Help me to resolve this in Java

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.


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Deepak Lal

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

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

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 ]

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.

;-)



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Deepak Lal

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

What you describe later is called diamond inheritance and looks a bit like this A / \ B C \ / DAnd Paul Beckett has explained how to deal with that. You cannot implement diamond inheritance in Java (nor in C#) by using the extends keyword.

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

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