Help me to resolve this in Java

[Deepak Lal]

Hi,
I have the below query.Please help me to resolve it.

Superclass: Class A.
Class B extends Class A and Class C extends Class A.
Class D extends Class B and Class C,But according to Java Concept you cannot extend more than 1 class.So how i achieve this in Java.?

Example provided will be highly appreciated to solve the above scenario
Help provided will be highly appreciated.


--
Deepak Lal

[Stephen Davies]

Originally posted by Deepak Lal:
Hi,
I have the below query.Please help me to resolve it.




Example provided will be highly appreciated to solve the above scenario
Help provided will be highly appreciated.


--
Deepak Lal

CLASS A
CLASS B EXTENDS A
CLASS C EXTENDS B
CLASS D EXTENDS C

[Deepak Lal]

Hi,

CLASS A -- Correct
CLASS B EXTENDS A -- Correct



CLASS C EXTENDS B
|
|-- The problem lies in this stmt i.e Class C is extending Class B but i want Class C to extend Class A not Class B.I dont want Class C to inherit properties of Class B...Hope its clear now. so how do i achieve it ??


CLASS D EXTENDS C -- this does not come in to picture if the previous step is invalid.

Please reply your comments on this...

--
Deepak Lal

[Paul Beckett]

classes can't use multiple inheritance but a class can implement multiple interfaces and an interface can extend multiple interfaces.

a possible solution for your problem is to use a combination of interfaces and composition. I've given an indicative solution below. Bear in mind that:
B instanceof A = true
BImpl instanceof A = true
BImpl instanceof AImpl = false

Also, to get this to work correctly, all methods exposed in the concrete implementation of A (AImpl) must be defined in the interface A.

  interface A{
   methodA();
}
interface B extends A{
   methodB();
}
interface C extends A{
   methodC();
}
interface D extends B,C{
   methodD();
}
 
 
class AImpl implements A{
   methodA(){}
}
 
class BImpl implements B{
   A a = new AImpl();
   
   methodA(){
     a.methodA();
   }
   
   methodB(){}   
}
 
class CImpl implements C{
   A a = new AImpl();
   
   methodA(){
     a.methodA();
   }
   
   methodB(){}   
   
   methodC(){}
}
 
class DImpl implements D{
   B b = new BImpl();
   C c = new CImpl();
   
   methodA(){
      b.methodA();
   }
   
   methodB(){
      b.methodB();
   }
   
   methodC(){
      c.methodC();   
   }
   
   methodD(){}
}  

[ November 26, 2008: Message edited by: Paul Beckett ]

[Deepak Lal]

There seems to be a little problem with the below code only.
Can you cross verify if methodB(){} will appear in CImpl class ??

Other parts of the code are perfectly fine.

class CImpl implements C{  
 
 A a = new AImpl();      
 methodA(){     
   a.methodA();   
 }  
    
methodB(){} //how come methodB(){} gets implemented in CImpl..Can you verify
   
methodC(){}
 
}
 

Thanks for your implementation.

;-)



--
Deepak Lal

[Campbell Ritchie]

I am not trying drawing on the screen, but if you draw a UML diagram of your classes A B C D you may see the relationship better. It will look something like this, only with nice squares and proper black heads to the arrows.

A
^
|
B
^
|
C
^
|
D

[Campbell Ritchie]

What you describe later is called diamond inheritance and looks a bit like this

 A
/  \
B  C
\  /
 D

And Paul Beckett has explained how to deal with that. You cannot implement diamond inheritance in Java (nor in C#) by using the extends keyword.

[Paul Beckett]

yes Deepak, you are right. C doesn't extend B so CImpl does not need to implement methodB.

[Deepak Lal]

Yeah Paul,
Thanks for the reply.Thanks all for your valuable answers and input.
--
Deepak Lal