Using inner classes for callbacks : Nested Classes « Class Definition « Java Tutorial

1. Java Tutorial
2. Class Definition
3. Nested Classes

interface InterfaceA {
  void increment();
}

class ClassA implements InterfaceA {
  private int i = 0;

  public void increment() {
    i++;
    System.out.println(i);
  }
}

class ClassB {
  void increment() {
    System.out.println("Other operation");
  }

  static void f(ClassB mi) {
    mi.increment();
  }
}

class ClassC extends ClassB {
  private int i = 0;

  private void incr() {
    i++;
    System.out.println(i);
  }

  private class Closure implements InterfaceA {
    public void increment() {
      incr();
    }
  }

  InterfaceA getCallbackReference() {
    return new Closure();
  }
}

class Caller {
  private InterfaceA callbackReference;

  Caller(InterfaceA cbh) {
    callbackReference = cbh;
  }

  void go() {
    callbackReference.increment();
  }
}

public class MainClass {
  public static void main(String[] args) {
    ClassA c1 = new ClassA();
    ClassC c2 = new ClassC();
    ClassB.f(c2);
    Caller caller1 = new Caller(c1);
    Caller caller2 = new Caller(c2.getCallbackReference());
    caller1.go();
    caller1.go();
    caller2.go();
    caller2.go();
  }
}