Callbacks in Java

java_callback.java

// From https://www.quora.com/What-are-callback-methods-in-Java
// Java program to illustrate synchronous callback

interface OnGeekEventListener {
  // this can be any type of method
  void onGeekEvent();
}

class B {
  private OnGeekEventListener mListener; // listener field
  // setting the listener
  public void registerOnGeekEventListener(OnGeekEventListener mListener){
    this.mListener = mListener;
  }

  // my synchronous task
  public void doGeekStuff(){
    // perform any operation
    System.out.println("Performing callback before synchronous Task");
    // check if listener is registered.
    if (this.mListener != null) {
    // invoke the callback method of class A
    mListener.onGeekEvent();
  }
}

 // Driver Function
 public static void main(String[] args){
   B obj = new B();
   OnGeekEventListener mListener = new A();
   obj.registerOnGeekEventListener(mListener);
   obj.doGeekStuff();
  }
}

class A implements OnGeekEventListener {
  @Override
  public void onGeekEvent(){
    System.out.println("Performing callback after synchronous Task");
    // perform some routine operation
  }
  // some class A methods
}