interface CallBack {
 void methodToCallBack();
}

class CallBackImpl implements CallBack {
 public void methodToCallBack() {
  System.out.println("I've been called back");
 }
}

class Caller {

 public void register(CallBack callback) {
  callback.methodToCallBack();
 }

 public static void main(String[] args) {
  Caller caller = new Caller();
  CallBack callBack = new CallBackImpl();
  caller.register(callBack);
 }
}

import java.util.ArrayList;
import java.util.List;

// For example: Let's assume that this interface is offered from your OS to be implemented
interface TimeUpdaterCallBack {
 void updateTime(long time);
}

// this is your implementation.
// for example: You want to update your website time every hour
class WebSiteTimeUpdaterCallBack implements TimeUpdaterCallBack {

 @Override
 public void updateTime(long time) {
  // print the updated time anywhere in your website's example
  System.out.println(time);
 }
}

// This is the SystemTimer implemented by your Operating System (OS)
// You don't know how this timer was implemented. This example just
// show to you how it could looks like. How you could implement a
// callback by yourself if you want to.
class SystemTimer {

 List<TimeUpdaterCallBack> callbacks = new ArrayList<TimeUpdaterCallBack>();

 public void registerCallBackForUpdatesEveryHour(TimeUpdaterCallBack timerCallBack) {
  callbacks.add(timerCallBack);
 }

 // ... This SystemTimer may have more logic here we don't know ...

 // At some point of the implementaion of this SystemTimer (you don't know)
 // this method will be called and every registered timerCallBack
 // will be called. Every registered timerCallBack may have a totally
 // different implementation of the method updateTime() and my be
 // used in different ways by different clients.
 public void oneHourHasBeenExprired() {

  for (TimeUpdaterCallBack timerCallBack : callbacks) {
   timerCallBack.updateTime(System.currentTimeMillis());
  }
 }
}

// This is our client. It will be used in our WebSite example. It shall update
// the website's time every hour.
class WebSiteTimeUpdater {

 public static void main(String[] args) {
  SystemTimer SystemTimer = new SystemTimer();
  TimeUpdaterCallBack webSiteCallBackUpdater = new WebSiteTimeUpdaterCallBack();
  SystemTimer.registerCallBackForUpdatesEveryHour(webSiteCallBackUpdater);
 }
}