sample random timer

App.java

public class App {

	public static void main(String[] args) {

		// create new timer
		MyTimer timer = new MyTimer();

		// add listener
		timer.addListener(new MyTimer.IMyTimerEventListener() {

			@Override
			public void onEventFired() {
				System.out.println("event fired");
			}
			
		});

		// start the timer
		timer.start();
	}

}

MyTimer.java

import java.util.ArrayList;
import java.util.Random;

public class MyTimer extends Thread {

	public interface IMyTimerEventListener {
		void onEventFired();
	}

	// list of listeners that will be notified about the event
	private ArrayList<IMyTimerEventListener> listeners;

	// maximum sleep time in seconds
	private static final int MAX_SLEEP_TIME = 30;

	// minimum sleep time in seconds
	private static final int MIN_SLEEP_TIME = 1;

	// create new random
	private static final Random rnd = new Random();

	public MyTimer() {
		this.listeners = new ArrayList<IMyTimerEventListener>();
	}

	public void addListener(IMyTimerEventListener listener) {
		if (listener == null)
			throw new IllegalArgumentException("listener");

		if (!listeners.contains(listener))
			listeners.add(listener);
	}

	@Override
	public void run() {

		// while the thread is alive
		while (!isInterrupted()) {

			try {
				// get random value between max and min
				int randomTimeToSleep = rnd.nextInt((MAX_SLEEP_TIME - MIN_SLEEP_TIME) + 1)
						+ MIN_SLEEP_TIME;

				// let the thread sleep for random amount of time
				Thread.sleep(randomTimeToSleep * 1000);
				
			} catch (InterruptedException e) {
				// thread couldn't sleep, maybe throw exception here
			}

			// execute event and notify listeners
			notifyListeners();
		}
	}

	private void notifyListeners() {
		// iterate all attached listeners and fire event
		for (IMyTimerEventListener listener : listeners) {
			listener.onEventFired();
		}
	}
}