What do the angle brackets mean in this Interface definition?

Question

I am relatively new to Java Programming and I came across this declaration of an interface and a class that implements it:

public interface Abcd<E extends Comparable<E>>{
......
......
......
}

public class AbcdImpl<E extends Comparable<E>> implements Abcd<E>{
......
......
......
}

Can you explain what <E extends Comparable<E>> stands for and typically what does <E> signify?

Why on Earth would you want to replace < and > with ( and ) in the question? Not interested in people answering maybe? — Romain, Aug 24 '12 at 11:01
Possible duplicate http://stackoverflow.com/questions/2231804/java-interface-extends-comparable & "E" stands for any data type — Venkata Krishna, Aug 24 '12 at 11:05
@upcoming_coder: actually that's not an exact duplicate, the linked-to asker already knew about generics, this is more basic. — Joachim Sauer, Aug 24 '12 at 11:07
is the template name of a generic class. Think of it as Collection_of_, for instance. — PypeBros, Aug 24 '12 at 11:09
I'm sorry for the syntax mistakes but whenever i was using the <...> it would get commented out from the question. — Jones, Aug 24 '12 at 11:14

score 6 · Answer 1 · edited Jan 16 '16 at 10:13

E stand for a type, the letter doesn't need to be E it can be anything, but usually people will use letters like T,K,E,V. It makes sense to use a letter that somehow signifies what the type is used for. For instance in the Map interface Map<K,V>, K is for key and V is for value.

As for <E extends Comparable<E>> it means that the type E that is the generic type of AbcdImpl needs to itself implement the Comparable interface (for itself). So any class E has to have a method

compareTo(E obj)

for comparing one instance of E to another instance of E.

score 2 · Answer 2 · answered Aug 24 '12 at 11:12

What you see is a generic type declaration.

See http://en.wikipedia.org/wiki/Generics_in_Java

Abcd<E> is normally read as "Abcd of E".

For example, List is a list of items, but it's not specified what kind of items. List is a list of strings, and is a parametrised version of the generic type List.

What do the angle brackets mean in this Interface definition?

2 Answers2

Linked