Design Patterns | Chain of Responsibility
  1. References
  2. Example
  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:

      • Implementation 1:

        public class FirstHandler extends Handler {
    FirstHandler(final Handler nextHandler) {
        super(nextHandler);
    }

    public void doRequest(final Request request) {
        request.append("FirstHandler ...");
    }
}

      • Implementation 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);
    }
}

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