Why is "extends T" allowed but not "implements T"?

Asked 2009-06-10T15:42:33.507

382

Is there a special reason in Java for using always "extends" rather than "implements" for defining bounds of type parameters?

For example:

public interface C {}
public class A<B implements C>{}

is prohibited, but

public class A<B extends C>{}

is correct. What is the reason for that?

java generics syntax

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2 Answers

350

There is no semantic difference in the generic constraint language between whether a class 'implements' or 'extends'. The constraint possibilities are 'extends' and 'super' - that is, is this class to operate with assignable to that other one (extends), or is this class assignable from that one (super).

answered 2009-06-10T15:45:51.503

Here is a more involved example of where extends is allowed and possibly what you want:

public class A<T1 extends Comparable<T1>>

answered 2012-07-31T13:48:57.567

Why is "extends T" allowed but not "implements T"?

2 Answers

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