Design Patterns | Chain of Responsibility

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1. References
2. Example
   • The Object
   • The Abstract Handler
   • The Concrete Handler (1)
   • The Concrete Handler (2)
   • A simple class to test the Chain of Responsibility design pattern

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1. References
   • Definition: (source: http://en.wikipedia.org/wiki/Chain-of-responsibility_pattern)
     
     The Chain of Responsibility pattern is a design pattern consisting of a source of command objects and a series of processing objects.
     Each processing object contains logic that defines the types of command objects that it can handle; the rest are passed to the next processing object in the chain.
     
2. Example
   • The Object:
     
     public class Request { final private StringBuilder data = new StringBuilder(); public void append(final String value) { data.append(value); } public String toString() { return data.toString(); } }
     
   • The Abstract Handler:
     
     public abstract class Handler { private Handler nextHandler; Handler(final Handler nextHandler) { this.nextHandler = nextHandler; } public void handleRequest(final Request request) { doRequest(request); if (nextHandler != null) { nextHandler.handleRequest(request); } } public abstract void doRequest(final Request request); }
     
   • The Concrete Handler (1):
     
     public class FirstHandler extends Handler { FirstHandler(final Handler nextHandler) { super(nextHandler); } public void doRequest(final Request request) { request.append("FirstHandler ..."); } }
     
   • The Concrete Handler (2):
     
     public class SecondHandler extends Handler { SecondHandler(final Handler nextHandler) { super(nextHandler); } public void doRequest(final Request request) { request.append("SecondHandler ..."); } }
     
   • A simple class to test the Chain of Responsibility design pattern:
     
     public class ChainOfResponsibilityPatternTest { public static void main(String[] args) { // create the request Request request = new Request(); // create the handlers Handler secondHandler = new SecondHandler(null); Handler firstHandler = new FirstHandler(secondHandler); // handle the request firstHandler.handleRequest(request); System.out.println(request); } }