Java Generics: set List of superclass using List of subclass

Question

If I have a method in MyClass such as

setSuperClassList(List<Superclass>)

...should I be able to do this:

new MyClass().setSuperClassList(new ArrayList<Subclass>())

It appears this won't compile. Why?

Answer 1

Try setSuperClassList(List<? extends Superclass>).

Also check PECS to see wether you should use ? extends or ? super.

Answer 2

You are just doing the generics a bit wrong. Add the ? extends bit, and that will allow the passed in list to contain the SuperClass or any of its subclasses.

setSuperClassList(List<? extends Superclass>)

This is called setting an upper bound on the generics.

The statement List<Superclass> says that the List can only contain SuperClass. This excludes any subclasses.

Answer 3

It won't compile sincejava.util.List is not covariant.

Try setSuperClassList(List<? extends Superclass>) instead.