clock.java

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/*
 * Copyright (c) 2000 David Flanagan.  All rights reserved.
 * This code is from the book Java Examples in a Nutshell, 2nd Edition.
 * It is provided AS-IS, WITHOUT ANY WARRANTY either expressed or implied.
 * You may study, use, and modify it for any non-commercial purpose.
 * You may distribute it non-commercially as long as you retain this notice.
 * For a commercial use license, or to purchase the book (recommended),
 * visit http://www.davidflanagan.com/javaexamples2.
 */
package com.davidflanagan.examples.applet;
import java.applet.*;         // Don't forget this import statement!
import java.awt.*;            // Or this one for the graphics!
import java.util.Date;        // To obtain the current time
import java.text.DateFormat;  // For displaying the time

/** 
 * This applet displays the time, and updates it every second 
 **/
public class Clock extends Applet implements Runnable {
    Label time;               // A component to display the time in
    DateFormat timeFormat;    // This object converts the time to a string
    Thread timer;             // The thread that updates the time
    volatile boolean running; // A flag used to stop the thread

    /**
     * The init method is called when the browser first starts the applet.
     * It sets up the Label component and obtains a DateFormat object
     **/
    public void init() {
	time = new Label();
	time.setFont(new Font("helvetica", Font.BOLD, 12));
	time.setAlignment(Label.CENTER);
	setLayout(new BorderLayout());
	add(time, BorderLayout.CENTER);
	timeFormat = DateFormat.getTimeInstance(DateFormat.MEDIUM);
    }

    /**
     * This browser calls this method to tell the applet to start running.
     * Here, we create and start a thread that will update the time each
     * second.  Note that we take care never to have more than one thread
     **/
    public void start() {
	running = true;                // Set the flag 
	if (timer == null) {           // If we don't already have a thread
	    timer = new Thread(this);  // Then create one
	    timer.start();             // And start it running
	}
    }

    /**
     * This method implements Runnable.  It is the body of the thread.  Once
     * a second, it updates the text of the Label to display the current time
     **/
    public void run() {
	while(running) {     // Loop until we're stopped
	    // Get current time, convert to a String, and display in the Label
	    time.setText(timeFormat.format(new Date()));  
	    // Now wait 1000 milliseconds
	    try { Thread.sleep(1000); }
	    catch (InterruptedException e) {}
	}
	// If the thread exits, set it to null so we can create a new one
	// if start() is called again.
	timer = null;
    }

    /**
     * The browser calls this method to tell the applet that it is not visible
     * and should not run.  It sets a flag that tells the run() method to exit
     **/
    public void stop() { running = false; } 

    /**
     * Returns information about the applet for display by the applet viewer
     **/
    public String getAppletInfo() {
	return "Clock applet Copyright (c) 2000 by David Flanagan";
    }
}