Question

I have something like:

List<? extends BaseClass> a = getMyList();

Any of the following two instructions are invalid, and the compiler says that it requires a “? extends BaseClass” as an argument.

a.add(new BaseClass());
a.add(new SubClass());

I guess the problem is that

How can this be solved?

Answer 1

The problem is that the compiler has no way of determining the actual type of the list.

It could be that the type is SubClass, in which case you would not be allowed to add a BaseClass.
It could even be SubSubClass, meaning you couldn’t add either BaseClass or SubClass.

However, this will compile:

List<BaseClass> a = getMyList();
a.add(new BaseClass());
a.add(new SubClass());