The compiler currently accepts the following program:

import java.util.*;

interface A{
List<? extends A> f();
}

interface B{
List<? extends B> f();
}

abstract class C implements A, B {} // should give error

However, the draft JLS 3rd edition says :


8.4.7.4 Inheriting Methods with the Same Signature

It is possible for a class to inherit more than one method with the same signature.
It is a compile time error if a class inherits more than one concrete method with the same signature.
This can happen, if a superclass is parameteric, and it has two methods that were distinct in the generic declaration, but have the same signature in the particular instantiation used.

Otherwise, there are two possible cases:

* If one of the inherited methods is not abstract, then there are two subcases:

 - If the method that is not abstract is static, a compile-time error occurs.
 - Otherwise, the method that is not abstract is considered to override, and therefore to implement, all the other methods on behalf of the class that inherits it. A compile-time error occurs if, comparing the method that is not abstract with each of the other of the inherited methods, for any such pair, either the return type of one the methods is not a subtype of the return type of the other or one has a return type and the other is void. Moreover, a compile-time error occurs if the inherited method that is not abstract has a throws clause that conflicts (§8.4.5) with that of any other of the inherited methods.

* If all the inherited methods are abstract, then the class is necessarily an abstract class and is considered to inherit all the abstract methods. A compile-time error occurs if, for any two such inherited methods, either the return type of one the methods is not a subtype of the return type of the other or one has a return type and the other is void. (The throws clauses do not cause errors in this case.)

****************************************

Therefore, the program given should be an error.

###@###.### 2004-03-03