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Implementing the Runnable Interface

The Clock applet shown below displays the current time and updates its display every second. You can scroll this page and perform other tasks while the clock continues to update because the code that updates the clock's display runs within its own thread.

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Note: If you don't see the applet running above, you need to install Java Plug-in, which happens automatically when you install the J2SE JRE or SDK. We strongly recommend that you install the latest version; at least 1.3.1 is required for all our applets. You can find more information in the Java Plug-in home page.

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The Clock applet uses a different technique than SimpleThread for providing the run method for its thread. Instead of subclassing Thread, Clock implements the Runnable interface (and therefore implements the run method defined in it). Clock then creates a thread and provides itself as an argument to the Thread's constructor. When created in this way, the Thread gets its run method from the object passed into the constructor. The code that accomplishes this is shown in bold here:

import java.awt.Graphics;
import java.util.*;
import java.text.DateFormat;
import java.applet.Applet;

public class Clock extends Applet implements Runnable {
    private Thread clockThread = null;
    public void start() {
        if (clockThread == null) {
            clockThread = new Thread(this, "Clock");
            clockThread.start();
        }
    }
    public void run() {
        Thread myThread = Thread.currentThread();
        while (clockThread == myThread) {
            repaint();
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e){
            // the VM doesn't want us to sleep anymore,
            // so get back to work
            }
        }
    }
    public void paint(Graphics g) {
        // get the time and convert it to a date
        Calendar cal = Calendar.getInstance();
        Date date = cal.getTime();
        // format it and display it
        DateFormat dateFormatter = DateFormat.getTimeInstance();
        g.drawString(dateFormatter.format(date), 5, 10);
    }
    // overrides Applet's stop method, not Thread's
    public void stop() {
        clockThread = null;
    }
}

The Clock applet's run method loops until the browser asks it to stop. During each iteration of the loop, the clock repaints its display. The paint method figures out what time it is, formats it in a localized way, and displays it. You'll see more of the Clock applet in The Life Cycle of a Thread which uses it to teach you about the life of a thread.

Deciding to Use the Runnable Interface

You have now seen two ways to provide the run method for a Java thread:

1. Subclass the Thread class defined in the java.lang package and override the run method.

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   Example:  See the SimpleThread class described in Subclassing Thread and Overriding run.

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2. Provide a class that implements the Runnable interface (also defined in the java.lang package) and therefore implements the run method. In this case, a Runnable object provides the run method to the thread.

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   Example:  See the Clock applet just shown.

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There are good reasons for choosing either of these options over the other. However, for most cases, including that of the Clock applet, the following rule of thumb will guide you to the best option.

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Rule of Thumb:  If your class must subclass some other class (the most common example being Applet), you should use Runnable as described in option #2.

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To run in a Java-enabled browser, the Clock class has to be a subclass of the Applet class. Also, the Clock applet needs a thread so that it can continuously update its display without taking over the process in which it is running. (Some browsers might create a new thread for each applet so as to prevent a misbehaved applet from taking over the main browser thread. However, you should not count on this when writing your applets; your applets should create their own threads when doing computer-intensive work.) But since the Java language does not support multiple class inheritance, the Clock class cannot be a subclass of both Thread and Applet. Thus the Clock class must use the Runnable interface to provide its threaded behavior.

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