Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.
Participants
- Handler: defines an interface for handling the requests. (optional) implements the successor link
- ConcreteHandler: handles requests it is responsible for. Can access its successor. If the ConcreteHandler can handle the request, it does so; otherwise, it forwards the request to its successor.
- Client: initiates the request to a ConcreteHandler object on the chain.
Code
public class Main { public static void main(String[] args) { Handler h1 = new ConcreteHandler1(); Handler h2 = new ConcreteHandler2(); Handler h3 = new ConcreteHandler3(); h1.setSuccessor(h2); h2.setSuccessor(h3); int[] requests = { 2, 5, 14, 22, 18, 3, 27, 20 }; for (int request : requests) { h1.handleRequest(request); } } } public abstract class Handler { protected Handler successor; public void setSuccessor(Handler successor) { this.successor = successor; } abstract void handleRequest(int request); } public class ConcreteHandler1 extends Handler { @Override public void handleRequest(int request) { if (request >= 0 && request < 10) { System.out.println(this.getClass().getSimpleName() + " handled request " + request); } else if (successor != null) { successor.handleRequest(request); } } } public class ConcreteHandler2 extends Handler { @Override public void handleRequest(int request) { if (request >= 10 && request < 20) { System.out.println(this.getClass().getSimpleName() + " handled request " + request); } else if (successor != null) { successor.handleRequest(request); } } } public class ConcreteHandler3 extends Handler { @Override public void handleRequest(int request) { if (request >= 20 && request < 30) { System.out.println(this.getClass().getSimpleName() + " handled request " + request); } else if (successor != null) { successor.handleRequest(request); } } } Output
ConcreteHandler1 handled request 2 ConcreteHandler1 handled request 5 ConcreteHandler2 handled request 14 ConcreteHandler3 handled request 22 ConcreteHandler2 handled request 18 ConcreteHandler1 handled request 3 ConcreteHandler3 handled request 27 ConcreteHandler3 handled request 20 
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