gistfile1.java

public interface CallbackInterface {
    void onSomethingHappens(int data);
    void onAnotherThingHappens();
}

public class CallbackHost {
    private CallbackInterface callback;

    public CallbackHost(CallbackInterface callback) {
        this.callback = callback;
    }

    public void setSomeData(int data) {
        if (callback != null)
            callback.onSomethingHappens(42);
    }
}

public class CallbackClient {
    public CallbackClient() {
        CallbackHost host = new CallbackHost(new CallbackInterface() {
            public void onSomethingHappens(int data) {
                System.out.println(data);
            }

            public void onAnotherThingHappens() {
                System.out.println("AnotherThingHappens");
            }
        });
    }
}