If the interface declares a method, then the declaration of that method is said to override any and all methods with the same signature in the superinterfaces of the interface that would otherwise be accessible to code in this interface.
If a method declaration in an interface overrides the declaration of a method in another interface, a compile-time error occurs if the methods have different return types or if one has a return type and the other is void. Moreover, a method declaration must not have a throws clause that conflicts (§8.4.4) with that of any method that it overrides; otherwise, a compile-time error occurs.
Methods are overridden on a signature-by-signature basis. If, for example, an interface declares two public methods with the same name, and a subinterface overrides one of them, the subinterface still inherits the other method.
An interface inherits from its direct superinterfaces all methods of the superinterfaces that are not overridden by a declaration in the interface.
It is possible for an interface to inherit more than one method with the same signature (§8.4.2). Such a situation does not in itself cause a compile-time error. The interface is considered to inherit all the methods. However, a compile-time error occurs if, for any two such inherited methods, either they have different return types or one has a return type and the other is void. (The throws clauses do not cause errors in this case.)
There might be several paths by which the same method declaration is inherited from an interface. This fact causes no difficulty and never of itself results in a compile-time error.