Multiple inheritance on Java interfaces

Question

I thought multiple inheritance was always illegal in Java, but this code compiles:

public interface A {   void a(); }  public interface B {   void b(); }  public interface AB extends A, B { } 

Would having an empty interface such as AB be considered a bad practice? Is there a way to achieve something similar while avoiding the empty interface (using generics or otherwise)?

Note: I'm not asking how to simulate multiple inheritance via interfaces. I realize I could do the following:

public class AbImpl implements A, B {   public void a() {}   public void b() {} } 

For various reasons I need an interface that has both methods.

Answer 1

Multiple inheritance of implementations is not allowed. Components can inherit multiple interfaces, though.

Inheriting multiple interfaces isn't problematic, since you're simply defining new method signatures to be implemented. It's the inheritance of multiple copies of functionality that is traditionally viewed as causing problems, or at the very least, confusion (e.g., the diamond of death).

Answer 2

An interface can extend one or more other interfaces. You can also implement more than one interface in your classes. It is legal because interface is only contract - there is no implementation. You're simply defining a contract for what a class is able to do, without saying anything about how the class will do it.