Applets

A Java program can be compiled to be either an:

application;
applet.

In these notes, we shall concern ourselves primarily with applets.

Swing

Applet GUI can be implemented using the Abstract Windowing Toolkit (AWT) or Swing. Swing is newer and provides enhanced functionality compared to the AWT. Swing sits on top of AWT and Java2D. The various AWT GUI components are inherited by Swing. All of the Swing classes have a "J" placed in front of them to distinguish them from their AWT super-class. Swing applets must be implemented as a subclass of JApplet.

JApplets Versus AWT Applets

For a standard GUI based applet, there is little difference between AWT and Swing. We shall develop our programs in Swing using JApplets. As JApplets and AWT applets and almost identical, the term "applet" is commonly used to refer to JApplet. This is the convention that will be used in these notes.

Example JApplet (Run Example Code)

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

public class DemoJApplet extends JApplet
{
    @Override
    public void init()
    {
        this.setContentPane(new View());
    }

    public class View extends JPanel
    {
        @Override
        protected void paintComponent(Graphics g)
        {
            super.paintComponent(g);
            g.drawString("JApplet demo", 100, 50);
        }
    }
}

Even if Swing JApplets is being used to provide the GUI functionality, it is quite common for to also use some of the AWT classes in the same applet. For example, in the example above, the Graphics class belongs to the AWT package, so the AWT package must be imported into the programme.

The differences between JApplets and AWT applets include:

Each top-level Swing container, such as a JApplet, has a single content root pane. You add components to a JApplet's content root pane, not directly to the JApplet. The most noticeable effects of the root pane's presence are support for adding a menu bar and the need to use a content pane.
You set the layout manager on a JApplet's content pane, not directly on the JApplet.
The default layout manager for a JApplet's content pane is BorderLayout. This differs from the default layout manager for Applet, which is FlowLayout.
JApplets implement double buffering as the default paining mode. Applets implement single buffering.

We shall deal with these issues as they arise.

Applets Versus Applications

Applets differ from stand-alone programs as follows:

applets are designed to be used in web browsers;
applets are wrapped in HTML code. Java creates an applet in response to a request issued by a web browser;
an applet's size is given in the HTML code that wraps the applet.
applets do not have access to the local file structure.
applets do not have a title bar. There is no title in the Applet constructor;
applets have an init() (and a start()) method instead of a main() method.
applets do not have a close button. They close automatically when the browser leaves the webpage that the applet is running on.
every applet is a subclass of the JApplet class;
applets use a layout manager to arrange component positions within the applet;

There can only be one occurrence of init() in any applet. After the class's constructor, init() is the first code to be called when an applet is loaded into a browser. A class's constructor does not have access to the GUI. Therefore, init() is the first place where GUI methods can be called. The init() method is called exactly once for any applet. It is not called every time the browser returns to the HTML webpage in which the applet resides.

There can only be one occurrence of start() in any applet. The start() method is called after the init() method when an applet is loaded into a browser. In addition, start() is called every time a browser returns to the HTML webpage in which the applet resides.

An example where the start() method can be used is for starting an audio clip every time a user returns to a webpage.

To get access to the JApplet class an applet must import (include) the JPpplet package (library).

As every applet is a subclass of the JApplet class, every applet must extend the JApplet class:

For example:

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

public class DemoJApplet extends JApplet
{
    @Override
    public void init()
    {
        this.setContentPane(new View());
    }

    public class View extends JPanel
    {
        @Override
        protected void paintComponent(Graphics g)
        {
            super.paintComponent(g);
            g.drawString("JApplet demo", 100, 50);
        }
    }
}

The HTML wrapper code for an applet is:

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
        <title>Demo</title>
    </head>
    <body>
        <applet code   = DemoJApplet.class
                name   = Demo
                width  = 640
                height = 480>
        </applet>
    </body>
</html>

Passing Parameters from HTML files

Using the HTML param tag and the Java getParameter() method we can read in applet parameters from a HTML file. The getParameter() method has the syntax:

String getParameter(String parameterName);

If the parameter does not exist in the HTML file, then NULL is returned.

For example:

import javax.swing.JApplet;

public class DemoJApplet extends JApplet
{
    private String imageName;
    private int scaleX;
    private int scaleY;

    @Override
    public void init()
    {
        this.imageName = this.getParameter("imageName");
        this.scaleX = Integer.getInteger(this.getParameter("scaleX"));
        this.scaleY = Integer.getInteger(this.getParameter("scale Y"));
    }
}

The HTML wrapper code for an applet with parameters is:

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
        <title>Demo</title>
    </head>
    <body>
        <applet code       = DemoJApplet.class
                name       = Demo
                width      = 640
                height     = 480 
                param name = "imageName" value = "myBitmap.gif"
                param name = "scaleX"    value = "2"
                param name = "scaleY"    value = "1"
        </applet>
    </body>
</html>