The Applet Lifecycle

An applet has no main method, so it never decides for itself when to start doing work. Instead, its host container - a browser plugin in the applet's original design, or appletviewer when you're following along today - calls a fixed set of methods on your applet class at well-defined moments. Together these five methods make up the applet lifecycle. Understanding exactly when each one fires, and which ones can fire more than once, is the single most important concept in this entire track.

The Five Lifecycle Methods

Call Order

On first load, the container calls the methods in this order. If the user navigates away and back (or the applet is otherwise hidden and shown again) without the page being reloaded, the bracketed portion below repeats before the applet is eventually destroyed:

init() -> start() -> paint()  ->  [ stop() -> start() -> paint() ]*  ->  stop() -> destroy()
 once      once     as needed        repeats each time hidden/shown         once      once

Notice that init() only ever appears once at the very start, and destroy() only ever appears once at the very end. Everything between them - start(), paint(), and stop() - can happen any number of times, in that start/paint/stop rhythm, for as long as the applet remains on the page.

Observing the Order Yourself

The easiest way to internalize this order is to override all five methods with a System.out.println call in each, run the applet with appletviewer, and watch the console while you minimize and restore the viewer window (or switch away and back):

import java.applet.Applet;
import java.awt.Graphics;

public class LifecycleDemoApplet extends Applet {

  public void init() {
    System.out.println("init() called");
  }

  public void start() {
    System.out.println("start() called");
  }

  public void paint(Graphics g) {
    System.out.println("paint() called");
    g.drawString("Watch the console!", 20, 30);
  }

  public void stop() {
    System.out.println("stop() called");
  }

  public void destroy() {
    System.out.println("destroy() called");
  }
}

Compile it with javac LifecycleDemoApplet.java, reference it from a minimal HTML file the same way you did in the previous lesson, and open it with appletviewer. You'll see init(), start(), and paint() print immediately; minimizing and restoring the viewer window prints stop() followed by another start() and paint(); closing the viewer prints a final stop() followed by destroy().