Java泛型：使用子类的列表设置超类的列表

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 ? extends or ? super ? super .

Answer 2

You are just doing the generics a bit wrong. Add the ? extends ? 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 since java.util.List is not covariant .

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

Answer 4

Do:

setSuperClassList(List<? extends Superclass> list)

This will allow a list of any subclass of Superclass.