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public interface CallBack {
  void callMethod();
}
 
class MyCallBack implements CallBack {
  public void callMethod() {
    System.out.println("CallBack invoked");
  }
}
 
class Client {
  CallBack callback;
  public void registerCallBack(CallBack callback) {
    this.callback = callback;
  }
 
  public void doSomething() {
    callback.callMethod();
  }
  public static void main(String[] args) {
    Client client = new Client();
    CallBack myCallBack = client.registerCallBack(new MyCallBack();
    client.registerCallBack( myCallBack);
    // ...
    client.doSomething(); // prints "CallBack invoked"
  }
}