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I am trying to understand what is this java class definition.

abstract public class A<P extends B<?, ?>,Input,Output> { ... // class defined ... }

A c++ programmer moving to java

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  • im a c++ programmer too, slightly dabbling in java, and to me that looks like STL template definitions :) Commented Apr 26, 2011 at 3:06
  • duplicate of many like: stackoverflow.com/questions/490091/java-generics Commented Apr 26, 2011 at 3:06
  • +1 for @MeBigFatGuy's username-lol Commented Apr 26, 2011 at 3:07

5 Answers 5

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This defines an abstract class called A, with three type parameters:

  • P, which must be of type B (with any type arguments) or any type derived from it
  • Input, of any type
  • Output, of any type

Of interest is the first type parameter. In C++, for a type-based template parameter, you can supply any type; in Java, you have the option to constrain the type by what class and/or interfaces such a type must also extend/implement.

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2 Comments

thanks, can u provide some links explaining java classes taking parameters
@Kazoom: Those classes are called "generic" classes. Read the link given by MeBigFatGuy in the question's comments.
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A bit of "translation":

"abstract" means this class may have abstract (~=pure virtual) methods.

class A is a generic (~template) definition

P extends ... is an extra constraint on generic parameter, should be subclass of ...

P extends B<?, ?> means that the generic parameter#1 is a subclass of another generic class

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It might not have pure virtuals. Abstract classes in Java only have the property of not being directly instantiatable in and of themselves.
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It's an abstract class definition (obviously) with 3 generic parameters.

The first parameter P has a constraint that it has to be of type (or that extends) class/interface B which has two generic parameters (no constraint on those) so it could be like 

public class B<T1, T2> { }

The second and third parameters namely Input and Output have no constraints.

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-1

The angle bracket notation is for Java Generics.

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-1 You'd think that C++ people already know what generics are, surely? C++ templates are much beefier than Java generics.
the main difference being in c++ each specialization creates a separate class, whereas in java the specialization is lost at compile time.
@MeBigFatGuy: well... after compilation phase is complete... Java generics just aren't reified like C++ generics are.
@Chris: Generics != C++ templates, and it's not clear the OP had run across the term before.
+1 to @Jim: Java Generics and C++ templates share only superficial similarity. They use a similar syntax and Java generics approach a subset of the problems approached by C++ templates, but their implementation and are quite different!
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Well, to really understand it you'd want to include more of the definition, but B is a class with generics itself, and A has three generic references in it; it's a bit pathological, but it's fairly easy to step through.

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