CopiesList.addAll method throws UnsupportedOperationException

Question

List hi = Collections.nCopies(10, \"Hi\");
List are = Collections.nCopies(10, \"Are\");

hi.addAll(are);

hi.forEach(System.out::println)         


        

Answer 1

Collections.nCopies returns an immutable List, so you can't add anything to it:

Returns an immutable list consisting of n copies of the specified object.

It returns an immutable List since it only contains a single reference of the element you pass to its constructor:

The newly allocated data object is tiny (it contains a single reference to the data object).

This is done for the benefit of performance and storage requirements - Collections.nCopies(10, "Hi") and Collections.nCopies(10000000, "Hi") occupy the same amount of space.

Therefore it would be very difficult to implement it as a mutable List. How would you set, for example, the 10000'th element to some new value if there's no storage allocated for that element?

If you need a mutable List that contains copies of the same object, you have to pass the immutable List to some mutable List constructor:

List<String> mutable = new ArrayList<>(Collections.nCopies(10, "Hi"));

Answer 2

As the JavaDoc says, the returned list is immutable, which means you can't modify it:

[nCopies] Returns an immutable list consisting of n copies of the specified object.

Maybe you didn't get the "in combination with the List.addAll" part. You must have a non-immutable list in which you can add all your elements:

List modifiableList = new ArrayList<String>();

List<String> hi = Collections.nCopies(10, "Hi");
List<String> are = Collections.nCopies(10, "Are");

modifiableList.addAll(are);
modifiableList.addAll(hi);

modifiableList.forEach(System.out::println);