java generics subtype in interface

Question

If I have the following interface and I want to implement it

public interface A<E extends Comparable<E>>{
    //code
}

What is the correct syntax for the implementing class declaration? I get an error when I do this

public class B<E extends Comparable<E>> implements A<E extends Comparable<E>>{}

Should it just read implements A<E> or just implements A?

Grundlefleck · Accepted Answer · 2011-06-24 17:07:49Z

3

Where the Comparable type E is, for example, String, you would want:

public class B implements A<String> { ... }

Where you want to retain the generic type parameter declaration in B, you would have:

public static class B<E extends Comparable<E>> implements A<E> { ... }

Note that the E in A is not related to the E in B, i.e. the following is valid:

public static class B<Foo extends Comparable<Foo>> implements A<Foo> { ... }

(whether you want to distinguish or not in your code I don't know, but it might help in understanding)

answered Jun 24, 2011 at 17:07

Grundlefleck

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Comments

dagnelies · Accepted Answer · 2011-06-24 17:02:35Z

2

public class B<E extends Comparable<E>> implements A<E>{}

answered Jun 24, 2011 at 17:02

dagnelies

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I think so too. You have to redeclare that this E is really a Comparable, before you can give it to your interface.