Every applet must define a subclass of the Applet or JApplet class. In the Hello World applet, this subclass is called HelloWorld. The following is the source for the HelloWorld class.

import javax.swing.JApplet;
import javax.swing.SwingUtilities;
import javax.swing.JLabel;

public class HelloWorld extends JApplet {
    //Called when this applet is loaded into the browser.
    public void init() {
        //Execute a job on the event-dispatching thread; creating this applet's GUI.
        try {
            SwingUtilities.invokeAndWait(new Runnable() {
                public void run() {
                    JLabel lbl = new JLabel("Hello World");
                    add(lbl);
                }
            });
        } catch (Exception e) {
            System.err.println("createGUI didn't complete successfully");
        }
    }
}

Applets inherit significant functionality from the Applet or JApplet class, including the capabilities to communicate with the browser and present a graphical user interface (GUI) to the user.

An applet that will be using GUI components from Swing (Java's GUI toolkit) should extend the javax.swing.JApplet base class, which provides the best integration with Swing's GUI facilities.

JApplet provides a root pane, which is the same top-level component structure as Swing's JFrame and JDialog components, whereas Applet provides just a basic panel. See How to Use Root Panes for more details on how to utilize this feature.

An applet can extend the java.applet.Applet class when it does not use Swing's GUI components.