Into to Applets

Intro to Applets

Intro

While not necessarily the most important use of Java, applets are a large part of the quick success and popularity of Java
An applet is a Java program that runs within the web browser environment
The web browser loads up an applet, which is a compiled Java program, and then calls up the Java Virtual Machine on the client computer to run the program
Regular Java applications have a main() method, used as a starting point for code execution
Applets do not have a main() method -- they have a different, but specific, structure

The `Applet` class

Every Java applet is derived from the base class called Applet. Typical form of an applet class declaration is:
```
    public class ThisApplet extends java.applet.Applet
```
Or if the package java.applet is imported at the top of a file, then the form:
```
    public class ThisApplet extends Applet
```
The browser controls the applet by calling some important methods derived from class Applet. The default versions are empty. The user's applet should define them (whichever ones are necessary in a given instance) to perform specific functions:
- public void init() -- invoked when the applet is first loaded by the browser
- public void start() -- invoked after the init method, once the applet has loaded, or when the user moves back to the page with the applet
- public void stop() -- opposite of start. Invoked when the user leaves the page with the applet
- public void destroy() -- invoked when the browser exits. (stop is always called before destroy).
- public void paint(Graphics) -- comes from java.awt.Component, and it's called when graphics elements of the applet need to be painted onto the screen.
class Applet is an AWT class, not designed for use with Swing components. To use Swing components in Applets, create an applet that extends the JApplet class, from the javax.swing package, which is a subclass of Applet. Example usage:
```
    public class MyApplet extends JApplet
```

Embedding an applet in a web page

In a web page (HTML file), use the <applet>...</applet> tags. Attributes:
- code=classfilename.class
- width=applet_width
- height=applet_height
- archive=archivefile - for loading an archive file with multiple classes, like a jar file
- codebase=url - to specify that the applet code is in a different location than this html file
- vspace=verticalmargin
- hspace=horizontalmargin
- align=alignmentvalue
- alt=alternate_text - what to print if applet cannot display
To pass parameters into the applet, use the html <param> tag, which goes between the <applet>...</applet> tags. Each parameter has a name and a value -- the syntax is:
```
    <param name=parametername value=parametervalue>
```
Examples:
```
    <param name=X value=100>
    <param name=LASTNAME value="Smith">
```

In the program, to read a parameter, use the following Applet class method:

    public String getParameter("parametername");

Sample usage, given above param tag examples (from inside applet):

    int x = Integer.parseInt(getParameter("X"));  // stores 100 in x
    String last = getParameter("LASTNAME");	  // stores "Smith" in last

A few examples of applet tags:

  <applet code="Welcome.class" width="350" height="175"></applet>

  <applet code="Thing.class" archive="myThings.jar"></applet>

  <applet code="StartGame.class" archive="game.jar">
    <param name=numPlayers value=2>
  </applet>

A few examples

Here are a few example pages of small applets. Each one embeds an applet, so while on the html page, select the browser option to view the html source code for each page -- this will show how the applet tag is used to embed the applet into the page. If any of these don't load up in your browser, then you probably need to download the Java Plug-in, which comes automatically with the J2SE Java Runtime Environment (JRE)

A small applet using only AWT components. Notice that it uses the paint() method to draw the applet.
- Welcome.java - the Java source code for the applet
- the web page embedding the applet
The WelcomeApplet example from the textbook. This applet uses Swing components. The applet is derived from class JApplet.
- WelcomeApplet.java - the Java source code for the applet
- MessagePanel.java - Java source code for a class used by the applet
- the web page embedding the applet
The MortgageApplet example from the textbook. This applet also uses Swing components, as well as the Mortgage class example from chapter 6.
- MortgageApplet.java - the Java source code for the applet
- Mortgage.java - Java source code for the Mortgage class
- the web page embedding the applet

Tutorials on the java.sun site:

A tutorial on writing Java applets (using the Applet class)

How to Make Applets -- using Swing components

Running Applets as Applications

Any class in Java can have a main() method
If an applet has a main() method, it can be run as a normal application, with the entry point being main(). When run as an applet, the main() method is not relevant
Not every application can be an applet. Applets are subject to some security restrictions (to prevent viruses or malicious code):
- Applets cannot read from or write to the local computer's file system. (There is a way to make an exception to this in Java 2)
- Applets cannot call up new programs on the browser's computer.
- Applets aren't allowed to establish connections between the browser's computer and other computers (other than the server where the applet is).

More Applet Examples

DisplayMessage - an example that takes in a parameter message from the html page and displays it in the applet
MoveMessageDemo - an example that illustrates mouse events, with a message that can be moved with the mouse
KeyboardEventDemo - an example demonstrating keyboard events
ScribbleDemo - an applet that allows the user to draw on a panel with the mouse
Tic Tac Toe - an example of a tic tac toe game, in an applet