Learning Java, 4th Edition

Timers

Java includes two handy classes for timed code execution. If you write a clock application, for example, you might want to update the display every second. You might want to play an alarm sound at some predetermined time. You could accomplish these tasks using multiple threads and calls to Thread.sleep(). But the java.util.Timer and java.util.TimerTask classes handle this for you.

The Timer class is a scheduler. Each instance of Timer has a single thread that runs in the background, watching the clock and executing one or more TimerTasks at appropriate times. You could, for example, schedule a task to run once at a specific time like this:

    import java.util.*;

    public class Y2K {
      public static void main(String[] args) {
        Timer timer = new Timer();

        TimerTask task = new TimerTask() {
          public void run() {
            System.out.println("Y2K!");
          }
        };

        Calendar cal = new GregorianCalendar( 2000, Calendar.JANUARY, 1 );
        timer.schedule( task, cal.getTime());
      }
    }

TimerTask implements the Runnable interface. To create a task, you can simply subclass TimerTask and supply a run() method. Here, we’ve created a simple anonymous subclass of TimerTask that prints a message to System.out. Using the schedule() method of Timer, we’ve asked that the task be run on January 1, 2000. If the scheduled time has already passed (as in our example), the task is run immediately.

There are some other varieties of schedule(); you can run tasks once or at recurring intervals. There are two kinds of recurring tasks—fixed delay ...