Java Timer

1. Schedule a One-Time Task With Timer in Java
2. Schedule a Repeating Timer in Java

Timer is a service available in the Java programming language that allows users to schedule a future event. These future events can be one time or repeated at regular time intervals. There can be triggers that we use to initiate future events.

Schedule a One-Time Task With Timer in Java

import java.util.Timer;
import java.util.TimerTask;

public class StopWatch {
  Timer timer;

  public StopWatch(int seconds) {
    timer = new Timer();
    timer.schedule(new StopTask(), seconds * 1000);
  }

  public static void main(String[] args) {
    new StopWatch(10);
    System.out.println("StopWatch Started.");
  }

  class StopTask extends TimerTask {
    public void run() {
      System.out.println("Time Up!");
      timer.cancel();
    }
  }
}

In the above code block, we have created a public class named the StopWatch class. It has its public constructor, which takes time in seconds. In the constructor block, a timer instance gets created. Over the timer instance, the schedule method gets called. This method task is to schedule the specific event at the given amount of time. It takes two parameters TimerTask instance and the delay. We define the delay in milliseconds. The method throws IllegalArgumentException when the given delay defined is invalid, IllegalArgumentException when the task provided is already scheduled or canceled. It also throws NullPointerException when the task instance provided is null.

Just below the constructor, we have the main method from where the actual execution starts. It creates a new instance of the StopWatch class with defined delay, which internally executes the statement in the constructor block.

Below the main method, an inner class named StopTask is defined that extends a TimerTask abstract class. TimerTask is an abstract class that has an abstract method called the run method. The method needs to get overridden in the StopTask class, and if not overridden, the class should itself become abstract. Now in the run method, we have stopped the timer with the cancel method. The method discards any currently scheduled task and terminates the timer task.

Below is the output of the above code block. The second line gets printed when the delay in the schedule method gets encountered.

StopWatch Started.
Time Up!

Schedule a Repeating Timer in Java

We will modify the above code to create a repeating timer. Repeated timer gets executed repeatedly at regular intervals of time.

In the below code block, we have created a Timer instance. The instance variable use is to call a method scheduleAtFixedRate method, which schedules the future events. The method has various implementations. The one we used takes three parameters. First, a TimerTask instance holds statements related to action we want to perform in the future. Next, the delay variable defines the time taken by the task or event to run. Last is the period that defines the time between successive calls of the event. This method throws exceptions that include the NullPointerException IllegalArgument and IllegalState Exceptions.

import java.util.Timer;
import java.util.TimerTask;

public class RepeatedTimer {
  public static void main(String[] args) {
    Timer timer = new Timer();
    System.out.println("Stop Watch Started.");
    timer.scheduleAtFixedRate(new RepeatedTask(), 500, 1000);
  }

  static class RepeatedTask extends TimerTask {
    public void run() {
      System.out.println("Running!");
    }
  }
}

Below is the never-ending outcome of the future event scheduled.

Running!
Running!
Running!
Running!