Java unbound wildcard generics

Question

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);
}

You might want to have a look at http://stackoverflow.com/questions/503721/generics-in-java-using-wildcards — assylias, Feb 06 '12 at 18:25

score 6 · Accepted Answer · answered Feb 06 '12 at 18:24

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.

1 Answers1