overloading?
• Differentiate between data abstraction and encapsulation.
• What are virtual functions?
• What is Garbage Collection(GC)
• What is a friend function?

Exception Handling

• What is an exception?
• What is exception handling?
• What is the difference between an error and an exception?
• What is a try/catch block?
• What is a finally block?
• What is the difference between throw, throws and throwable in Java?
• What are the types of Exception in Java
• Can we override a super class method which is throwing an unchecked exception with checked exception in the sub class?
• Can we keep the statements after finally block If the control is returning from the finally block itself?
• Why an exception occurs in the program?

Multithreading Concepts(Advanced)

• What do you mean by Multithreading? Why is it important?
• What is a thread?
• What is the difference between Thread and Process?
• What are the ways to implement Thread in java?
• Difference between Thread and Runnable in Java
• What are the thread states?
• What is the difference between sleep and wait method in java?
• What is Daemon thread and how to create a Daemon thread in java?
• What is synchronization?
• Which is more preferred - synchronization block or synchronization method?
• What is deadlock?
• What is context-switching in multi-threading?
• What is thread starvation?
• What is thread priority
• What is race condition in java and how we can solve it?
• Is it possible to call the run() method directly to start a new thread?
• What is the lock interface? Why is it better to use a lock interface rather than a synchronized block?
• What is Semaphore? Why do we use them?
• Why wait and notify method are called from synchronized block?
• What’s the difference between class lock and object lock?
• What is ThreadPool?
• What is BlockingQueue?
• What do you mean by inter-thread communication?
• What is Thread Scheduler and Time Slicing?
• What will happen if we don’t override the thread class run() method?

⌚ What is Java?

Java is a high-level, object-oriented, reliable, secure, high-performance, multithreaded, and portable programming language. It is also platform-independent. In June 1991, James Gosling created it. Because it offers its own JRE and API, it can also be referred to as the platform. discussing the origins of the language, which was originally dubbed Oak after an oak tree that was located outside of Gosling's office. Later, the project was known as Green before being renamed Java after the Indonesian coffee variety Java

⌚ List the features of Java Interview Questions.

The Java programming language has the following features.

Simple: Learning Java is simple. Because Java's grammar is based on C++, writing programmes in it is simpler.

Java's object-oriented paradigm enables us to maintain our code as a collection of various types of objects that include both data and behaviour.

Java supports the read-once, write-anywhere paradigm. On every machine, the Java programme can be run. Java programmes (.java) are transformed into executable bytecode (.class), which can be used on any machine.

Platform Independent: The programming language Java is cross-platform. When compared to other programming languages like C and C++, it does not require a platform to run. Java comes with a platform that is used to run its programmes. Java can be run independently of an operating system.

Secure: Java's lack of explicit references makes it secure. Java is made more secure by the addition of the ByteCode and Exception handling concepts.

Strong memory management makes Java a robust programming language. It is more resilient because of ideas like automatic trash collection and exception handling, among others.

⌚ What does Java virtual machine mean to you?

The computer can run Java programmes thanks to the Java Virtual Machine. The main method contained in the Java code is called by JVM, which functions as a run-time engine. The computer system needs to implement the JVM specification. The Java code is converted by the JVM into a Bytecode that is similar to native code and independent of the target machine.

⌚ What distinguishes JDK, JRE, and JVM from one another?

JVM JVM, which stands for Java Virtual Machine, is an abstract machine that offers the runtime environment needed to execute Java bytecode. It is a specification that details how the Java Virtual Machine operates. Oracle and other businesses have given its implementation. JRE is the name of its implementation. For numerous hardware and software systems, JVMs are accessible (so JVM is platform dependent). When we run the Java class, a runtime instance is created. The JVM has three concepts: instance, implementation, and specification.

JRE The Java Runtime Environment is known as JRE. It is how the JVM is implemented. A collection of software tools called the Java Runtime Environment is used to create Java programmes. To offer the runtime environment, it is employed. It is how the JVM is implemented. It really does exist. It has a collection of libraries and additional files that the JVM uses during runtime.

JDK The term "Java Development Kit" is an abbreviation. It is a setting for creating software that is used to create Java applets and applications. It really does exist. JRE and development tools are included. JDK is an implementation of any of the Java Platforms listed below, all of which were made available by Oracle Corporation:

⌚ What is the platform?

The hardware or software environment in which a piece of software is run is known as a platform. Platforms come in two flavours: hardware- and software-based. The platform is software-based and uses Java.

⌚ What distinguishes the Java platform from other platforms in terms of its primary features?

The Java platform differs from other platforms in the ways listed below. 🚀 Java is a software-based platform, as opposed to other platforms that could be either hardware- or software-based. 🚀 While other platforms can only have the hardware components, Java runs on top of other hardware platforms.

⌚ How many different kinds of memory spaces does JVM allocate?

🚀 Class(Method) Area: The Class Area is used to hold structures specific to each class, including the runtime constant pool, fields, method data, and method code.

🚀 Heap: The runtime data region is where the objects' memory is allocated.

🚀 Java stack is used to store frames. It participates in method invocation and return and holds local variables and incomplete results. Each thread has a unique, concurrently constructed private JVM stack. Every time a method is called, a fresh frame is made. Once its method has been invoked, a frame is destroyed.

🚀 Program Counter Register: The address of the Java virtual machine instruction presently being executed is stored in the PC (programme counter) register.

🚀 Native Method Stack: It contains all the native methods used in the application.

⌚ Why Java consider a platform independent language?

Because the compiler compiles the code and then converts it to platform-independent byte code that can be run on various platforms, the Java language was created in a way that it is independent of all hardware and software. The installation of a runtime environment (JRE) on the machine is the only prerequisite for running that byte code.

⌚ What do you understand by an instance variable and a local variable?

Instance variables All methods in the class can access variables known as instance variables. They are stated both inside the class and outside the methods. These variables, which are inextricably linked to an object, characterise its characteristics. All the objects of the class will have their copy of the variables for utilization. If any modification is done on these variables, then only that instance will be impacted by it, and all other class instances continue to remain unaffected.

Local variables Each class object will have a copy of the variables available for use. All other instances of the class continue to be untouched if any changes are made to these variables; just that instance will be affected.

⌚ What gives Java its 'write once and run anywhere' nature?

⌚ what are the various access specifiers in java.

The terms used to specify the access scope of a method, class, or variable in Java are called access specifiers. The four access specifiers in Java are listed below.

🚀 Public Any class or method can access the classes, methods, or variables that are specified as public.

🚀 Protected may be accessible by classes belonging to the same package, by subclasses of this class, or by classes that are members of the same class.

🚀 Default can only be accessed inside the package. All classes, methods, and variables have default scopes by default.

🚀 Private Only other members of the class may access the private class, methods, or variables.

⌚ Why are static variables and methods used?

The static methods and variables are shared by all of the class's objects. The static is a class component, not an object. We don't need to build the object to access the static variables because they are kept in the class area. Therefore, static is utilised when it's necessary to specify variables or methods that apply to all class objects.

For instance, the name of the college is what all the students have in common in the class that simulates a group of college students. As a result, the term " college name" will be considered static.

⌚ What do you mean by object?

The Object is a living thing with a state and behaviour. In Java, an object is an instance of the class with instance variables serving as the object's state and methods serving as the object's activity. The new keyword can be used to construct a class object.

⌚ What do you mean by constructor?

The specific kind of method used to initialise an object's state is known as the constructor. When the class is created and the object's memory is allocated, it is called. When an object is created with the new keyword, the class's default constructor is used each time. The constructor's name must resemble the class name. There can be no explicit return type in the constructor.

⌚ What variety of constructors does Java support?

There are two sorts of constructors in Java, depending on the inputs supplied to them.

Default Constructor: The default Constructor is the one that does not take any arguments. Instance variables are typically initialised with default values using the default constructor. It can also be utilised to carry out various beneficial tasks related to object generation. If no constructor is defined in the class, the compiler implicitly calls the default constructor.

Parameterized Constructor: This constructor is able to initialise instance variables with specified values. In other words, we can say that parameterized constructors are constructors that can receive arguments.

⌚ Does constructor return any value?

 No, constructor does not return any value.

 • While declaring a constructor you will not have anything like return type.

 • In general, Constructor is implicitly called at the time of instantiation.

 • And it is not a method, its sole purpose is to initialize the instance variables.

⌚ What are the advantages of Packages in Java?

There are various advantages of defining packages in Java.

 • Packages avoid the name clashes.

 • The Package provides easier access control.

 • We can also have the hidden classes that are not visible outside and used by the package.

It is easier to locate the related classes.

⌚ What do you understand by copy constructor in Java?

There is no copy constructor in java. However, we can copy the values from one object to another like copy constructor in C++.

There are many ways to copy the values of one object into another in java. They are:

 • By constructor

 • By assigning the values of one object into another

 • By clone() method of Object class

⌚ What are the differences between the constructors and methods?

There are many differences between constructors and methods. They are given below.

⌚ What is the static variable?

The static variable is used to refer to the common property of all objects (that is not unique for each object), e.g., The company name of employees, college name of students, etc. Static variable gets memory only once in the class area at the time of class loading. Using a static variable makes your program more memory efficient (it saves memory). Static variable belongs to the class rather than the object.

⌚ What are the restrictions that are applied to the Java static methods?

Two main restrictions are applied to the static methods.

The static method can not use non-static data member or call the non-static method directly.

this and super cannot be used in static context as they are non-static.

⌚ What is the difference between static (class) method and instance method?

There are many differences between static and instance methods. They are given below.

⌚ Can we make constructors static?

As we know that the static context (method, block, or variable) belongs to the class, not the object. Since Constructors are invoked only when the object is created, there is no sense to make the constructors static. However, if you try to do so, the compiler will show the compiler error.

⌚ Explain the concept of constructor overloading

Constructor overloading is the process of creating multiple constructors in the class consisting of the same name with a difference in the constructor parameters. Depending upon the number of parameters and their corresponding types, distinguishing of the different types of constructors is done by the compiler.

three constructors are defined here but they differ on the basis of parameter type and their numbers.

⌚ What is the static block?

Static block is used to initialize the static data member. It is executed before the main method, at the time of classloading.

⌚ Can we execute a program without main() method?

Yes, we can execute a java program without a main method by using a static block. Static block in Java is a group of statements that gets executed only once when the class is loaded into the memory by Java ClassLoader, It is also known as a static initialization block. Static initialization block is going directly into the stack memory. But it was possible till JDK 1.6. Since JDK 1.7, it is not possible to execute a Java class without the main method.

⌚ Explain method overloading and overriding by citing relevant examples.

In Java, method overloading is made possible by introducing different methods in the same class consisting of the same name. Still, all the functions differ in the number or type of parameters. It takes place inside a class and enhances program readability. The only difference in the return type of the method does not promote method overloading. The following example will furnish you with a clear picture of it.

Method overriding is the concept in which two methods having the same method signature are present in two different classes in which an inheritance relationship is present. A particular method implementation (already present in the base class) is possible for the derived class by using method overriding. Let’s give a look at this example:

⌚ Can single try block and multiple catch blocks can co-exist in a Java Program. Explain.

Yes, multiple catch blocks can exist but specific approaches should come prior to the general approach because only the first catch block satisfying the catch condition is executed. The given code illustrates the same:

⌚ Explain the use of final keyword in variable, method and class.

Java final keyword is a non-access specifier that is used to restrict a class, variable, and method. If we initialize a variable with the final keyword, then we cannot modify its value. If we declare a method as final, then it cannot be overridden by any subclasses. And, if we declare a class as final, we restrict the other classes to inherit or extend it.

If any value has not been assigned to that variable, then it can be assigned only by the constructor of the class.

final method: A method declared as final cannot be overridden by its children's classes.

A constructor cannot be marked as final because whenever a class is inherited, the constructors are not inherited. Hence, marking it final doesn't make sense. Java throws compilation error saying - modifier final not allowed here

⌚ Explain public static void main(String args[]) in Java.

Main() in Java is the entry point for any Java program. It is always written as public static void main(String[] args). public: Public is an access modifier, which is used to specify who can access this method. Public means that this Method will be accessible by any Class. static: It is a keyword in java which identifies it is class-based. main() is made static in Java so that it can be accessed without creating the instance of a Class. In case, main is not made static then the compiler will throw an error as main() is called by the JVM before any objects are made and only static methods can be directly invoked via the class. void: It is the return type of the method. Void defines the method which will not return any value. main: It is the name of the method which is searched by JVM as a starting point for an application with a particular signature only. It is the method where the main execution occurs. String args[]: It is the parameter passed to the main method.

⌚ What are wrapper classes in Java?

Wrapper classes convert the Java primitives into the reference types (objects). Every primitive data type has a class dedicated to it. These are known as wrapper classes because they “wrap” the primitive data type into an object of that class.

⌚ What is the difference between Array list and vector in Java?

⌚ what is the difference between this() and super() in java.

In Java, super() and this(), both are special keywords that are used to call the constructor.

⌚ Why Java Strings are immutable in nature?

In Java, string objects are immutable in nature which simply means once the String object is created its state cannot be modified. Whenever you try to update the value of that object instead of updating the values of that particular object, Java creates a new string object. Java String objects are immutable as String objects are generally cached in the String pool. Since String literals are usually shared between multiple clients, action from one client might affect the rest. It enhances security, caching, synchronization, and performance of the application.

⌚ What is singleton class in Java and how can we make a class singleton?

Singleton class is a class whose only one instance can be created at any given time, in one JVM. A class can be made singleton by making its constructor private.

Example: Java Singleton Class Syntax

⌚ Can you call a constructor of a class inside another constructor?

Yes, we can call a constructor of a class inside another constructor. This is also called as constructor chaining. Constructor chaining can be done in 2 ways- Within the same class: For constructors in the same class, the this() keyword can be used. From the base class: The super() keyword is used to call the constructor from the base class. The constructor chaining follows the process of inheritance. The constructor of the sub class first calls the constructor of the super class. Due to this, the creation of sub class’s object starts with the initialization of the data members of the super class. The constructor chaining works similarly with any number of classes. Every constructor keeps calling the chain till the top of the chain.

⌚ What happens when the main() isn't declared as static?

When the main method is not declared as static, then the program may be compiled correctly but ends up with a severe ambiguity and throws a run time error that reads "NoSuchMethodError."

⌚ In Java, how do you decide whether to use a string literal or a string object?

To hard-code a string in Java, we have two options. A string literal and a string object. So, what's different about these two options? When you use a string literal, the string is interned. That means it's stored in the "string pool" or "string intern pool". In the string pool, each string is stored no more than once. So if you have two separate variables holding the same string literals in a Java program. Those two Strings don't just contain the same objects in the same order, they are in fact both pointers to the same single canonical string in the string pool. This means they will pass an '==' check. If our Strings were instantiated as objects, however, they would not be "interned," so they would remain separate objects (stored outside of the string pool).

⌚ Can we override private methods in Java?

No, we cannot override private or static methods in Java. Private methods in Java are not visible to any other class which limits their scope to the class in which they are declared. The program gives a compile time error showing that display() has private access in Parent class and hence cannot be overridden in the subclass. If a method is declared as static, it is a member of a class rather than belonging to the object of the class. It can be called without creating an object of the class. A static method also has the power to access static data members of the class.

⌚ What is the difference between StringBuffer and String?

String is an Immutable class, i.e. you can not modify its content once created. While StringBuffer is a mutable class, means you can change its content later. Whenever we alter content of String object, it creates a new string and refer to that, it does not modify the existing one. This is the reason that the performance with StringBuffer is better than with String.

⌚ What is OOPS (Object Oriented Programming) and what are the four pillars of oops?

Object-oriented Programming (OOP) is a programming paradigm based on the concept of "objects", which may contain data, in the form of fields, often known as attributes; and code, in the form of procedures, often known as methods.

⌚ What is Inheritance?

Inheritance means one class can extend to another class. So that the codes can be reused from one class to another class. The existing class is known as the Super class whereas the derived class is known as a sub class.

Inheritance is only applicable to the public and protected members only. Private members can’t be inherited.

⌚ What do you mean by data encapsulation?

Data encapsulation is a concept of binding data and code in single unit called object and hiding all the implementation details of a class from the user. It prevents unauthorized access of data and restricts the user to use the necessary data only. Data encapsulation refers to sending data where the data is augmented with successive layers of control information before transmission across a network. The reverse of data encapsulation is de-capsulation, which refers to the successive layers of data being removed (essentially unwrapped) at the receiving end of a network.

⌚ What is Polymorphism?

⌚ What is runtime polymorphism or dynamic method dispatch?

In Java, runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. In this process, an overridden method is called through the reference variable of a superclass. Let’s take a look at the example below to understand it better.

⌚ What do you mean by an interface in Java?

An interface in Java is a blueprint of a class or you can say it is a collection of abstract methods and static constants. In an interface, each method is public and abstract but it does not contain any constructor. Thus, interface basically is a group of related methods with empty bodies. Example:

⌚ What are the different types of inheritance in Java?

⌚ what is multiple inheritance is it supported by java.

If a child class inherits the property from multiple classes is known as multiple inheritance. Java does not allow to extend multiple classes. The problem with multiple inheritance is that if multiple parent classes have the same method name, then at runtime it becomes difficult for the compiler to decide which method to execute from the child class. Therefore, Java doesn’t support multiple inheritance. The problem is commonly referred to as Diamond Problem.

⌚ What is an association?

Association is a relationship where all object have their own lifecycle and there is no owner. Let’s take the example of Teacher and Student. Multiple students can associate with a single teacher and a single student can associate with multiple teachers but there is no ownership between the objects and both have their own lifecycle. These relationships can be one to one, one to many, many to one and many to many.

⌚ What do you mean by aggregation?

An aggregation is a specialized form of Association where all object has their own lifecycle but there is ownership and child object can not belong to another parent object. Let’s take an example of Department and teacher. A single teacher can not belong to multiple departments, but if we delete the department teacher object will not destroy.

⌚ What is the difference between abstract classes and interfaces?

⌚ why we can’t create the object of abstract class in java.

Because an abstract class is an incomplete class (incomplete in the sense it contains abstract methods without body and output) we cannot create an instance or object; the same way we say for an interface.

⌚ If a child class inherits base class then are the constructor of the base class also inherited by the child class?

Constructors are not properties of a class. Hence they cannot be inherited. If one can inherit constructors then it would also mean that a child class can be created with the constructor of a parent class which can later cause referencing error when the child class is instantiated. Hence in order to avoid such complications, constructors cannot be inherited. The child class can invoke the parent class constructor by using the super keyword.

⌚ What happens if we make the constructor final?

If we make the constructors final then the class variables initialized inside the constructor will become unusable. Their state cannot be changed.

⌚ Are there any limitations of Inheritance?

Inheritance in Java is a powerful concept to enable reusability. However, it too comes with a few limitations:

 • Firstly, there is an increase in program execution time due to constant to and fro between parent and child classes.

 • Next, Inheritance leads to tight coupling between the superclass and the subclass.

 • Thirdly, in case of modifications in the program, the developer needs to make changes in both the superclass and the subclass.

 • Lastly, it requires careful implementation otherwise, the program would yield incorrect functionality.

⌚ What is a superclass?

A superclass or base class is a class that acts as a parent to some other class or classes. For example, the Vehicle class is a superclass of class Car.

⌚ What is a subclass?

A class that inherits from another class is called the subclass. For example, the class Car is a subclass or a derived of Vehicle class.

⌚ What is enum in java

Enumerations serve the purpose of representing a group of named constants in a programming language. For example, the 4 suits in a deck of playing cards may be 4 enumerators named Club, Diamond, Heart, and Spade, belonging to an enumerated type named Suit. Other examples include natural enumerated types (like the planets, days of the week, colors, directions, etc.). Enums are used when we know all possible values at compile time, such as choices on a menu, rounding modes, command line flags, etc. It is not necessary that the set of constants in an enum type stay fixed for all time.

⌚ Pointers are used in C/ C++. Why does Java not make use of pointers?

Pointers are quite complicated and unsafe to use by beginner programmers. Java focuses on code simplicity, and the usage of pointers can make it challenging. Pointer utilization can also cause potential errors. Moreover, security is also compromised if pointers are used because the users can directly access memory with the help of pointers. Thus, a certain level of abstraction is furnished by not including pointers in Java. Moreover, the usage of pointers can make the procedure of garbage collection quite slow and erroneous. Java makes use of references as these cannot be manipulated, unlike pointers.

⌚ Can you tell the difference between equals() method and equality operator (==) in Java?

⌚ what are the differences between hashmap and hashtable in java, ⌚ java works as “pass by value” or “pass by reference” phenomenon.

Java always works as a “pass by value”. There is nothing called a “pass by reference” in Java. However, when the object is passed in any method, the address of the value is passed due to the nature of object handling in Java. When an object is passed, a copy of the reference is created by Java and that is passed to the method. The objects point to the same memory location. 2 cases might happen inside the method:

Case 1: When the object is pointed to another location: In this case, the changes made to that object do not get reflected the original object before it was passed to the method as the reference points to another location.

Case 2: When object references are not modified: In this case, since we have the copy of reference the main object pointing to the same memory location, any changes in the content of the object get reflected in the original object.

⌚ What is operator overloading?

Operator overloading refers to implementing operators using user-defined types based on the arguments passed along with it.

⌚ Differentiate between data abstraction and encapsulation.

⌚ what are virtual functions.

A virtual function or virtual method in an OOP language is a function or method used to override the behavior of the function in an inherited class with the same signature to achieve the polymorphism.

The virtual keyword is not used in Java to define the virtual function; instead, the virtual functions and methods are achieved using the following techniques:

 • We can override the virtual function with the inheriting class function using the same function name. Generally, the virtual function is  defined in the parent class and override it in the inherited class.

 • A virtual function should have the same name and parameters in the base and derived class.

 • For the virtual function, an IS-A relationship is necessary, which is used to define the class hierarchy in inheritance.

 • The Virtual function cannot be private, as the private functions cannot be overridden.

 • The virtual functions can be used to achieve oops concepts like runtime polymorphism.

In a nutshell, the methods or functions that can be used to achieve the polymorphism are the virtual functions or methods in Java.

⌚ What is Garbage Collection(GC)

Garbage Collection is process of reclaiming the runtime unused memory automatically. In other words, it is a way to destroy the unused objects. To do so, we were using free() function in C language and delete() in C++. But, in java it is performed automatically. So, java provides better memory management.

⌚ What is a friend function?

A function prototype preceded with the keyword “friend” and defined outside the class's scope allows it to access all private and protected members of the class. Even though the prototype for friend function appears in the class definition, friends are not member functions. Coming to the second half of the question, JAVA doesn't support “friend” keyword but the concept of friend function can be implemented in JAVA by choosing appropriate access specifiers for the class and it's members.

⌚ What is an exception?

An exception is a kind of notification that interrupts the normal execution of a program. Exceptions provide a pattern to the error and transfer the error to the exception handler to resolve it. The state of the program is saved as soon as an exception is raised.

⌚ What is exception handling?

Exception handling in Object-Oriented Programming is a very important concept that is used to manage errors. An exception handler allows errors to be thrown and caught and implements a centralized mechanism to resolve them.

⌚ What is the difference between an error and an exception?

⌚ what is a try/catch block.

A try/catch block is used to handle exceptions. The try block defines a set of statements that may lead to an error. The catch block basically catches the exception.

⌚ What is a finally block?

A finally block consists of code that is used to execute important code such as closing a connection, etc. This block executes when the try block exits. It also makes sure that finally block executes even in case some unexpected exception is encountered.

⌚ What is the difference between throw, throws and throwable in Java?

Throw: The throw keyword in Java is used to explicitly throw an exception from a method or any block of code. We can throw either checked or unchecked exception. The throw keyword is mainly used to throw custom exceptions.

throws: throws is also a keyword in java which is used in the method signature to indicate that this method may throw mentioned exceptions. The caller to such methods must handle the mentioned exceptions either using try-catch blocks or using throws keyword. Below is the syntax for using throws keyword.

Throwable: Throwable is a super class for all types of errors and exceptions in java. This class is a member of java.lang package. Only instances of this class or it’s sub classes are thrown by the java virtual machine or by the throw statement. The only argument of catch block must be of this type or it’s sub classes. If you want to create your own customized exceptions, then your class must extend this class.

⌚ What are the types of Exception in Java

Exceptions can be categorized into two ways:

Built-in Exceptions

Checked Exception

Unchecked Exception

User-Defined Exceptions

Checked Exceptions

These are the exceptions that are checked at compile time. If some code within a method throws a checked exception, then the method must either handle the exception or it must specify the exception using the throws keyword.

Unchecked Exceptions

These are the exceptions that are not checked at compile time. In C++, all exceptions are unchecked, so it is not forced by the compiler to either handle or specify the exception. It is up to the programmers to be civilized, and specify or catch the exceptions. In Java exceptions under Error and RuntimeException classes are unchecked exceptions, everything else under throwable is checked.

⌚ Can we override a super class method which is throwing an unchecked exception with checked exception in the sub class?

No. If a super class method is throwing an unchecked exception, then it can be overridden in the sub class with same exception or any other unchecked exceptions but can not be overridden with checked exceptions.

⌚ Can we keep the statements after finally block If the control is returning from the finally block itself?

No, it gives unreachable code error. Because, control is returning from the finally block itself. Compiler will not see the statements after it. That’s why it shows unreachable code error.

⌚ Why an exception occurs in the program?

There can be many reasons that might generate an exception in a Java program.

1. Opening a non-existing file in your program.

2. Reading a file from a disk but the file does exist there.

3. Writing data to a disk but the disk is full or unformatted.

4. When the program asks for user input and the user enters invalid data.

5. When a user attempts to divide an integer value by zero, an exception occurs.

When a data stream is in an invalid format, etc

⌚ What do you mean by Multithreading? Why is it important?

Multithreading means multiple threads and is considered one of the most important features of Java. As the name suggests, it is the ability of a CPU to execute multiple threads independently at the same time but sharing the process resources simultaneously. Its main purpose is to provide simultaneous execution of multiple threads to utilize the CPU time as much as possible. It is a Java feature where one can subdivide the specific program into two or more threads to make the execution of the program fast and easy.

⌚ What is a thread?

A Thread is a concurrent unit of execution. We can say that it is part of the process which can easily run concurrently with other parts of the process.

⌚ What is the difference between Thread and Process?

Following are the differences between thread and process:

A process can have many threads. Threads can execute any part of process.And same part of Process can be executed by multiple threads.

Processes have their own address while Thread share the address space of the process that created it.

Thread has its own stack while in process all threads share a common system resource like heap memory.

⌚ What are the ways to implement Thread in java?

There are two ways to implement Thread in java.

1. By implementing Runnable interface in java and then creating Thread object from it.

2. By extending the Thread class.

⌚ Difference between Thread and Runnable in Java

There are two ways to create a new thread of execution. One is to declare a class to be a subclass of the Thread class. This subclass should override the run method of the Thread class. An instance of the subclass can then be allocated and started.

The other way to create a thread is to declare a class that implements the Runnable interface. That class then implements the run method. An instance of the class can then be allocated, passed as an argument when creating Thread, and started.

Every thread has a name for identification purposes. More than one thread may have the same name. If a name is not specified when a thread is created, a new name is generated for it.

Example of Runnable

Example of Thread

⌚ What are the thread states?

Following are the different thread states:

New: A thread which is just instantiated is in the new state. When a start() method is invoked, the thread becomes the ready state. Then it is moved to the runnable state by the thread scheduler.

Runnable: A thread which is ready to run

Running: A thread which is executing is in running state.

Blocked: A blocked thread is waiting for a monitor lock is in this state. This thing can also happen when a thread performs an I/O operation and moves to the next state.

Waiting: It is a thread that is waiting for another thread to do the specific action.

Timed_waiting: It is a thread that is waiting for another thread to perform.

Terminated: A thread that has exited is in this state.

⌚ What is the difference between sleep and wait method in java?

⌚ what is daemon thread and how to create a daemon thread in java.

Daemon thread in Java is a low-priority thread that runs in the background to perform tasks such as garbage collection. Daemon thread in Java is also a service provider thread that provides services to the user thread. Its life depends on the mercy of user threads i.e. when all the user threads die, JVM terminates this thread automatically.

By setting the setDaemon(true) , we can create a daemon thread in java.

⌚ What is synchronization?

Synchronization is the capability to control the access of multiple threads to any shared resource.

The main advantage of synchronization is:

a. to avoid consistency problem

b. to avoid thread interference

⌚ Which is more preferred - synchronization block or synchronization method?

Synchronized block is the more preferred way because it doesn't lock the object while synchronized methods lock the object. Synchronized method will stop multiple synchronized blocks in the class, even though they are not related, from the execution and put them in the wait state to get the lock on the object.

⌚ What is deadlock?

Deadlock is a situation where two threads are waiting for each other to release locks holded by them on resources.For example

Thread 1 : locks resource A, waits for resource B

Thread 2 : locks resource B, waits for resource A

⌚ What is context-switching in multi-threading?

It is the process of storing and restoring of CPU state. This helps to resume thread execution from the same point at a later point in time. It is one of the essential features for multitasking operating system and support for the multi-threaded environment.

⌚ What is thread starvation?

Thread starvation is basically a situation or condition where a thread won’t be able to have regular access to shared resources and therefore is unable to proceed or make progress. This is because other threads have high priority and occupy the resources for too long. This usually happens with low-priority threads that do not get CPU for its execution to carry on.

⌚ What is thread priority

Thread priority simply means that threads with the highest priority will get a chance for execution prior to low-priority threads. One can specify the priority but it's not necessary that the highest priority thread will get executed before the lower-priority thread. Thread scheduler assigns processor to thread on the basis of thread priority. The range of priority changes between 1-10 from lowest priority to highest priority.

⌚ What is race condition in java and how we can solve it?

Race condition in Java is a type of concurrency bug or issue that is introduced in your program because of parallel execution of your program by multiple threads at the same time, Since Java is a multi-threaded programming language hence the risk of Race condition is higher in Java which demands a clear understanding of what causes a race condition and how to avoid that. Anyway, Race conditions are just one of the hazards or risks presented by the use of multi-threading in Java just like deadlock in Java. Race conditions occur when two threads operate on the same object without proper synchronization and their operation interleaves on each other.

In order to fix this race condition in Java, you need to wrap this code inside the synchronized block which makes them atomic together because no thread can go inside the synchronized block if one thread is already there.

⌚ Is it possible to call the run() method directly to start a new thread?

No, it's not possible at all. You need to call the start method to create a new thread otherwise run method won't create a new thread. Instead, it will execute in the current thread.

⌚ What is the lock interface? Why is it better to use a lock interface rather than a synchronized block?

Lock interface was introduced in Java 1.5 and is generally used as a synchronization mechanism to provide important operations for blocking.

Advantages of using Lock interface over Synchronization block:

Methods of Lock interface i.e., Lock() and Unlock() can be called in different methods. It is the main advantage of a lock interface over a synchronized block because the synchronized block is fully contained in a single method.

Lock interface is more flexible and makes sure that the longest waiting thread gets a fair chance for execution, unlike the synchronization block.

⌚ What is Semaphore? Why do we use them?

A Semaphore is an integer variable shared among multiple processes. The main aim of using a semaphore is process synchronization and accesss control for a common resource in a concurrent environment. The intial value of a semaphore depends on the problem at hand but mostly, we use the number of resources available as the intial value. As resources start getting occupied we decrement the value of this variable.

There are two types of semaphore:

Binary Semaphore: It can have only two integer values: 0 & 1. It's simpler to implement and provides mutual exclusion. We can use a binary semaphore to solve the critical section problem.

Counting Semaphore: It is again an integer value which can range over an unrestricted domain. We can use it to resolve synchronization problems like resource allocation.

⌚ Why wait and notify method are called from synchronized block?

Wait() is called, so that thread can wait on some condition. When condition is met, then thread has to give up the lock. To give up the lock, thread has to own it first. Thread can acquire lock by enter into synchronized context.

If wait method is called outside of synchronized context, then it will throw IllegalMonitorStateException .

⌚ What’s the difference between class lock and object lock?

Class Lock: In java, each and every class has a unique lock usually referred to as a class level lock. These locks are achieved using the keyword ‘static synchronized’ and can be used to make static data thread-safe. It is generally used when one wants to prevent multiple threads from entering a synchronized block.

Object Lock: In java, each and every object has a unique lock usually referred to as an object-level lock. These locks are achieved using the keyword ‘synchronized’ and can be used to protect non-static data. It is generally used when one wants to synchronize a non-static method or block so that only the thread will be able to execute the code block on a given instance of the class.

⌚ What is ThreadPool?

ThreadPool is a pool of threads that reuses a fixed number of threads to execute the specific task.

⌚ What is BlockingQueue?

BlockingQueue basically represents a queue that is thread-safe. Producer thread inserts resource/element into the queue using put() method unless it gets full and consumer thread takes resources from the queue using take() method until it gets empty. But if a thread tries to dequeue from an empty queue, then a particular thread will be blocked until some other thread inserts an item into the queue, or if a thread tries to insert an item into a queue that is already full, then a particular thread will be blocked until some threads take away an item from the queue.

⌚ What do you mean by inter-thread communication?

Inter-thread communication, as the name suggests, is a process or mechanism using which multiple threads can communicate with each other. It is especially used to avoid thread polling in java and can be obtained using wait(), notify(), and notifyAll() methods.

⌚ What is Thread Scheduler and Time Slicing?

Thread Scheduler: It is a component of JVM that is used to decide which thread will execute next if multiple threads are waiting to get the chance of execution. By looking at the priority assigned to each thread that is READY, the thread scheduler selects the next run to execute. To schedule the threads, it mainly uses two mechanisms: Preemptive Scheduling and Time slicing scheduling.

Time Slicing: It is especially used to divide CPU time and allocate them to active threads. In this, each thread will get a predefined slice of time to execute. When the time expires, a particular thread has to wait till other threads get their chances to use their time in a round-robin fashion. Every running thread will get executed for a fixed time period.

⌚ What will happen if we don’t override the thread class run() method?

Nothing will happen as such if we don’t override the run() method. The compiler will not show any error. It will execute the run() method of thread class and we will just don’t get any output because the run() method is with an empty implementation.

🚀 Conclusion

We have now reached the end of this page. With the knowledge from this page, you should be able to create your own programmes with some research, and it's in fact recommended to hone your programming skills with small projects. There is no way to cover all the information you need to be a successful programmer in one course. In fact, programming is a constant learning process, regardless of whether you are a seasoned professional developer or a newbie.

TEST YOUR KNOWLEDGE!

1. which component is used to compile, debug and execute the java programs, 2. which one of the following is not a java feature, 3. which environment variable is used to set the java path, 4. what will be the output of the following java program, 5. which of the following is not a segment of memory in java, 6. what will be the output of the following java program, 7. which of these class is a superclass of every class in java, 8. which of these keywords can be used to prevent method overriding in java, 9. which of these packages contains the exception stack overflow in java, 10. java extension used in threads, 11. a thread become non runnable when, 12. a method used to force one thread to wait for another thread to finish.

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10 Tricky Programming Questions to Ace Your Java Interview

Team CodeQuotient / June 9, 2022 / Software Engineering Bootcamp

If you are preparing for a Java job interview, you must be familiar with these tricky programming questions. Every interview usually comes with tricky questions that trick candidates into thinking either too much or not at all.

With over 45 billion active virtual machines globally , Java is one of the most popular programming languages in the world. It is the number one development language for microservices and is the most-used language for DevOps, AI, VR, Big Data, Continuous Integration, Analytics, Mobile, Chatbots, and much more.

With over 9 million active Java developers globally , the demand for talent in this domain is at an all-time high. As per Mercer, one of the global leaders in talent assessment, the role of a Java developer is one of the most in-demand roles in 2022, thanks to the ever-increasing adoption of Java in multiple domains, devices, and organisations.

However, clearing an interview for this job role isn’t all that easy, given that most interview panellists tend to throw tricky questions at the candidates to find the best talent for the job. Let’s see some of those.

Java Programming – Tricky Interview Questions

Let us show you the ten most tricky Java programming questions you can expect in an interview, with solutions.

1. What will the following Java program print?

public class Test { public static void main(String[] args) {

System.out.println(Math.min(Double.MIN_VALUE, 0.0d)); } }

This question is tricky because, unlike the Integer class, where the MIN VALUE is negative, the Double class’s MAX VALUE and MIN VALUE are both positive integers. Double.MIN VALUE is 2^(-1074), a double constant with the least magnitude of all double values.

Because of Double, this program will print 0.0 instead of the obvious answer. The value of MIN VALUE is greater than zero.

If you’d like to understand the difference between a compiler and an interpreter in Java, Click here .

2. What will happen if you put the return statement or System.exit () on the ‘try‘ or ‘catch‘ block? Will the ‘finally’ block execute?

Many experienced Java programmers think the ‘finally’ block will always execute. However, that reasoning is challenged here by putting a return statement in the ‘try’ or ‘catch’ block or calling System.exit() from the ‘try’ or ‘catch’ block.

The solution for this mind-bender is that the ‘finally’ block will execute successfully, even if you put a return statement in the ‘try’ or ‘catch’ block. However, the ‘finally’ block will fail to execute if you call System.exit() from the ‘try’ or ‘catch’ block.

3. Can you override a private or static method in Java?

Method overriding is always an excellent topic to confuse developers. You cannot override a static method in Java. It will hide the superclass method if you create an identical method with the same return type and method arguments as the ones in the child class. This is also known as method hiding.

Likewise, overriding a private method in the subclass is impossible as it is not accessible there. As a workaround, you can create a new private method with the same name in the child class.

If you’re new to software development and overwhelmed with new terms, here are the 19 most important terms you should know!

4. What will the expression 1.0 / 0.0 return? Will it throw an exception or any compile-time errors?

It will compile successfully and not throw ArithmeticException. It will instead return Double.Infinity.

5. Does Java support multiple inheritances?

This is the most challenging problem in Java. Interviewers frequently argue that since C++ can allow multiple direct inheritances, why can’t Java? 

Because Java supports multiple inheritances of Type by enabling one interface to extend other interfaces, the answer is considerably more complicated than it appears. Java does not allow multiple inheritances of implementation.

6. What will happen if we put a key object in a HashMap already there?

HashMap does not allow duplicate keys, so putting the same key again will overwrite the original mapping. The same key will generate the same hashcode and end up at the same bucket position. Each bucket contains a linked list of maps.

This is an entry object that has both a Key and a Value. Java will compare each entry’s Key object to this new key using the equals() method, and if the comparison returns true, the value object in that entry will be replaced by the new value.

7. What does the following Java program print?

public class Test { public static void main(String[] args) throws Exception { char[] chars = new char[] {‘\u0097’}; String str = new String(chars); byte[] bytes = str.getBytes(); System.out.println(Arrays.toString(bytes)); } }

This question is particularly tricky because this program’s output depends on the operating system and locale. On a Windows XP with the US locale, the above program prints 63, but if you run this program on Linux or Solaris, you will get different values.

8. If a method throws NullPointerException in the superclass, can we override it with a method that throws RuntimeException?

Yes, you can throw a superclass of RuntimeException in overridden method, but you can not do that if it’s a checked Exception.

9. What is the difference between CyclicBarrier and CountDownLatch in Java?

You can reuse CyclicBarrier even if the Barrier is broken, but you can not reuse CountDownLatch in Java.

10. Can you access a non-static variable in the static context?

No, you cannot access a non-static variable from the static context in Java, as it will give a compile-time error.

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50+ Important Java Interview Questions and Answers to Know

Study these essential Java interview questions and answers to prepare for upcoming technical interviews and land the Java job you want.

Need to interview a Java developer for a freelance project or job? Here are 37 essential Java interview questions and answers provided by some of our top Java experts here at Arc.

Although technical interviews can’t gauge how well a candidate would perform on a real-life project, this is still an integral part of the hiring process. Here are some Java interview questions that you can ask a developer to evaluate their understanding of the language.

Basic Java Interview Questions

Intermediate java interview questions.

• Advanced Java Interview Questions

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1. What’s the difference between  String ,  StringBuffer , and StringBuilder ?

String  is an immutable class. In older JDKs the recommendation when programmatically building a String was to use  StringBuffer  since this was optimized to concatenate multiple Strings together.

However, the methods  StringBuffer  were marked as synchronized, which meant that there was a performance penalty, hence  StringBuilder  was introduced to provide a non-synchronized way to efficiently concatenate and modify  Strings .

2. How do you run a Java application on the command line and set the  classpath  with multiple jars?

This is one of those Java interview questions where some people will be thinking what!? But, I’ve met a lot of Java developers who’ve not run a Java application outside of an IDE for years.

3. What is the difference between  final ,  finalize  and  finally ?

final  is a Java keyword used to indicate that either a method can not override in a subclass, or a class can not be extended or a field can not be modified.  finalize  is a method that gets called on an instance of an Object when it is garbage collected.  finally  is a Java keyword used in exception handling to indicate a block of code that should always be run whether an exception is thrown or not.

4. How does Garbage Collection prevent a Java application from going out of memory?

This is a tricky Java interview question… it doesn’t have to be!

Garbage Collection simply cleans up unused memory when an object goes out of scope and is no longer needed. However, an application could create a huge number of large objects that causes an OutOfMemoryError.

5. What’s the difference between a  ClassNotFoundException  and  NoClassDefFoundError ?

A ClassNotFoundException means the class file for a requested class is not on the classpath of the application. A NoClassDefFoundErrormeans that the class file existed at runtime, but for some reason the class could not be turned into a Class definition.

A common cause is an exception being thrown in static initialization blocks.

Check out our entire set of software development interview questions to help you hire the best developers you possibly can.

• JavaScript Interview Questions
• Machine Learning Interview Questions
• MongoDB Interview Questions
• TypeScript Interview Questions
• Selenium Interview Questions
• Spring Interview Questions
• Data Engineer Interview Questions
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• SQL Interview Questions
• DevOps Interview Questions
• Engineering Manager Interview Questions
• Java Interview Questions
• PHP Interview Questions
• Ruby on Rails Interview Questions
• Angular Interview Questions
• Android Interview Questions
• Data Warehouse Interview Questions

If you’re a developer, familiarize yourself with the non-technical interview questions commonly asked in the first round by HR recruiters and the questions to ask your interviewer !

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6. Why isn’t  String ‘s .length() accurate?

It isn’t accurate because it will only account for the number of characters within the String. In other words, it will fail to account for code points outside of what is called the BMP (Basic Multilingual Plane), that is, code points with a value of  U+10000  or greater.

The reason is historical: when Java was first defined, one of its goal was to treat all text as Unicode; but at this time, Unicode did not define code points outside of the BMP. By the time Unicode defined such code points, it was too late for char to be changed.

This means that code points outside the BMP are represented with two chars in Java, in what is called a  surrogate pair . Technically, a char in Java is a UTF-16 code unit.

The correct way to count the real numbers of characters within a String, i.e. the number of code points, is either:

or, with Java 8:

7. Given two double values  d1 ,  d2 , why isn’t it reliable to test their equality using:

Because of  Double.NaN  (literally: “Not a Number”).

will print  false .

The most accurate way to tell whether two double values are equal to one another is to use  Double.compare()  and test against 0, as in:

8. What is the problem with this code:

There are, in fact, two problems:

• the code relies on the default Charset of the JVM;
• it supposes that this default Charset can handle all characters.

While the second problem is rarely a concern, the first certainly is a concern.

For instance, in most Windows installations, the default charset is  CP1252 ; but on Linux installations, the default charset will be UTF-8.

As such, such a simple string as “é” will give a different result for this operation depending on whether this code is run on Windows or Linux.

The solution is to always specify a Charset, as in, for instance:

The what is the problem with this code? question is one of the most popular Java interview questions, but it’s not necessarily going to be this one above, of course. Be prepared to do some detective work to identify the issue.

Also, keep in mind: while the problem may be exception handling, method overloading, an access specifier issue, or something else, it could also be nothing at all! This is one of those trick Java interview questions where the answer will rely on your gut that everything is perfect with the code already.

9. What is the JIT?

The JIT is the JVM’s mechanism by which it can optimize code at runtime.

JIT means Just In Time. It is a central feature of any JVM. Among other optimizations, it can perform code inlining, lock coarsening or lock eliding, escape analysis etc.

The main benefit of the JIT is on the programmer’s side: code should be written so that it just works; if the code can be optimized at runtime, more often than not, the JIT will find a way.

(On a more advanced note: the JIT is such a complex piece of machinery that it makes it complicated to do accurate performance benchmarks for JVM code; this is why such frameworks as JMH exist.)

10. How do you make this code print  0.5  instead of  0 ?

prints  0 . Why? How do you make this code print  0.5  instead?

The problem here is that this expression:

has integer literals on both sides of the operator:  1  and  2 . As a consequence, an integer division will be performed, and the result of  1  divided by  2  in an integer division is  0 .

In order for the result to be a double as expected, at least one operand of the operation needs to be a double. For instance:

11. What is the inferred type of the method reference System.out::println?

In this code:

what is the inferred type of the method reference  System.out::println?

It is an  IntConsumer .

IntStream.range(0, 10)  returns an  IntStream , and  IntStream  defines a .forEach() method accepting an IntConsumer as an argument, whose prototype is:

System.out  is a  PrintStream , and a  PrintStream  has a method named  println  which takes an int as an argument and returns void. This matches the signature of an  IntConsumer , hence the result.

12. What is the problem with this code?

The problem is that the Stream returned by  Files.lines()  is not closed.

This should be used instead:

Stream  extends  BaseStream , and  BaseStream  extends  AutoCloseable . While this has no influence on streams you obtain from collections for instance, the stream returned by  Files.lines()  is I/O bound. Neglecting to close it correctly may lead to a resource leak if an error occurs while processing the stream.

13. What will be the contents of a list after a given operation and why?

Consider the following piece of code: (Question provided by Francis Galiegue)

What will be the contents of the list after this operation and why?

The contents will be:

The reason is that there are two removal operations on a List:

• remove(int index)
• remove(Object obj)

The JVM will always select the most specific overload of a method; and here we pass an int as an argument, the code therefore removes the element at index 2.

To remove the _element_ 2 from the list, the following needs to be written:

14. Write a function to detect if two strings are anagrams (for example, SAVE and VASE)

This is my go-to first interview question. It helps me gauge a candidate’s ability to understand a problem and write an algorithm to solve it.

If someone has not solved the problem before, I expect to see some code with loops and if/then’s. Maybe some  HashMaps . I look for the ability to break down the problem to see what you need to check, what the edge cases are, and whether the code meets those criteria.

The naive solution is often to loop through the letters of the first string and see if they’re all in the second string. The next thing to look for is that the candidate should also do that in reverse too (check string 1 for string 2’s letters)? The next thing to look for is, what about strings with duplicate letters, like VASES?

If you can realize that these are all required and create a functional, non-ridiculous solution, I am happy.

Of course, one can solve it trivially by sorting and comparing both strings. If someone catches this right away, they usually have seen the problem before. But that’s a good sign that someone cares enough to do prep work. Then we can tackle a harder problem.

The details of the implementation are not important; what’s important is that the candidate understands what they need to do, and also understands why their solution works or doesn’t work. If the candidate can demonstrate this, they’re on the right track.

Here is one way to implement a better solution, comparing sorted strings:

15. What is the contract between equals and hashCode of an object?

The only obligation is that for any objects  o1  and  o2  then if  o1.equals(o2)  is  true  then  o1.hashCode() == o2.hashCode()  is true.

Note that this relationship goes only one way: for any o1, o2 of some class C, where none of o1 and o2 are null, then it can happen that o1.hashCode() == o2.hashCode() is true BUT o1.equals(o2) is false.

16. Can an  enum  be extended?

No. Enum types are final by design.

17. How threadsafe is  enum  in Java?

Creation of an  enum  is guaranteed to be threadsafe. However, the methods on an  enum  type are not necessarily threadsafe

18. How does the JVM handle storing local variables vs storing objects?

Objects are stored on the heap. Variables are a reference to the object.

Local variables are stored on the stack.

19. Identify the problem in the below Java code:

A classic example of escaping references.

When an object of  Bar  is created, the super constructor in  Foo  gets called first, which in turn calls the ‘overridden’  doSomething  method.

The  doSomething  method passes the this instance to the class  Zoom .  Zoom  now can use the ‘ this ‘ instance before it is created entirely. BAD!!!

20. When do you use volatile variables?

When a member variable is accessed by multiple threads and want the value of a volatile field to be visible to all readers (other threads in particular) after a write operation completes on it.

More Important Basic Questions for Java Developers

Keep in mind that, although Java is already an object-oriented programming language, you may want to ask questions about object-oriented programming that are more theoretical, conceptual, and outside general Java programming.

Consider including the following additional core Java interview questions on OOP:

• What are classes / objects / abstractions / inheritances in object-oriented programming?
• Can you name the 5 SOLID object-oriented programming design principles?
• How do method overloading and method overriding work in OOP or Java?
• What is an abstract class in Java?

21. Why do you need to use synchronized methods or blocks?

If threads are being used and a number of threads have to go through a synchronized section of code, only one of them may be executed at a time. This is used to make sure shared variables are not updated by multiple threads.

22. What is the difference between HashMap and ConcurrentHashMap ?

ConcurrentHashMap  is thread-safe; that is the code can be accessed by single thread at a time while  HashMap  is not thread-safe.  ConcurrentHashMap  does not allow NULL keys while  HashMap  allows it.

23. When do you need to override the equals and  hashCode  methods in Java?

By defining  equals()  and  hashCode()  consistently, the candidate can improve the usability of classes as keys in hash-based collections such as  HashMap .

24. What is a Service?

A service is a function that is well-defined, self-contained, and does not depend on the context or state of other services.

25. What is a good use case of calling System.gc() ?

One may call  System.gc()  when profiling an application to search for possible memory leaks. All the profilers call this method just before taking a memory snapshot.

26. What is the marker interface in Java?

The marker interface in Java is an interface with no field or methods. In other words, it an empty interface in java is called a marker interface. An example of a marker interface is a Serializable, Clonable, and Remote interface. These are used to indicate something to the compiler or JVM.

27. How are Annotations better than Marker Interfaces?

Annotations allow one to achieve the same purpose of conveying metadata about the class to its consumers without creating a separate type for it. Annotations are more powerful, too, letting programmers pass more sophisticated information to classes that “consume” it.

28. What are checked and unchecked exceptions? When do you use them?

A  checked  exception is an exception that must be catch, they are checked by the compiler. An  unchecked  exception is mostly runtime exception, and is not required to be catch. In general, use checked exception when the situation is recoverable (retry, display reasonable error message).

29.  int a = 1L; won’t compile and int b = 0; b += 1L;  compiles fine. Why?

When  +=  is used, that’s a compound statement and the compiler internally casts it. Whereas in the first case, the compiler straightaway shouts at you since it is a direct statement.

Compiler behavior and statement types can be confusing, so questions like this will test a candidate’s grasp of these concepts.

30. Why aren’t you allowed to extend more than one class in Java but are allowed to implement multiple interfaces?

Extending classes may cause ambiguity problems. On the other hand, in terms of interfaces, the single method implementation in one class can serve more than one interface.

Other Intermediate Interview Questions for Java Developers

Be sure you ask about multithreading, as it’s one of Java’s most important features. Here are a few Java multithreading questions you want to ask:

• How does multithreading work?
• How to implement a thread in Java?
• How to create daemon threads?
• What is thread starvation?
• What is the ExecutorService interface and how does it work?

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Advanced Java Interview Questions for Experienced Developers

31. why doesn’t the following code generate a  nullpointerexception  even when the instance is  null .

There is no need for an instance while invoking a static member or method since static members belong to a class rather than an instance.

A null reference may be used to access a class (static) variable without causing an exception.

32. Look at the below code. Why is the code printing  true  in the second and  false  in the first case?

JVM’s cache behavior can be confusing, so this question tests that concept. The second output is  true  as we are comparing the references because the JVM tries to save memory when the Integer falls within a range (from -128 to 127).

At point 2, no new reference of type Integer is created for ‘d’. Instead of creating a new object for the Integer type reference variable ‘d’, it is only assigned with a previously created object referenced by ‘c’. All of these are done by JVM.

33. How do you check if the given two strings below are anagrams or not?

34. how do you reverse  string("java programming")  without using iteration and recursion, 35. give real-world examples of when to use an  arraylist  and when to use  linkedlist ..

ArrayList  is preferred when there are more  get(int) , or when search operations need to be performed as every search operation runtime is  O(1) .

If an application requires more  insert(int)  and  delete(int)  operations, then  LinkedList  is preferred, as  LinkedList  does not need to maintain back and forth to preserve continued indices as  arraylist  does. Overall this question tests the proper usage of collections.

36. What is the difference between an Iterator and a ListIterator ?

This question tests the proper usage of collection iterators. One can only use  ListIterator  to traverse  Lists , and cannot traverse a  Set  using  ListIterator .

What’s more, one can only traverse in a forward direction using  Iterator s. Using  ListIterator , one can traverse a  List  in both the directions (forward and backward).

One cannot obtain indexes while using  Iterator . Indexes can be obtained at any point of time while traversing a list using  ListIterator . The methods  nextIndex()  and  previousIndex()  are used for this purpose.

37. What is the advantage of a generic collection?

They enable stronger type checks at compile time.

A Java compiler applies strong type checking to generic code, and issues errors if the code violates type safety. Fixing compile-time errors is easier than fixing runtime errors, which can be difficult to find.

Hopefully, you’ve found these interview questions useful when vetting Java developers.

Keep in mind that the technical interview is just one portion of the hiring process. Whether you’re hiring freelance or full-time Java developers, you also want to evaluate their soft skills like communication, problem-solving, time management, and more.

The Arc team publishes insightful articles and thought leadership pieces related to software engineering careers and remote work. From helping entry-level developers land their first junior role to assisting remote workers struggling with working from home to guiding mid-level programmers as they seek a leadership position, Arc covers it all and more!

Further reading

20 Spring Interview Questions and Answers to Know (With MVC & Boot)

8 Questions to Ask Recruiters Before Committing to the Dev Hiring Process

29 Angular Interview Questions and Answers to Practice & Prepare For

How to Write a Great Thank-You Email After an Interview

11 Ruby on Rails Interview Questions and Answers

50+ Essential Vue Interview Questions & Answers (Easy to Advanced)

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Is Java platform-independent? How?

Yes, Java is a platform-independent language. This is because the Java compiler converts the source code into bytecode, which can be executed by any Java Virtual Machine (JVM), regardless of the underlying hardware and operating system​​.

What is Java?

Java is a high-level, object-oriented, robust, and secure programming language that is platform-independent and portable. Developed by James Gosling at Sun Microsystems (which is now a subsidiary of Oracle Corporation) in 1991, Java is designed for high performance, with a strong emphasis on runtime safety and multithreading​​​​.

Can you list the features of the Java programming language?

• The core features of Java include object-oriented structure, high performance, robustness, security, multithreading capabilities, architectural neutrality (platform independence), and portability. Java is also known for its extensive library support​​.

What is the Java Virtual Machine (JVM)?

• The JVM is a part of the Java Runtime Environment (JRE) that provides a runtime environment for executing Java bytecode. It is the component of the Java platform that makes Java a platform-independent language by enabling Java bytecode to be executed on any system that has the JVM installed​​.

Describe the Spring framework.

• The Spring framework is a comprehensive tool for supporting the development of Java applications. It provides an extensive programming and configuration model for modern Java-based enterprise applications. Its core features include dependency injection (IoC), aspect-oriented programming, and transaction management, among others. Spring is designed to work with any Java application and is widely used for building scalable, robust, and maintainable systems​​.

How do you reverse a string in Java?

• In Java, you can reverse a string by using a StringBuilder or StringBuffer class and calling the reverse() method. Java does not have a built-in reverse method for the String class due to the immutable nature of strings​​.

Explain JDK, JRE, and JVM.

• JDK stands for Java Development Kit and is a software development environment used for developing Java applications. It includes the JRE and development tools. The JRE, or Java Runtime Environment, is the installation package that provides the environment to run Java applications and applets. It includes the JVM, libraries, and other components to run applications written in Java. The JVM, as mentioned earlier, is the Java Virtual Machine, which actually executes Java bytecode​​.

What is the public static void main(String[] args) in Java?

• This method is the entry point for any Java application. public denotes that this method can be accessed from anywhere, static allows the method to be called without an instance, void means it does not return any value, and String[] args is an array of String objects that accepts arguments passed to the application during runtime​​.

What is a class loader in Java?

• The classLoader in Java is a part of the Java Runtime Environment that dynamically loads Java classes into the JVM. Java applications aren’t a single block of code but are composed of many individual classes. ClassLoaders load these classes when required, typically the ones not part of the Java core API.

How is Java different from C++?

• Java is purely object-oriented, meaning it doesn’t support global functions or variables outside of a class structure. It has a garbage collector for automatic memory management, whereas C++ requires manual memory management. Java also has a single inheritance model using interfaces, unlike C++ which allows for multiple inheritance. Java’s platform independence makes it distinct, as compiled Java code can run on all platforms that have a JVM, which is not the case with C++.

What are wrapper classes in Java?

• Wrapper classes in Java provide a way to use primitive data types (int, char, etc.) as objects. The primitive data type is wrapped in an object of its corresponding class, such as Integer for int, Character for char, and so on. Wrapper classes are used in Java classes that require an object as an argument, like in collections.

What is the difference between equals() and == in Java?

• In Java, == operator is used to compare primitives and check if two references point to the same object, not comparing the content inside the objects. The equals() method is intended to check the actual contents of an object for equality, rather than their references.

Explain exception handling in Java.

• Exception handling in Java is a robust mechanism that allows a program to deal with runtime errors without crashing. It’s done using four keywords: try, catch, finally, and throw. The try block encloses code that might throw an exception, while catch is used to handle the exception that occurs. finally contains code that must be executed whether an exception occurs or not, and throw is used to explicitly throw an exception.

What are Java Streams?

• Java Streams represent a sequence of elements on which one or more operations can be performed. Stream operations are either intermediate (returning a Stream) or terminal (returning a result or side-effect). They provide a high-level abstraction for Java collection manipulation, using functional-style operations.

What is the purpose of System.out.println() in Java?

• System.out.println() is used to print messages to the console. It is often used for debugging purposes, allowing developers to output text, variables, and other data types to the standard output stream, which is commonly the console or screen.

Explain generics in Java.

• Generics enable types (classes and interfaces) to be parameters when defining classes, interfaces, and methods. They provide a way to ensure type safety by making the code generic, so it can work with different data types while providing compile-time type checking.

What is multithreading in Java?

• Multithreading is a Java feature that allows concurrent execution of two or more parts of a program for maximum utilization of CPU. Each part of such a program is called a thread, and threads run in parallel within the process. Multithreading is used in Java to perform complicated tasks in a background thread without affecting the main thread’s performance.

Explain the concept of the Java Memory Model.

• The Java Memory Model specifies how the Java Virtual Machine works with the computer’s memory. It defines the interaction between threads and memory and ensures that shared variables are consistently updated across all threads.

What is a Java Package and how is it used?

• A Java package is a namespace that organizes a set of related classes and interfaces. Conceptually you can think of packages as being similar to different folders on your computer. They are used to avoid name conflicts and to control access, making it easier to locate and use classes, interfaces, enumerations, and annotations.

What is an abstract class?

• An abstract class in Java is a class that cannot be instantiated and is used to declare common characteristics for subclasses. It can contain abstract methods (without bodies) as well as concrete methods (with bodies). Abstract classes are used as a base for other classes to extend and implement the abstract methods.

Explain the concept of Constructors in Java.

• Constructors in Java are special methods that are called when an object is instantiated. They have the same name as the class and are used to set up initial values for object attributes.

What is the purpose of the final keyword in Java?

• In Java, the final keyword can be used to mark a variable as unchangeable (constant), a method as not overrideable, or a class as not inheritable.

Describe the Singleton pattern in Java.

• The Singleton pattern is a design pattern that ensures a class has only one instance and provides a global point of access to it. In Java, this can be implemented by making the constructor private, defining a static method that returns the instance, and ensuring that only one instance is created by controlling the instantiation logic.

What is the difference between Abstract Class and Interface in Java?

• Abstract classes can have both abstract and concrete methods, provide a base for subclasses, and can have constructors. Interfaces can only declare methods (before Java 8 all methods are public and abstract by default) and constants (public static final). With Java 8 and onwards, interfaces can also have default and static methods with a body.

How does Java achieve memory management?

• Java achieves memory management through its garbage collection, which automatically deallocates memory that is no longer in use or reachable by an application. This helps prevent memory leaks and other related problems.

What is synchronization in Java?

• Synchronization in Java is a mechanism that ensures that only one thread can access the resource at a time. It is used to prevent thread interference and memory consistency errors in multithreaded applications.

Explain method overloading and method overriding in Java.

• Method overloading occurs when multiple methods have the same name but different parameters (type or number). It is a compile-time polymorphism. Method overriding occurs when a subclass provides a specific implementation of a method that is already provided by its parent class or interface. It is a runtime polymorphism.

Can you explain the static keyword in Java?

• The static keyword in Java denotes that a member variable or method belongs to the class, rather than instances of the class. Static methods can be called without creating an object of the class, and static variables are shared among all instances of the class.

What are Java Annotations?

• Annotations in Java are used to provide metadata for Java code. They do not directly affect the operation of the code they annotate but can be used by the development tools and runtime libraries to generate code, XML files, and so forth.

What is a transient variable in Java?

• A transient variable is one that is not serialized during the serialization process. This keyword is used to indicate that a field should not be part of the serialization process and be ignored when objects are serialized and deserialized.

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Welcome to the Scenario-Based Programming Questions repository! 🚀

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This repository is a collection of scenario-based programming questions designed to help developers enhance their problem-solving and critical thinking skills. Each question presents a real-world scenario or problem, challenging you to write code that solves it effectively.

Why Scenario-Based Questions?

Scenario-based questions go beyond basic coding exercises by simulating real-world challenges. They encourage you to consider various factors, make decisions, and implement solutions that are not only correct but also efficient and maintainable. These questions are perfect for honing your coding abilities and preparing for technical interviews.

• Diverse set of programming questions covering different domains and difficulty levels.
• Detailed explanations and sample solutions to help you understand the problem-solving process.
• A platform for collaboration, where you can contribute your own scenario-based questions and solutions.
• A space for discussion, where you can share your insights and learn from others.

Getting Started

• Explore the list of scenario-based questions in the repository.
• Choose a question that interests you.
• Attempt to solve the problem in your preferred programming language.
• Compare your solution with the provided sample solutions and explanations.
• Join the discussion and share your thoughts or alternative solutions in the Issues section.

Contributing Your contributions are welcome! If you have a scenario-based programming question or a solution to share, please follow the Contribution Guidelines.

Feedback Your feedback is invaluable. If you have any suggestions, ideas, or issues to report, please open an issue in the repository, and I'll be happy to address it.

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1. Write Hello World Program in Java

2. write a program in java to add two numbers., 3. write a program to swap two numbers, 4. write a java program to convert integer numbers and binary numbers..

• 5. Write a Program to Find Factorial of a Number in Java
• 6. Write a Java Program to Add two Complex Numbers

7. Write a Program to Calculate Simple Interest in Java

• 8. Write a Program to Print the Pascal’s Triangle in Java

9. Write a Program to Find Sum of Fibonacci Series Number

10. write a program to print pyramid number pattern in java., 11. write a java program to print pattern., 12. write a java program to print pattern., 13. java program to print patterns., 14. write a java program to compute the sum of array elements., 15. write a java program to find the largest element in array, 16. write java program to find the tranpose of matrix, 17. java array program for array rotation, 18. java array program to remove duplicate elements from an array, 19. java array program to remove all occurrences of an element in an array, 20. java program to check whether a string is a palindrome, 21. java string program to check anagram, 22. java string program to reverse a string, 23. java string program to remove leading zeros, 24. write a java program for linear search., 25. write a binary search program in java., 26. java program for bubble sort..

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28. Java Program for Selection Sort.

29. java program for merge sort., 30. java program for quicksort., java exercises – basic to advanced java practice programs with solutions.

Test your Java skills with our topic-wise Java exercises. As we know Java is one of the most popular languages because of its robust and secure nature. But, programmers often find it difficult to find a platform for Java Practice Online. In this article, we have provided Java Practice Programs. That covers various Java Core Topics that can help users with Java Practice.

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5. write a program to find factorial of a number in java., 6. write a java program to add two complex numbers., 8. write a program to print the pascal’s triangle in java.

Time Complexity: O(N) Space Complexity: O(N)

Time Complexity: O(logN) Space Complexity: O(N)

Time Complexity: O(N 2 ) Space Complexity: O(1)

27. Write a Program for Insertion Sort in Java.

Time Complexity: O(N logN) Space Complexity: O(N)

Time Complexity: O(N logN) Space Complexity: O(1)

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As a programming language, Java is considered moderately easy to learn. It is unique from other languages due to its lengthy syntax. As a beginner, you can learn beginner to advanced Java in 6 to 18 months.

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Top 10 Tricky Java interview questions and Answers

10 tricky java interview question - answered.

More Trick Java Questions for Practice?

• When doesn't Singleton remain Singleton in Java?
• is it possible to load a class by two ClassLoader?
• is it possible for equals() to return false, even if the contents of two Objects are the same?
• Why compareTo() should be consistent to equals() method in Java?
• When do Double and BigDecimal give different answers for equals() and compareTo() == 0. 
• How does "has before" apply to volatile work?
• Why is 0.1 * 3 != 0.3,
• Why is (Integer) 1 == (Integer) 1 but (Integer) 222 != (Integer) 222 and which command arguments change this.
• What happens when an exception is thrown by a Thread?
• Difference between notify() and notifyAll() call?
• Difference between System.exit() and System.halt() method?
• Does following code legal in Java? is it an example of method overloading or overriding?

While I'm an certain these questions are getting asked, I find it ironic that being able to answer them in no way indicates any ability to be a successful programmer. That is why they are trick questions I guess.

Good work done! It will definitely help me in my interviews... :) thanks .. :)

Hi , I am able to override public static method declared in base class in its subclass. It didn't throw any compilation or runtime exception

It won't because compiler will treat it as different method. This is called method hiding and its indeed one of the tricky Java question. Remember that this method is not overriding super class method as static method bonded using type information. That's the reason this Java question is tricky.

One of the tricky Java question I faced is "What is difference between Collection and Generics in Java", its not tricky because I don't know either collection or Generics but How can you compare Generics with Collection ?

Correct both are diff things and they are not related at all..

Hi Is the line in codes correct. It's one of ur question. Can you access non static variable in static context? Another tricky Java question from Java fundamentals. "No you can not access static variable in non static context in Java". Read why you can not access non-static variable from static method to learn more about this tricky Java questions.

Hi Ankur, Thanks for pointing out, you should read opposite i.e non static variable can not accessible from static context. This always confuse if you don't remember it and that's why its one of the tricky question.

Hi This line is not correct. "No you can not access static variable in non static context in Java".

you can not access a static variable inside the non-static one,other-way its so true.

all are tricky questions?

its really nice tricky question to read to face interview.

these are not tricky rather easy questions ...

i agree nice question to prepare before appearing for interview

These are definitely a good set of tricky questions that a candidate may face during an interview. I certainly enjoyed going through them but I have a different opinion regarding the last question: Can you access non static variable in static context? I think the answer to that question is a YES albeit one should note that you need an object reference of the associated class to access the variable (I assume this variable is an instance variable). And likewise you can directly access a static variable in non static context too. Please refer to this link for more information http://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html

Is the compareTo code correct ? the references look to be wrong.

The explanation of the last question is little bit confusing. Actually static variables are always accessible in non static context however the reverse is not true.

In static context, you can access non-static variables - by creating a new instance of the object. Hence the reverse is partly true.

public int compareTo(Object o){ Employee emp = (Employee) emp; return this.id - o.id; } I think this is what it should be... public int compareTo(Object o){ Employee emp = (Employee) o; return this.id - o.id; }

Wrong, it should be: public int compareTo(Object o){ Employee emp = (Employee) o; return this.id - emp.id; }

I should also mention that you shouldn't cast to Employee without knowing it's an Employee via instanceof, or just using Comparable generic interface.

Try some more new

Q 1. about try and catch block class Main { public static void main(String args[]) { try { int a=2/0; System.exit(0); } catch(Exception e) { System.out.println("i am in catch block"); } finally { System.out.println("finally"); } } } /*OUTPUT i am in catch block finally*/ finally block get executed if we place System.exit(0) in try block

Before calling System.exit(0) you raised excetion "int a=2/0". Onces system.exit(0) is called finally will never called. plz Don't confused others :)

2/0 is arithmetic exception so before going to system.exit it will jump to catch block and then finally but once system.exit(0) executes ...finally will not execute then .. try it with class Main { public static void main(String args[]) { try { int a=2/1; System.exit(0); } catch(Exception e) { System.out.println("i am in catch block"); } finally { System.out.println("finally"); } } }

as per my java knowledge,try,finally blocks will execute parallely,because if single statement(inside try block first line or empty block) is executed corresponding catch may or may not execute but finally block will execute automatically,otherwise it wont execute,so without try&catch blocks we cant write finally block in java.

System.exit(0) is unreachable statement here.

I was asked during an interview the difference between "arraylist" and "linkedlist" and when I should use them Also, found a site where you can give online Java mock interviews. Its www.preparestreet.com

linklist is dynamic is called at runtime to save memory. where as arrylist is not dynamic whose memory locations are allocated in memory so memory is already stored for that so memory is waste. if u have shortage of memory can use linklist it is envoke at runtime and if u have enough memory can use arraylist..

Wrong. LinkedList uses more memory, because, apart from values, it stores a reference to the previous and next entry. The difference between them in the ways of access to the elements of the list, and add / remove items in the list

What is so tricky about these question, to me they look most simplest question, which doesn't even qualify for Interviews. Tricky questions are those, who challenge your notion e.g. When Singleton doesn't remain Singleton in Java? is it possible to load a class by two ClassLoader? is it possible for equals() to return false, even if contents of two Objects are same? Why compareTo() should be consistent to equals() method in Java? is Following code legal in Java? is it example of method overloading or overriding? public String getDescription(Object obj){ return obj.toString; } public String getDescription(String obj){ return obj; } and public void getDescription(String obj){ return obj; } Anyone disagree with my questions not being tricky?

Actually your questions are better than Javin's Questions...

Trickier) When do Double and BigDecimal give different answers for equals() and compareTo() == 0.

Hi, Good collection of tricky questions, However I feel that the questions in the interview becomes tricky because we need to answer the same as what interviewer in thinking is correct, this is the most tricky part. It is always better to clarify the question correctly by rephrasing it.

Some trickier questions. 1a) how do you prevent a return statement from calling finally. 1b) how do you make a System.exit() call the finally block. 3) Not sure multiple inheritance is the trickest question in Java. How about; 3a) how does "has before" apply to volatile work? 3b) why is 0.1 * 3 != 0.3, 3c) why is (Integer) 1 == (Integer) 1 but (Integer) 222 != (Integer) 222 and which command arguments change this. 7) how can you release synchronized locks in an order which is not the reverse order? (You can do this BTW) Good answers to some common questions, I look forward to some trickier questions. ;)

I would add : What happens when exception is thrown in a Thread? Difference between notify and notifyAll call? Difference between System.exit() and System.halt() method? do you agree these are much trickier question for an average Java programmers?

Is there a System.halt() method exists in Java? I think it is Runtime.getRuntime().halt(status), right?

Thanks for sharing these. I need some good Java interview questions and a few of these might help me out.

No problem Tom, thanks for dropping by, If you have not read already, you may find my article about 10 Questions to make Programming Interview Less Costly useful as well.

Answer to the question "is it possible to load a class by two ClassLoader?" is Yes, it quite possible if you are using a custom class loader or working on managed environment which uses classloader e.g. web and application server. This is one more reason why you should use instanceof instead of getClass() while overriding equals() mehtod, otherwise equals() will return false even if object are same but classloader is different.

hi.. this is hari.can you any one send me jsp&servlets 1.6 year interview questions. this is my id:[email protected] please help me...

How would you describe Enum<E extends Enum<E> > ?

About Q2: What will happen if you put return statement or System.exit () on try or catch block ? Will finally block execute? Here is a code fragment (Java class) I found a long time ago on some C++ forum. It is called "JavaSucks" (sorry) and its content says it all: public class JavaSucks { public static void main(String[] args) { for (;;) { try { System.out.println("Java sucks"); } catch (Exception e) { System.exit(0); } finally { continue; } } } } It will compile without errors? If yes, what do you think it will produce on console as output? Compile it and run it. Result: no compilation errors, just one warning. It will print infinitely the string "JavaSucks"...

Pass by value and pass by reference is also a good tricky Java question . Btw, Java is always pass by value, even for objects its pass by value, its just that reference or handle to the object is passed.

can we declare constructor as private??? if yes then for which purpose??

We declare constructor as private if we want to implement singleton pattern. Also, If you want to create only 1 instance of your custom class then you can use private constructor. Please,write me if I am missing anything.

Hi Sandeep, Yes, you can declare a private constructor in Java. You can do to prevent instantiation of class outside the class, for example, Singleton pattern is one of the prime examples of the private constructor. In Singleton, the class itself is responsible for creating the instance and managing it so constructor is made private.

A fiend who is a really good programmer was given some of these in an interview a few months ago. His response was: "I'd fire who every wrote such lousy code. I never have to worry about such things, because I write defensive and don't allow such nonsense in my code base" He didn't get the job, but he's right.Most of this "trick" questions are are just academic and should never occur to in real life. It's lousy people think these type of questions some how gauge the productivity and ability of a programmer who avoids such pitfalls to begin with and hence doesn't know the answers to the way out of them.

Some questions are really tricky especially "Why 01*3 != 0.3 in Java?", I doubt even experienced Java developers can answer with clarity and confidence. I have asked this question to my friends having 5 and 6 years of experience and my technical lead having 10 years of experience Java, but they couldn't provide me good answer. All they could say is, since some floating point numbers cannot be represented precisely in Java, hence 0.1*0.3 != 0.3

I think in practice questions "How does "has before" apply to volatile work?", you mean how "happens before" concept work with volatile variables? right? as far as I know there are certain rules which decides which update will happen first. I remember reading about it on Java concurrency in Practice, which says that a volatile write will happen before volatile read.

I have recently shared few more Java interview questions especially for developer with 1 to 4 years of experience, you can see it here

Is null key allowed to stored in HashMap? If yes, how it is handled? What will happen if two different HashMap key objects have same hashcode? Visit http://modernpathshala.com/Learn/Java/Interview for more java interview questions and answers.

public class Test { public static void main(String[] args) { System.out.println(Math.min(Double.MIN_VALUE, 0.0d)); } } These type of questions will get you people in the industry who have crammed the language and don't know the design practices. No way to gauge a good programmer from a bad one

What would the following program will print public class Main { public static void main(String[] args) { Collection c = new HashSet(); print(c); } public static void print(Collection c){ System.out.println("Collection"); } public static void print(Set s){ System.out.println("Set"); } public static void print(HashSet hs){ System.out.println("HashSet"); } } I said "HashSet" but it was wrong.

Overriding static methods is called as method hiding.In method hiding method resolution is always based on reference type so it will print Collection.

It will print "Collection" because methods are static and so they are bonded during compile time and at that time only type information is available because object is created at runtime.

Honestly, these questions are poor indicators of how effective they will be on the job. They may be entertaining, but they are really bad questions when screening job candidates for real work. On the job you want people who are creative, capable of exploring available options, and capable of executing well on the tasks that they are assigned to them, large or small. These sort of "gotcha" questions do not actually tell you if they are good developers - just whether they have "happened" to stumbled upon obscure facts or details in their careers or have spend an inordinate amount of time reading java docs.

Hello @Jake, I don't believe the go/no-go decision is based upon these tricky questions. They are mainly used to add the surprise element or check the depth of candidates knowledge in any particular topic. It's a good indicator of candidate's deep and thorough understanding but as you said, don't expect every Java developer knows these subtle details.

Wow Really Nice Collections of Interview Questions . Thank you :)

Hello @Jang Bahadur, thanks you like these tricky Java questions, If you have any, please share with us as well :-)

One of these questions, CyclicBarrier vs CountDownLatch are asked to me on screening round, I managed to answer it well, thanks to internet and you guys.

web services consume or produce the web services if consume explain how

It can do both. for example, HTTP GET request return something from server but by using POST and PUT you can also create new objects into Server.

Very helpful

public int compareTo(Object o){ Employee emp = (Employee) o; return this.id - e.id; } the issue with this code: Compilation error.. E is undefined.

What is E? there is no E here, btw, yes it is not using generics which it could and that way you don't need that cast.

TreeSet allowed null? if it is allowed what is the senario. if it is not allowed what is the senario? please tell me with example

I don't think TreeSet allows null becuase it's a SortedSet and it has to compare elements so if you try to compare null with anything it will result in NullPointerException, btw, it might just allow in case you have just one element. not sure but you can check that by running it yourself.

Really good collection of interview quetion. Thanks

Well, three is nothing wrong ---> Well, there is nothing wrong

yes, indeed, the spelling is wrong :-)

Feel free to comment, ask questions if you have any doubt.

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500+ Java MCQs

Prepare for your Java interviews and exams with our comprehensive collection of 500+ multiple-choice questions. This resource is designed to help you test and improve your knowledge of Java programming concepts, including core Java, object-oriented programming, data structures, algorithms, and more. Whether you are a beginner or an experienced Java developer, these MCQs will challenge your understanding and help you identify areas for improvement. Each question is carefully crafted to assess your problem-solving skills and critical thinking abilities. With this extensive question bank, you can confidently prepare for any Java-related interview or exam. Take advantage of this valuable resource to enhance your Java expertise and increase your chances of success.

Q1. Who is considered the father of Java?

A. Dennis Richie

B. Ken Thompson

C. James Gosling

D. Bjarne Stroupstrup

Ans:: C. James Gosling

Q2. Which of the following statements about the Java language is true?

A. Java supports only Procedural Oriented Programming approach

B. Both Procedural and Object Oriented Programming are supported in Java

C. Java supports only Object Oriented Programming approach

D. None of the Above

Ans:: C. Java supports only Object Oriented Programming approach

Q3. JRE stands for

A. Java Realtime Environment

B. Java Rapid Enterprise

C. Java Runtime Environment

D. None of the above

Ans:: C. Java Runtime Environment

Q4. Java source files are compiled and converted to

A. Object code

B. machine code

C. Bytecode

D. executable file

Ans:: C. Bytecode

Q5. JVM is ___________for bytecode.

A. a compiler

B. an interpreter

C. assembler

D. none of the above

Ans:: B. an interpreter

Q6. What is the size of the int data type in Java?

Ans:: C. 4 bytes

Q7. Java is a __________ language?

A. Platform Dependent

B. Platform Independent

D. Both A & B

Ans:: B. Platform Independent

Q8. In which year Java was developed?

Ans:: A. 1995

Q9. How many access modifiers are present in Java?

Ans:: D. 4 (public, private, default, protected)

Q10.Which of the following is not an access modifier in Java?

B. protected

Ans:: C. Secure

Q11.Which of the following is not an access modifier in Java?

Q12.Which is a valid float literal?

Ans:: D. 1.23f

Q13.Which of these values can a boolean variable contain?

A. True & False

B. 0 & 1

C. Any integer value.

D. Both a & b

Ans:: A. True & False

Q14.What are the types of memory allocated in memory in java?

A. Heap memory

B. Stack memory

C. Both A and B

D. None of these

Ans:: C. Both A and B

Q15.What makes the Java platform independent?

A. Advanced programming language

B. It uses bytecode for execution

C. Class compilation

D. All of these

Ans:: B. It uses bytecode for execution

Q16.Which of these is an incorrect array declaration?

A. int arr[] = new int[5];

B. int [] arr = new int[5];

C. int arr[]; arr = new int[5];

D. int arr[] = int [5] new;

Ans:: D. int arr[] = int [5] new;

Q17.The keyword used to create a constant variable

D. none of these

Ans:: A. final

Q18.What is stored in the object obj in the following lines of code?

A. Memory address of allocated memory of the object.

C. Any arbitrary pointer

Ans:: B. Null

Q19.What is the entry point of a program in Java?

A. main() method

B. The first line of code

C. Last line of code

D. main class

Ans:: A. main() method

Q20.Can the main() method be overloaded in Java?

Ans:: A. Yes(Note: We can overload the main method in Java but JVM only calls the original main method, it will never call our overloaded main method.)

Q21.Which class in Java is used to take input from the user?

Ans:: A. Scanner

Q22. Is string mutable in Java?

Ans:: B. No

Q23.Which of these is a type of variable in Java?

A. Instance Variable

B. Local Variable

C. Static Variable

Ans:: D. All of these

Q24. What will be the output of the below program?

public class Test {

public static void main(String[] args) {

String str = "Hello";

str = "World";

System.out.println(str);

Ans:: B. World

Q25.What is typecasting in Java?

A. It is converting the type of a variable from one type to another

B. Casting variable to the class

C. Creating a new variable

Ans:: A. It is converting the type of a variable from one type to another

Q26.Which class cannot have a subclass in Java?

A.abstract class

B.parent class

C.final class

D.None of the above

Ans:: C. final class (We can not extend a class if it is a final class)

Q27.Which is the keyword used to inherit a class from another class?

B.implements

Ans:: B. extends

Q28.What is the use of the final keyword with method definition?

A.to supports method overriding

B.implements dynamic method dispatch

C.Prevent method overriding

D.None of these

Ans:: C.Prevent method overriding

Q29.Identify the type of inheritance when two or more subclasses inherit the properties of a super class.

A.Multiple inheritance

B.Single inheritance

C.Multilevel inheritance

D.Hierarchical inheritance

Ans:: D. Hierarchical inheritance

Q30.Which keyword is used inside a class to refer to its immediate super class?

D.none of these

Ans:: A. super

Q31.Which of the following is true in the case of abstract class?

A. Abstract constructors cannot be created.

B. Abstract classes can not be inherited.

C. An abstract class contains only abstract methods.

D. All of the above.

Ans:: A. Abstract constructors cannot be created.

Q32. Which of these keywords are used to define an abstract class?

B. abstract

C. Abstract

D. abstract class

Ans:: B. abstract

Q33. Which of these is not a correct statement?

A. Every class containing abstract method must be declared abstract.

B. Abstract class defines only the structure of the class not its implementation.

C. Abstract class can be initiated by new operator.

D. Abstract class can be inherited.

Ans:: C. Abstract class can be initiated by new operator.

Q34. Which method defined in the Integer class can be used to convert a whole number in string type to primitive int type?

A. valueOf()

B. intValue()

C. parseInt()

D. getInteger()

Ans:: C. parseInt()

Q35.The method sqrt() is defined in the class

D. Arithmetic

Answer» C. Math

Q36. Name the keyword that makes a variable belong to a class, rather than being defined for each instance of the class.

C. abstract

Ans:: A. static

Q37. Variables declared within a class are called

A. Identifier

B. local variable

C. instance variable

D. global variable

Ans:: C. instance variable

Q38. Variables declared within a method or block are called

A. Static variable

Ans:: B. local variable

Q39. Defining methods with the same name and different numbers of parameters are called

A. Method overriding

B. method overloading

C. Dynamic method dispatch

Ans:: B. method overloading

Q40. Which is used to initialize the objects.

B. arguments

C. constructors

D. new keyword

Ans:: C. constructors

Q41. What is the return type of Constructors?

D. None of the these

Ans:: D. None of the these

Q42. Which of the following is a method having the same name as that of its class?

A. finalize

D. constructor

Ans:: D. constructor

Q43. Which operator is used by Java run time implementations to free the memory of an object when it is no longer needed?

Ans:: D. None of these

Q44. Which of these access specifiers must be used for main() method?

C. protected

Ans:: B. public

Q45. Which of these is used to access the member of a class before the object of that class is created?

D. protected

Ans:: C. static

Q46. Which keyword is used to create an object?

Ans:: C. new

Q47. Which keyword is used to refer to the current object?

Ans:: B. this

Q48. A Method that is automatically invoked during garbage collection.

A. destructor

B. terminate()

C. finalize()

D. destroy()

Ans:: C. finalize()

Q49. A primitive data type can be passed as an argument into a method.

A. By Value

B. by reference

C. both a & b

Ans:: A. By Value

Q50. Which of these is used as a default for a member of a class if no access specifier is used for it?

C. public, within its own package

Ans:: C. public, within its own package

Q51. Which of these keywords is used to refer to a member of the base class from a sub-class?

Ans:: B. super

Q52. Which of these is the correct way of inheriting class A by class B?

A. class B + class A {}

B. class B inherits class A {}

C. class B extends A {}

D. class B extends class A {}

Ans:: C. class B extends A {}

Q53. Which of the following statements are true about interfaces?

A. Methods declared in interfaces are implicitly private.

B. Variables declared in interfaces are implicitly public, static, and final.

C. An interface contains any number of method definitions.

D. The keyword implements indicate that an interface inherits from another.

Ans:: B. Variables declared in interfaces are implicitly public, static, and final.

Q54. Which of these keywords is used to define interfaces in Java?

A. interface

B. Interface

D. implements

Ans:: A. interface

Q55. Which of these can be used to fully abstract a class from its implementation?

B. Packages

C. Interfaces

Ans:: C. Interfaces

Q56. Which of these access specifiers can be used for an interface?

B. Protected

Ans:: A. Public

Q57. Which of these keywords is used by a class to use an interface defined previously?

C. implements

D. Implements

Ans:: C. implements

Q58. Which of the following is the correct way of implementing an interface Department by a class Employee?

A. class Employee extends Department{}

B. class Employee implements Department{}

C. class Employee imports Department{}

Ans:: B. class Employee implements Department{}

Q59. Which keyword is used to create a package?

C. classpath

Ans:: B. package

Q60. The modifier which specifies that the member can only be accessed in its own class is.

Ans:: B. private

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The Java (coding): debugging assessment evaluates candidates’ proficiency in identifying and resolving issues in Java code.

Skills measured

• Understanding of Java Programming Language
• Knowledge of Debugging Techniques
• Error Identification and Analysis
• Troubleshooting and Problem Solving
• Proficiency in Debugging Tools
• Attention to Detail and Methodical Approach

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Programming Skills

About the Java (coding): debugging test

The Java (coding): debugging test is designed to assess a candidate’s proficiency in identifying and resolving issues in Java code. This assessment is highly relevant when hiring for roles that involve Java development, software engineering, and troubleshooting.

The test evaluates various sub-skills, including the candidate’s understanding of the Java programming language, knowledge of debugging techniques and tools, error identification and analysis abilities, troubleshooting and problem-solving skills, proficiency in debugging tools, and attention to detail in a methodical approach.

Employers conduct this assessment to identify candidates who possess strong debugging skills and can effectively maintain and improve code quality. Proficiency in these areas is crucial for ensuring stable and reliable software applications. Candidates who excel in this assessment demonstrate their ability to analyze code, identify bugs, trace errors, and apply appropriate debugging techniques to resolve issues.

By evaluating these sub-skills, employers can assess the candidate’s ability to effectively debug Java code and their overall competency in problem-solving within a programming context. This assessment provides valuable insights into a candidate’s critical thinking, attention to detail, knowledge of debugging tools, and their commitment to producing high-quality code.

Hiring candidates with strong skills in Java debugging ensures that they can effectively troubleshoot and fix issues, contributing to the stability and reliability of software applications. These skills are highly sought after in roles such as Java Developer, Software Engineer, Software Developer, and various technical positions.

Conducting the Java (coding): debugging assessment allows employers to make informed hiring decisions by evaluating the candidate’s ability to debug Java code, which is a fundamental skill in software development. This assessment helps identify individuals who can efficiently identify and resolve bugs, ensuring the smooth functioning of software applications.

Relevant for

• Java Developer
• Software Engineer
• Software Developer
• Application Developer
• Full-stack Developer
• Back-end Developer
• Front-end Developer
• Quality Assurance Engineer
• Test Engineer
• Support Engineer
• Technical Support Specialist
• Systems Analyst

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This sub-skill assesses the candidate’s overall understanding of the Java programming language, including syntax, control flow, data types, object-oriented concepts, and exception handling. It is crucial to assess this skill as a strong understanding of Java fundamentals is necessary for effective debugging.

This sub-skill evaluates the candidate’s familiarity with various debugging techniques and tools available in the Java ecosystem. It includes knowledge of breakpoints, step-by-step execution, variable inspection, logging, and stack trace analysis. Assessing this skill is important as it determines the candidate’s ability to identify and isolate issues in the code.

This sub-skill focuses on the candidate’s ability to identify errors, bugs, and exceptions in Java code. It includes understanding error messages, interpreting stack traces, and tracing the flow of execution to pinpoint the root cause of issues. Assessing this skill is crucial as it demonstrates the candidate’s proficiency in finding and understanding errors during the debugging process.

This sub-skill assesses the candidate’s problem-solving abilities while debugging. It involves analyzing complex scenarios, devising strategies to identify and resolve issues efficiently, and considering possible solutions. Assessing this skill is important as it reflects the candidate’s critical thinking and logical reasoning skills, essential for effective debugging.

This sub-skill evaluates the candidate’s proficiency in using debugging tools and frameworks specific to Java development. This includes knowledge of integrated development environments (IDEs) like Eclipse or IntelliJ IDEA and their debugging features. Assessing this skill is crucial as it indicates the candidate’s familiarity with industry-standard tools and their ability to leverage them for efficient debugging.

This sub-skill focuses on the candidate’s attention to detail and their ability to follow a systematic and methodical approach while debugging. It includes skills like code review, examining relevant documentation, and maintaining a structured workflow. Assessing this skill is important as it demonstrates the candidate’s thoroughness, precision, and ability to identify even subtle errors or inconsistencies in the code.

The Java (coding): debugging test is created by a subject-matter expert

Testlify’s skill tests are designed by experienced SMEs (subject matter experts). We evaluate these experts based on specific metrics such as expertise, capability, and their market reputation. Prior to being published, each skill test is peer-reviewed by other experts and then calibrated based on insights derived from a significant number of test-takers who are well-versed in that skill area. Our inherent feedback systems and built-in algorithms enable our SMEs to refine our tests continually.

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Elevate your recruitment process with Testlify, the finest talent assessment tool. With a diverse test library boasting 1000+ tests, and features such as custom questions, typing test, live coding challenges, google suit questions and psychometric tests, finding the perfect candidate is effortless.

Enjoy seamless ATS integrations, white label features and multilingual support, all in one platform. Simplify candidate skill evaluation and make informed hiring decisions with Testlify.

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Top five hard skills interview questions for Java (coding): debugging

Here are the top five hard-skill interview questions tailored specifically for Java (coding): debugging . These questions are designed to assess candidates’ expertise and suitability for the role, along with skill assessments.

1. Explain the difference between a syntax error and a runtime error in Java debugging.

Why this matters.

This question assesses the candidate’s understanding of different types of errors that can occur in Java code. It tests their knowledge of syntax errors that occur during compilation and runtime errors that occur during program execution.

What to listen for?

Listen for the candidate to accurately define syntax errors and runtime errors, provide examples of each, and explain how they can impact the program’s behavior. Look for their ability to demonstrate knowledge of identifying and resolving these errors during the debugging process.

2. Describe how you would approach debugging a Java program that throws a NullPointerException.

This question evaluates the candidate’s debugging skills and their ability to handle a common error like a NullPointerException. It tests their understanding of identifying the cause of the exception and fixing it.

Listen for the candidate to outline a systematic approach to debug the issue, including analyzing the stack trace, identifying the null reference, and locating the code where the exception is thrown. Look for their ability to propose solutions and preventive measures to avoid such exceptions.

3. What debugging techniques and tools do you use to identify and resolve performance bottlenecks in Java applications?

This question assesses the candidate’s knowledge of debugging techniques and tools specifically related to performance optimization in Java applications. It tests their understanding of profiling, code analysis, and performance monitoring.

Listen for the candidate to mention common debugging techniques for performance optimization, such as profiling tools like VisualVM or JProfiler, analyzing memory usage, identifying slow database queries, and optimizing algorithms. Look for their ability to explain how these techniques can improve the performance of Java applications.

4. How do you handle an infinite loop situation while debugging a Java program?

This question evaluates the candidate’s ability to handle and resolve infinite loop issues, which can cause programs to hang or crash. It tests their understanding of techniques to detect and break out of infinite loops.

Listen for the candidate to describe methods for identifying infinite loops, such as observing program behavior, using breakpoints, or adding print statements. Look for their ability to propose strategies like adjusting loop conditions or using break statements to break out of the loop.

5. Discuss a situation where you encountered a difficult bug in a Java program and explain how you approached and resolved it.

This question allows the candidate to showcase their real-life debugging experience and problem-solving abilities. It tests their ability to analyze complex scenarios, use systematic approaches, and resolve challenging bugs.

Listen for the candidate to provide a clear and concise description of the bug they encountered, explain the steps they took to identify the cause, and describe the debugging techniques or tools they used. Look for their ability to demonstrate persistence, critical thinking, and effective problem-solving skills throughout the debugging process.

Frequently asked questions (FAQs) for Java (coding): debugging

What is java (coding): debugging assessment.

A Java (coding): debugging assessment is an evaluation tool used to assess a candidate’s proficiency in identifying and resolving issues in Java code. This assessment typically includes questions, scenarios, and tasks that require candidates to analyze and debug problematic code. It tests their knowledge of debugging techniques, error identification and analysis, troubleshooting skills, and their ability to effectively use debugging tools and methods specific to Java programming.

How to use Java (coding): debugging assessment for hiring?

The Java (coding): debugging assessment can be used as an effective tool in the hiring process for technical roles that require Java programming and debugging expertise. It allows employers to evaluate a candidate’s practical skills in debugging, their ability to identify and resolve issues in Java code, and their overall problem-solving capabilities. The assessment results can be used to assess a candidate’s proficiency level, compare their skills with other applicants, and make informed hiring decisions. It can also be combined with other evaluation criteria to assess a candidate’s overall suitability for a specific role or project.

What roles can I use the Java (coding): debugging assessment for?

What topics can be covered in java (coding): debugging assessment, why is the java (coding): debugging assessment important.

The Java (coding): debugging assessment holds significant importance in the hiring process for roles that involve Java development and troubleshooting. It allows employers to gauge a candidate’s ability to effectively identify and resolve issues in Java code, ensuring the quality and stability of software applications. This assessment helps assess a candidate’s understanding of debugging techniques, their proficiency in using debugging tools, their problem-solving abilities, and their attention to detail. By evaluating these key skills, the assessment ensures that the selected candidates possess the necessary expertise to debug Java code efficiently, contributing to the development of reliable and robust software solutions.

Frequently asked questions (FAQs)

Want to know more about Testlify? Here are answers to the most commonly asked questions about our company.

Can I try a sample test before attempting the actual test?

Yes, Testlify offers a free trial for you to try out our platform and get a hands-on experience of our talent assessment tests. Sign up for our free trial and see how our platform can simplify your recruitment process.

How can I select the tests I want from the Test Library?

To select the tests you want from the Test Library go to the Test Library page and browse tests by categories like role-specific tests, Language tests, programming tests, software skills tests, cognitive ability tests, situational judgment tests, and more. You can also search for specific tests by name.

Ready-to-go tests are pre-built assessments that are ready for immediate use, without the need for customization. Testlify offers a wide range of ready-to-go tests across different categories like Language tests (22 tests), programming tests (57 tests), software skills tests (101 tests), cognitive ability tests (245 tests), situational judgment tests (12 tests), and more.

Can you integrate with our existing ATS?

Yes, Testlify offers seamless integration with many popular Applicant Tracking Systems (ATS). We have integrations with ATS platforms such as Lever, BambooHR, Greenhouse, JazzHR, and more. If you have a specific ATS that you would like to integrate with Testlify, please contact our support team for more information.

What are the basic technical requirements needed to take your tests?

Testlify is a web-based platform, so all you need is a computer or mobile device with a stable internet connection and a web browser. For optimal performance, we recommend using the latest version of the web browser you’re using. Testlify’s tests are designed to be accessible and user-friendly, with clear instructions and intuitive interfaces.

Are your tests valid and reliable?

Yes, our tests are created by industry subject matter experts and go through an extensive QA process by I/O psychologists and industry experts to make sure that the tests have a good reliability and validity and give accurate results.

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Java online test: Pre-employment coding assessment to hire the best entry-level developers

Summary of the java (coding): entry-level algorithms test.

This Java online test assesses entry-level candidates’ basic programming skills and evaluates their ability to program a small algorithm in Java. This candidate skills test uses a short and straightforward coding task to help you identify developers with the most essential Java skills.

This pre-employment Java online test is designed by a subject matter expert to screen entry-level developers who have a clear understanding of the fundamentals of the Java language and its functionality. You can send this online coding test to your prospective job applicants before inviting them to an interview. With accurate results, now you can hire quality software developers with the push of a button.

Covered skills

Entry-level algorithms

Use the Java (coding): Entry-Level Algorithms test to hire

Entry-level developers using Java in their technology stack.

This test is part of our Coding: Entry-Level Algorithms skills test

This Java (coding) test forms part of our Coding: Entry-Level Algorithms test . 

All entry-level coding languages are now centralized in a single, language-agnostic test to ensure hiring developers is simpler than ever. Offer your candidates a choice of 19+ entry-level coding languages in one test. 

Explore the latest addition to our programming skills test library.

About the Java (coding): Entry-Level Algorithms test

Java is a general-purpose language that is geared for distributed environments. It is one of the most popular programming languages for Android app development, embedded systems, desktop applications, and enterprise-grade systems.

Strong foundational knowledge of programming in Java is essential for junior programmers. Those candidates are able to hit the ground running and set themselves up for further professional growth.

This test gives candidates 10 minutes of time to complete a straightforward coding task. The code is evaluated against a set of test cases, some of which are available to the candidate to determine if they are on the right track.

This is a great initial screening test that allows you to effectively screen candidates based on essential skills.

We recommend combining coding tests with at least one of or cognitive ability tests evaluating numerical or analytical skills.

The test is made by a subject-matter expert

Alfred is an experienced software engineer with deep expertise in Python development, data engineering, and cloud solutions architecture. He has built out a cloud native data platform for a $16B hedge fund that systematically ingested TBs/PBs of data to drive quantitative trading strategies.

Alfred is also a certified AWS Solutions Architect and DevOps Engineer at the Professional. In his free time, he enjoys reading about personal psychology and new trends in technology.

Crafted with expert knowledge

TestGorilla’s tests are created by subject matter experts. We assess potential subject-matter experts based on their knowledge, ability, and reputation. Before being published, each test is peer-reviewed by another expert, then calibrated using hundreds of test takers with relevant experience in the subject. Our feedback mechanisms and unique algorithms allow our subject-matter experts to constantly improve their tests.

What our customers are saying

TestGorilla helps me to assess engineers rapidly. Creating assessments for different positions is easy due to pre-existing templates. You can create an assessment in less than 2 minutes. The interface is intuitive and it’s easy to visualize results per assessment.

VP of engineering, mid-market (51-1000 FTE)

Any tool can have functions—bells and whistles. Not every tool comes armed with staff passionate about making the user experience positive.

The TestGorilla team only offers useful insights to user challenges, they engage in conversation.

For instance, I recently asked a question about a Python test I intended to implement. Instead of receiving “oh, that test would work perfectly for your solution,” or, “at this time we’re thinking about implementing a solution that may or may not…” I received a direct and straightforward answer with additional thoughts to help shape the solution.

I hope that TestGorilla realizes the value proposition in their work is not only the platform but the type of support that’s provided.

For a bit of context—I am a diversity recruiter trying to create a platform that removes bias from the hiring process and encourages the discovery of new and unseen talent.

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Our screening tests identify the best candidates and make your hiring decisions faster, easier, and bias-free.

Learn how each candidate performs on the job using our library of 400+ scientifically validated tests.

Test candidates for job-specific skills like coding or digital marketing, as well as general skills like critical thinking. Our unique personality and culture tests allow you to get to know your applicants as real people – not just pieces of paper.

Give all applicants an equal, unbiased opportunity to showcase their skills with our data-driven and performance-based ranking system.

With TestGorilla, you’ll get the best talent from all walks of life, allowing for a stronger, more diverse workplace.

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Add your company logo, color theme, and more to leave a lasting impression that candidates will appreciate.

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Building assessments is a breeze with TestGorilla. Get started with these simple steps.

Building assessments is quick and easy with TestGorilla. Just pick a name, select the tests you need, then add your own custom questions.

You can customize your assessments further by adding your company logo, color theme, and more. Build the assessment that works for you.

Send email invites directly from TestGorilla, straight from your ATS, or connect with candidates by sharing a direct link.

Have a long list of candidates? Easily send multiple invites with a single click. You can also customize your email invites.

Discover your strongest candidates with TestGorilla’s easy-to-read output reports, rankings, and analytics.

Easily switch from a comprehensive overview to a detailed analysis of your candidates. Then, go beyond the data by watching personalized candidate videos.

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The Java (coding): Entry-Level Algorithms test will be included in a PDF report along with the other tests from your assessment. You can easily download and share this report with colleagues and candidates.

What is a Java online coding test?

The Java online test is a valuable tool utilized by hiring managers to assess candidates' proficiency in core Java concepts. This technical skill test enables a thorough evaluation of candidates' fundamental knowledge in areas like object-oriented programing, operators, interfaces, multithreaded programming, error handling, etc. By conducting the Java online test, hiring managers can effectively screen and evaluate candidates' Java skills, ensuring a more informed decision-making process in the hiring journey.

Java in business

Java is a high-level and general-purpose programming language used across many industries. This means the language is similar to human language, making it easier for a developer to understand and interpret, and as a general-purpose language, it can be used to build secure solutions for a variety of business needs. This contributes to Java being one of the most widely used development platforms and the preferred choice of developers across the board.

Its widespread use and open-source nature contribute to the large, supportive community that builds and shares Java application programming interfaces or APIs and other tools that make it easier to write code to accomplish complicated tasks. Some of the benefits of Java include:

• The Java programming language was created to be easy to use and because of this, it is also easy to learn, program, compile, and debug

• Programmers can use Java to build modular programs and reuse code because it's object-oriented

• Java is platform-independent, which means it can move easily from one computer system to another enables programmers to run the same program on many different systems

How to hire a Java developer

Java’s multitude of applications make it a popular programming language in the software development world. This means that many candidates will be skilled in the language, but, as mentioned before, the demand for Java developers is also high.

So before deciding to hire a new Java developer, you need to define the skill set that fits your business needs. Assuming you already have an in-house development team, you probably know where the missing link is and where to add more resources. However, if you're setting up your team from scratch, formulating a recruitment plan and defining your business needs will help you recruit the essential skills and avoid costly hiring mistakes.

Hiring a developer means you’re looking for someone who will contribute to your business goals and play an essential part in the team. So when you hire, candidates must have both the skills and the personality attributes that will contribute to team strength and the organization’s culture.

Determine the skills you need

For any job, some hard skills are non-negotiable. This means the skills play a significant part in the success of the individual and the organization. Before you start recruiting, it is important to determine what these skills are.

Another consideration is soft skills. These play an equally important role in the recruitment process because soft skills such as communication and interpersonal skills will contribute to the success of the individual as well as their team. Soft skills help team members to build bonds and effectively collaborate to achieve goals, overcome obstacles and grow within their roles.

Pinning down which hard and soft skills you need for the role will help you to create a clear job description that will resonate with potential candidates.

Write a clear job description

Once you’ve established the skills required for the position, you need to describe it in a way that will appeal to potential applicants. Job descriptions communicate your company's requirements so that candidates with the right expertise can apply for those positions. They serve as a guiding light during the hiring process, and the most substantial reference for candidates to fall back on.

Share your job post on various platforms

With many businesses competing to get the best technical talent, only posting on your company's careers page may not be enough. The pool of people who see your job can increase significantly if you use social-media platforms, networking websites, and job boards. In addition, this can increase the chances of getting your job description in front of candidates with the technical skills for the position.

Where in your recruitment process should you use the Java online coding test?

The Java online coding test is ideally used at a stage in your recruitment process where you want to assess candidates' practical coding skills and their ability to apply Java concepts in real-world scenarios. It is recommended to incorporate the Java online coding test after initial resume screening and any initial assessments of candidates' theoretical knowledge of Java.

Ultimately, the Java online coding test serves as a valuable assessment tool to validate candidates' Java coding skills and ensure their compatibility with the technical requirements of the position you are seeking to fill.

Add tests to your assessment

It is not enough to hire the most qualified candidate using a Java coding test. You also need to consider things like personality, culture, time management, and attention to detail when hiring.

Adding personality tests to your assessment can help you find the right fit for your team, based on the traits of your candidates. Other tests, like the culture-add test, can help you recruit a diverse workforce.

To streamline your recruitment process, TestGorilla’s pre-screening assessments help you evaluate candidates on hard and soft skills, and give you an unbiased view of individual skill competencies. Considering that it's nearly impossible to keep up with current developments and best practices in programming and software development, a Java coding test can help you identify the best candidate for the job.

An assessment is the total package of tests and custom questions that you put together to evaluate your candidates. Each individual test within an assessment is designed to test something specific, such as a job skill or language. An assessment can consist of up to 5 tests and 20 custom questions. You can have candidates respond to your custom questions in several ways, such as with a personalized video.

Yes! Custom questions are great for testing candidates in your own unique way. We support the following question types: video, multiple-choice, coding, file upload, and essay. Besides adding your own custom questions, you can also create your own tests.

A video question is a specific type of custom question you can add to your assessment. Video questions let you create a question and have your candidates use their webcam to record a video response. This is an excellent way to see how a candidate would conduct themselves in a live interview, and is especially useful for sales and hospitality roles. Some good examples of things to ask for video questions would be "Why do you want to work for our company?" or "Try to sell me an item you have on your desk right now."

Besides video questions, you can also add the following types of custom questions: multiple-choice, coding, file upload, and essay. Multiple-choice lets your candidates choose from a list of answers that you provide, coding lets you create a coding problem for them to solve, file upload allows your candidates to upload a file that you request (such as a resume or portfolio), and essay allows an open-ended text response to your question. You can learn more about different custom question types here .

Yes! You can add your own logo and company color theme to your assessments. This is a great way to leave a positive and lasting brand impression on your candidates.

Our team is always here to help. After you sign up, we’ll reach out to guide you through the first steps of setting up your TestGorilla account. If you have any further questions, you can contact our support team via email, chat or call. We also offer detailed guides in our extensive help center .

It depends! We offer five free tests, or unlimited access to our library of 400+ tests with the price based on your company size. Find more information on our pricing plans here , or speak to one of our sales team for your personalized demo and learn how we can help you revolutionize hiring today.

Yes. You can add up to five tests to each assessment.

We recommend using our assessment software as a pre-screening tool at the beginning of your recruitment process. You can add a link to the assessment in your job post or directly invite candidates by email.

TestGorilla replaces traditional resume screening with a much more reliable and efficient process, designed to find the most skilled candidates earlier and faster.

We offer coding tests for the following languages: Python, JavaScript, PHP, Java, C, C++, C#, and SQL. These tests feature small coding tasks for a candidate to complete. We also offer multiple-choice tests for web and mobile development framework, such as our Angular, React and Django tests.

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Verbal Ability

• Interview Q
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Logical Reasoning

Verbal reasoning, non-verbal reasoning.

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