Task : To implement a call-back facility

• Myclass m creates a Timer t
• Start t to run in parallel
  • Then m continues to run.
• Timer t notifies Myclass m when the timer ends.
  • Assume that Myclass m has a function timerdone.

• Code for Myclass:

public class Myclass{
	public void f(){
		...
		Timer t = new Timer(this);  // Timer t should know who to notify. Myclass m passes its identity when it creates a Timer.
		...
		t.start();
		...
	}
	public void timerdone() { ... }
}

• Code for Timer

public class Timer implements Runnable{
	//Now timer can be invoked in parallel
		private Myclass owner; //instance variable

	public Timer(Myclass o){
		owner = o;
	}

	public void start(){
		...
		owner.timerdone();
	}
}

Here the Timer is specific to Myclass .

• Is it possible to create a generic Timer?

Generic Timer

• To do this, make use of Java Class hierarchy.

• Change Parameter of the Timer constructor to type Object → Now compatible with any object that calls the timer.

• Problem: Since we need to notify the calling object that the timer is done, we use timerdone() function.

  • So we need to cast owner back to Myclass to call the timerdone() function.

    

• Solution : Use Interfaces.

Use interfaces