unchecked assignment 'java.util.collection' to 'java.util.collection java.lang.string '

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Collections unmodifiableList() method in Java with Examples

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The unmodifiableList() method of java.util.Collections class is used to return an unmodifiable view of the specified list. This method allows modules to provide users with “read-only” access to internal lists. Query operations on the returned list “read through” to the specified list, and attempts to modify the returned list, whether direct or via its iterator, result in an UnsupportedOperationException.

The returned list will be serializable if the specified list is serializable. Similarly, the returned list will implement RandomAccess if the specified list does.

Parameters: This method takes the list as a parameter for which an unmodifiable view is to be returned.

Return Value: This method returns an unmodifiable view of the specified list.

Below are the examples to illustrate the unmodifiableList() method

Example 2: For UnsupportedOperationException

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Since the return collection/map is unmodifiable, attempting to modify it (adding/removing new elements) throws UnsupportedOperationException .

These methods create shallow copy of the provided collection. That means the collection elements (or key/values in case of map) are not copied/cloned, so if the original elements (if mutable) are changed they will reflect the changes in the new collection and vice-versa.

These methods throw NullPointerException , if the provided collection has null elements.

List.copyOf(collection)

Set.copyof(collection), map.copyof(map), when to use copyof() methods.

Let's see the List.CopyOf() code snippet:

As seen above, this method is just a helper around List.of() method. Note that the List.of() also return a type AbstractImmutableList , so if the provided collection is created by List.of() methods then it is return as it is.

Also the copyOf() methods allow us to assign the provided collection to a collection having elements of super type ( covariant assignment ). In above snippet, there are also 'unchecked' castings, but since the returned collection is immutable this cannot lead to heap pollution situation.

Consider following example without CopyOf method:

Above ClassCastException will be a surprise for a developer who is not aware of aLibOperation() code.

Now let's use CopyOf() method:

As seen in above output, the UnsupportedOperationException is thrown at the right place (aLibOperation) where a wrong operation is performed, instead of very late unexpected ClassCastException .

Set.CopyOf() and Map.copyOf() have the similar behavior as we saw above.

List.CopyOf() vs Collections.unmodifiableList()

Collections.unmodifiableList() (and other similar methods) returns a unmodifiable view of the source collection, so changes made to the source collection reflects in it. Whereas, in case of List.copyOf() method, if the source collection is subsequently modified, the returned List will not reflect such modifications.

Example Project

Dependencies and Technologies Used:

• ListCopyOfExample.java
• ListExample1.java
• ListExample2.java
• MapCopyOfExample.java
• SetCopyOfExample.java
• WithCollectionsUtilExample.java

Unchecked assignment: ‘java.util.List’ to ‘java.util.Collection’

I am having an adapter where i have two lists one list is for InvestorsList where it comes with the list of investors and the other list is called investorListFull which is used to filter results when searching.

Below is how i have declared the lists

Below is how the lists are assigned in my recyclerview adapter constructor

Below is how i am filtering results in investors list

I am getting Unchecked assignment error in publish results investorList.addAll((List) filterResults.values);

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I am getting Unchecked cast error in publish results investorList.addAll((List) filterResults.values);

That’s because you’re doing an unchecked cast. Actually, you’re doing both a checked and an unchecked cast there.

(List) filterResults.values is a checked cast. This means that the Java compiler will insert a checkcast instruction into the bytecode to ensure that filterResults.values (an Object ) really is an instance of List .

However, investorList.addAll expects a List<Investor> , not a List . List is a raw type . You can pass a raw-typed List to a method expecting a List<Something> ; but this is flagged as unsafe because the compiler can’t guarantee that it really is a List<Something> – there is nothing about the List that makes it a “list of Something “, because of type erasure. The compiler can insert a checkcast instruction to ensure it’s a List ; but not one to ensure it’s a List<Something> : this fact is unchecked .

What it’s saying with the warning is “there may be something wrong here; I just can’t prove it’s wrong”. If you know for sure – by construction – that filterResults.values really is a List<Investor> , casting it to List<Investor> is safe.

You should write the line as:

Note that this will still give you an unchecked cast warning, because it’s still an unchecked cast – you just avoid the use of raw types as well.

If you feel confident in suppressing the warning, declare a variable, so you can suppress the warning specifically on that variable, rather than having to add the suppression to the method or class; and document why it’s safe to suppress there:

CollectionUtils.java

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Class Collections

• java.lang.Object
• java.util.Collections

Field Summary

Method summary, methods inherited from class java.lang. object, field detail, method detail, binarysearch, indexofsublist, lastindexofsublist, unmodifiablecollection, unmodifiableset, unmodifiablesortedset, unmodifiablelist, unmodifiablemap, unmodifiablesortedmap, synchronizedcollection, synchronizedset, synchronizedsortedset, synchronizedlist, synchronizedmap, synchronizedsortedmap, checkedcollection, checkedsortedset, checkedlist, checkedsortedmap, emptyiterator, emptylistiterator, emptyenumeration, singletonlist, singletonmap, reverseorder, enumeration, newsetfrommap, aslifoqueue.

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