Take the 2-minute tour ×

This question already has an answer here:

I am trying to learn Java generics. I am not clear when you will use <T extends Foo> and when you will use <T super Foo>. What do each one of these things mean about T? Lets say I have <T extends Comparable> and <T super Comparable>, what do each of these mean?

I've read several tutorials at sun.com but I'm still lost. Can somebody illustrate with examples?


share|improve this question
(Note Comparable is a generic type, so using it as a raw type in a generic bound will cause misery. It should, at the very "simplest" be Comparable<?>.) –  Tom Hawtin - tackline Dec 15 '09 at 23:00
there is no "<T super Whatever>", only "<? super Whatever>" –  newacct Dec 15 '09 at 23:45
add comment

marked as duplicate by Tala, Dan Hulme, Werner Vesterås, Adam Arold, Jakob S Sep 4 '13 at 8:25


6 Answers

up vote 12 down vote accepted

See Effective Java 2nd Edition, Item 28:


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 ;)

share|improve this answer
This doesn't really explain how they work or what they mean, whereas R Samuel Klatchko's answer does. –  ArtB Nov 26 '12 at 18:22
Dont get it, doesn't explain the difference. –  Basil Musa Dec 15 '12 at 18:02
This is how Bloch explains it: Suppose you want to add bulk methods to Stack<E>: •void pushAll(Collection<? extends E> src);void popAll(Collection<? super E> dst);User can pushAll from a Collection<Long> or a Collection<Number> onto a Stack<Number>User can popAll into a Collection<Object> or a Collection<Number> from a Stack<Number> –  18446744073709551615 Mar 12 '13 at 9:12
add comment

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.

share|improve this answer
This is the best answer to this question I've ever seen, and ought to be the accepted answer. Simple and works perfectly. –  ArtB Nov 26 '12 at 18:21
add comment

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.
share|improve this answer
FYI: this is extracted from java.sun.com/developer/technicalArticles/J2SE/generics In the future please mention your sources instead of writing as if they're yours. –  BalusC Dec 15 '09 at 23:52
@BalusC's link to the article doesn't work anymore. Here's a current one: oracle.com/technetwork/articles/javase/generics-136597.html –  Dathan Aug 9 '13 at 2:32
add comment

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.

share|improve this answer
add comment

If you are asking about type parameters, then there is no <T super X> construct in Java. Bounded parameter can only extend, but it can extend more than one type. E.g

public class MyClass< T extends Closeable & Runnable >
  // Closeable and Runnable are chosen for demonstration purposes only

In this case if you see MyClass< ConcreteT > then ConcreteT must be declared as

public class ConcreteT
  implements Closeable, Runnable

For bounded wildcards, read this article. Read section on get-put principle. Basically, super corresponds to write semantics, and extends corresponds to read semantics.

share|improve this answer
add comment

Remember PECS - Producer Extends Consumer Support. Also, uncle Bob discusses it well in his craftsman series. Check out http://objectmentor.com/resources/articles/The%5FCraftsman%5F44%5F%5FBrown%5FBag%5FI.pdf

share|improve this answer
add comment
