OO design with Java

Hello I am new to Java and would like to learn more about the OO features of this language.
I am currently working on a personal project and here are the details:
I am currently working with a servlet which in tail will communicate with another class. This class which I will call A acts as a hub between serveral other classes B,C,D,....etc.
So the servlet never interacts with anyother class other than A.
Also the other classes also communicate with each other through class A.
I hope I painted this picture vividly.
Here is my problem or I should say question. Say I have a class C and I want to replace that class with another class C2 which has a similar behavior, how can I design this so that I do not need to change any code in the "hub" class A. In other words I can just plug in a new class into A and remove it with minimal code change.
I already have found a method using inner classes and interfaces but is there a better approach to this or does anyone know of an alternative method.
thank you for any advice/response.
P�man M.

What you have is sometimes called a Mediator pattern with A beeing the mediator. Class A mediates contact with other classes B, C, D etcetera which don't know about each other.
You can make A a Singleton meaning there will only be one A object available in your application.
When any of the classes B, C, D are instantiated they immediately call A to become listeners. Whenever something happens A broadcasts a message to all its listeners which can take some action if they want to. This is called the Observer pattern. The message could consist of a message name and a "state" class with information about the current state of A.

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Hi Suraj,
> Since its graphical the blocks will take space, but
> there is always an adavntage of processing time.
> Ebven though it may appear bigger, it will take less
> time as compared with Java code (for the same
> mapping).
Could you explain more why the graphical mapping has better performance? I thought the graphical mapping is compiled into a Java class in the runtime anyway.
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> everyone may know basic java) .
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Message was edited by: Christoph Borst
Message was edited by: Christoph Borst
Message was edited by: Christoph Borst

Hi ,
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Development speed:
C / C++ (DJGPP):
downloading many parts - 1 week
installing - 1 weekend
looking for basic tutorials - 1 week
getting hello world to work - 1 afternoon
getting more tutorials - 1 week
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doing something worth doing - never
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Java
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installing - 1/2 hour
basic tuts - 1 hour
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user9046632 wrote:
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OO design with Java

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