Implementing an Interface Down the Inheritance Tree Question

[Russ Russell]

Hi,
The following code compiles:

interface Iface{
   public void implementMe();
}

class Test1 implements Iface{
   public void implementMe(){
      System.out.println("You have been implemented in Test1.");
   }
}


class Test2 extends Test1 implements Iface{
   //Even though I am implementing Iface, 
   //Don't need to implement the method implementMe() because my superclass already does?
}

My questions are:
1) Even though Test2 implements Iface, why doesn't it need to provide implementation for the method implementMe() ?
2) Even if I do provide a method with the same signature and return type as implementMe(), is it really an implementation of the interface method?
3) What am I gaining (if anything) by adding "implements Iface" to the Test2 class"?

Thank you very much,
-Russ

[Steve Luke]

Russ Russell wrote:...
My questions are:
1) Even though Test2 implements Iface, why doesn't it need to provide implementation for the method implementMe() ?

Test2 inherits an implementation of the implementMe() method from its parent class, Test1. Therefore it does not need to re-implement it.

2) Even if I do provide a method with the same signature and return type as implementMe(), is it really an implementation of the interface method?

Yes.

3) What am I gaining (if anything) by adding "implements Iface" to the Test2 class"?

You are clearly telling anyone who uses the class Test2 that it is a valid and intended implementation of Iface. I generally do not do that, preferring to only add the implements clause on the class which actually does do the method implementation.

[Russ Russell]

Thank you Steve.

It seems unnecessary to me to add the implements clause if this class is not explicitly providing implementation. However I saw an example similar to this in a sample certification question, and I just wanted to understand why anyone would do such a thing.

Thanks,
-Russ

[Paul Clapham]

You need to add the "implements" clause to specify that the class implements the interface. That's what the "implements" clause is for, no more and no less. If you don't declare that a class implements an interface, then it doesn't implement that interface.

That all may seem rather trivial, and it is. However from that one trivial statement you should be able to infer why somebody would want to "do such a thing", as you put it. Unless you were asking why somebody would want to implement an interface, that is...

[Joanne Neal]

Paul Clapham wrote:You need to add the "implements" clause to specify that the class implements the interface.

No you don't. If a class extends another class that implements the interface there is no need to have an explicit implements clause in it's declaration.
If you remove the implements Iface from the Test2 declaration then Test2 still implements Iface. That is what was being asked.

class Test2 extends Test1
{  
   //Don't need to implement the method implementMe() because my superclass already does?  
}  

[Campbell Ritchie]

Try

Test2 t2 = new Test2();
System.out.printf("Test2 implements IFace: %b%n", t2 instanceof IFace);

. . . and you will see Joanne is correct.

[Campbell Ritchie]

It’s all part of inheritance. If a Test1 IS-An IFace, and a Test2 IS-A Test1, then a Test2 IS-An IFace, too.