Wildcard

Wildcard ? means everything. For wildcard we can just use without having to declare before return type like Type Parameter

public interface Authenticator<SomethingGeneric> {  
    SomethingGeneric get();  
    void remove(Authenticator<?> user);  
    <T extends SomethingGeneric> void connect(T user);  
}

However the meaning in here is a bit different, since it's capturing everything, the logic does not necessary hold true for type safe

For example

public static <T extends Number> void copy(List<T> dest, List<T> src)

// vs

public static void copy(List<? extends Number> dest, List<? extends Number> src)

In here the first example guarantee that both dest and src will be of the same type whereas the second example does not.

Note: In some case it might be the same, for example

public <T extends Animal> void takeThing<ArrayList<T> list)

public void takeThing<ArrayList<? extends Animal> list)