Threads - JavaTech: An Introduction to Scientific and Technical Computing with Java - page 282

For periodic events, if the duration of processing in between the sleep periods
varies significantly, then the overall timing will vary with respect to a clock. Also,
if you need several timer events, the program will require several threads and this
will use up system resources.
Java provides two timer classes [5-7]:
javax.swing.Timer came with the Swing packages and is useful for such tasks as
prompting the updating of a progress bar
java.util.Timer and its helper class java.util.TimerTask provide for general
purpose timers with more features than the Swing timer

These timers can provide multiple timed events from a single thread and thus
conserve resources. They also have useful methods such as scheduleAt-
FixedRate(TimerTask task, long delay, long period) in java.
util.Timer . This method will set events to occur periodically at a fixed rate
and ties them to the system clock. This is obviously useful for many applications
such as a countdown timer and an alarm clock where you don't want the timing
to drift relative to absolute time.

8.7.1 java.util.Timer and TimerTask
The Timer and TimerTask combo in java.util offers the most general
purpose timing capabilities and includes a number of options. A Timer object
holds a single thread and can control many TimerTask objects. The TimerTask
abstract class implements the Runnable interface but it does not provide a
concrete run() method. Instead you create a TimerTask subclass to provide
the concrete run() method with the code to carry out the task of interest.
In the example below, we create a digital clock using a timer to redraw a
time display every second. The clock display uses DateFormatPanel ,which
we describe in Chapter 10 when discussing the date classes. Whenever this
panel is drawn it displays the current time. The applet adds an instance of this
panel to its content pane and in the start() method creates an instance of
java.util.Timer .

A subclass of TimerTask called UpdateTask overrides the run() method
and simply tells the panel to redraw itself. UpdateTask is defined as an inner
class here and has access to the clock panel reference. The timer schedules
calls to the UpdateTask every 1000 milliseconds. Figure 8.9 shows the clock
display.

Figure 8.9 The
ClockTimer1 and
ClockTimer2 programs,
which both provide a
current time display like
that shown here, illustrate
the use of
java.util.Timer and
javax.swing.Timer ,
respectively.

import javax.swing.*;
import java.awt.*;
import java.util.*;

/** This applet implements Runnable and uses a thread