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

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();

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

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

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