What is the difference between 'super' and 'extends' in Java Generics [duplicate]

Solution 1:

It depends which way on the inheritance hierarchy it allows. Assume you have a class "Child" which inherits from "Parent" which inherits from "Grandparent".

<T extends Parent> accepts either Parent or Child while <T super Parent> accepts either Parent or Grandparent.

Solution 2:

There are three types of wildcards:

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

Solution 3:

See Effective Java 2nd Edition, Item 28:

PECS

Producer extends, Consumer super

If your parameter is a producer, it should be <? extends T>, if it's a consumer it has to be <? super T>.

Take a look at the Google Collections, they know how to use it, because they got Bloch ;)