Timer and TimerTask in Java

Timer and TimerTask, in java.util, are an older way to run code later or repeatedly. A TimerTask is the job: a class with a run() method. A Timer owns one background thread that runs its tasks at the scheduled times. You can schedule a task once after a delay, or at a fixed interval. They are now considered legacy, replaced by ScheduledExecutorService, but you will still meet them in older code. In this lesson, you will learn how to use them and why they were replaced.

Methods of Timer

Example: A Reminder and a Repeating Task

Let's see an example that runs a one-time reminder and a repeating tick.

import java.util.Timer;
import java.util.TimerTask;
 
public class TimerDemo {
    public static void main(String[] args) throws InterruptedException {
        Timer timer = new Timer("reminders");
 
        timer.schedule(new TimerTask() {
            @Override
            public void run() {
                System.out.println("Reminder: Navika's class starts soon");
            }
        }, 350);
 
        TimerTask tick = new TimerTask() {
            private int count = 0;
 
            @Override
            public void run() {
                count++;
                System.out.println("tick " + count);
                if (count == 3) {
                    cancel();
                }
            }
        };
        timer.scheduleAtFixedRate(tick, 100, 200);
 
        Thread.sleep(800);
        timer.cancel();
        System.out.println("Timer canceled");
    }
}

tick 1
tick 2
Reminder: Navika's class starts soon
tick 3
Timer canceled

Explanation: The tick runs at 100, 300 and 500 milliseconds and cancels itself after the third run. The reminder runs once at 350 milliseconds, between the second and third ticks. Finally, timer.cancel() stops the timer's thread; without it, the non-daemon timer thread would keep the program running.

Why Timer Is Legacy

Common Mistakes

Forgetting to cancel a non-daemon Timer. The program never exits.

Letting a task throw an exception. The whole timer dies silently.

Scheduling a TimerTask object twice. It throws IllegalStateException; create a new task.

Practice

1. Easy: Use a Timer to print "Water the plants" every second, three times, then cancel it.
2. Harder: Rewrite the example with ScheduledExecutorService, using lambdas instead of TimerTask classes.

Key Takeaways