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

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 ;)

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.

For me the best answer came from @BSingh, when i read the article of Uncle Bob. I resume here, the conclusion of article.

Use List< T super Suit> whenever you are going to write into the list.

When you put an Object to the List, all you care about is that the object is of a type that is compatible with type held by the list. So you want the list to take the type of that object or any of the superclasses of that object.

Use List< T extends Suit> whenever you are going to read from a list.

On the other hand, when you read from a list, you want the type you are reading to be the type contained byt the list, or a derivative from that type.

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.