Question

What’s the difference between:

SomeClass<T extends OtherClass>

and

SomeClass<T super OtherClass>

Answer 1

Sun says (from Using and Programming Generics in J2SE 5.0)

There are three types of wildcards:

1. “? extends Type”: Denotes a family of subtypes of type Type. This is the most useful wildcard
2. “? super Type”: Denotes a family of supertypes of type Type
3. “?”: Denotes the set of all types or any

More Fun with Wildcards gives some good examples to all three types.

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Another good example (taken from the video recommended by @Adrian) is:

class Stack<E> {
    void pushAll( Collection<? extends E> src );
    void popAll( Collection<? super E> dest );
}

Here you can see the difference between super and extends:

For the method pushAll imagine you have a Stack<Number>. You want to push all elements of src to the Stack. But src should be able to be a Collection<Integer>. This is why you choose ? extends E here.

For the method popAll imagine you have a Stack<Integer>. You want to pop all elements of the Stack to dest. But dest should be able to be a Collection<Number>. This is why you choose ? super E here.