Implementing The Observer Pattern In Java

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// This is an interface, describing what method a Listener class
// should contain
public interface HelloListener {
    void notify(String msg);
}

// Let's make a class which listens to "Hello" events
public class MyClass implements HelloListener {
    void notify(String msg) {
        System.out.println(msg);
    }
}

// Let's make a class which generates "Hello" events
public class HelloEventGenerator {

    List<HelloListener> helloListeners = new ArrayList<HelloListener>();

    public void addHelloListener(HelloListener helloListener) {
        helloListeners.add(helloListener);
    }

    public void raiseEvent() {
        for(HelloListener helloListener : helloListeners) {
            helloListener.notify("Hello, world!);
        }
    }

}