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?
Thanks!
It depends which way on the inheritance hierarchy it allows. Assume you a class "Child" which inherits from "Parent" which inherits from "Grandparent".
<T extends Parent> would accept either Parent or Child while <T super Parent> would accept either Parent or Grandparent.
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
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.
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 ;)
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