Java unbound wildcard generics

问题:

Are there any advantages of using wildcard-type generics in the Bar class over completely skipping them?

public class Foo<T> {}

public interface Bar {
    public void addFoo(Foo<?> foo);
    public Foo<?> getFoo(String name);
}

回答1:

There are multiple advantages.

• They won't produce compiler warnings like using the raw type would

• They give more type safety. For example, consider if Foo was List instead. If you used List instead of List<?>, you could do this:

  myBar.getFoo("numbers").add("some string");
  

  even if the list was only supposed to contain Numbers. If you returned a List<?>, then you would not be able to add anything to it (except null) since the type of list is not known.

• They document something completely different than a raw type, namely that Foo is typed on some unknown but specific type.