# Unit 5: Applets (CA214) ## Introduction - An **applet** is a small Java program embedded in a web page and run inside a browser or `appletviewer`. It extends `java.applet.Applet` (or `javax.swing.JApplet`). - Applets have no `main()` method and run under a security sandbox. - Note: applets are now deprecated and removed from modern browsers, but are still part of the syllabus. ## Local and Remote Applets | Local applet | Remote applet | |---|---| | Stored on the local machine | Stored on a remote server | | Loaded without Internet | Downloaded over the Internet using its URL | | Path in `CODE` attribute | Needs `CODEBASE` attribute giving the URL | ## Applets versus Applications - Applications use `main()` and run standalone; applets are run by a browser, have no `main()`, and cannot normally read/write local files or open network connections to other hosts. ## Writing Applets - Import `java.applet.Applet` and `java.awt.*`; extend `Applet`; override `paint(Graphics g)`. - Example: `public class Hello extends Applet { public void paint(Graphics g){ g.drawString("Hello Applet", 20, 20); } }` - Other drawing methods: `drawLine`, `drawRect`, `drawOval`, `fillRect`, `setColor`, `setFont`. ## Applet Life Cycle Methods called by the browser in order: 1. `init()`: called once, initialise variables, load resources. 2. `start()`: called after init and every time the page is revisited. 3. `paint(Graphics g)`: draws output; called when display needs refresh. 4. `stop()`: called when the user leaves the page. 5. `destroy()`: called once at final removal to release resources. - States: Born -> Running -> Idle -> Dead. **[Figure: Applet life cycle]** Flow from Born (init) to Running (start/paint) to Idle (stop), back to Running (start), and finally Dead (destroy). ## Creating an Executable Applet 1. Write source `Hello.java` containing the applet class. 2. Compile: `javac Hello.java` to get `Hello.class`. 3. Create an HTML file that references the class. 4. Run with `appletviewer Hello.html` or in a Java-enabled browser. ## Designing a Web Page and the Applet Tag - A page uses HTML tags ``, ``, ``. - **Applet tag**: `` - Attributes: `CODE`, `WIDTH`, `HEIGHT` (required); `CODEBASE`, `ALT`, `NAME`, `ALIGN`, `VSPACE`, `HSPACE`, `PARAM`. ## Adding Applet to HTML and Running - Place the `` tag inside ``. Keep the `.class` file in the same folder (or give `codebase`). - A comment `/* */` at the top of the source lets `appletviewer Hello.java` run it directly. ## Passing Parameters to Applets - In HTML: ` ` - In Java: `String s = getParameter("user");` in `init()`. Values are strings; convert with `Integer.parseInt()` when required. ## Aligning the Display - `ALIGN` attribute values: `left`, `right`, `top`, `middle`, `bottom`, `texttop`, `absmiddle`, `baseline`, `absbottom`. - `HSPACE` and `VSPACE` give horizontal and vertical padding around the applet. ## HTML Tags and Applets - Text around the applet tag (such as `

`, `
`, ``) is displayed by browsers that do not support applets, so it works as alternate content (`ALT`). ## Getting Input from the User - Applets receive input through AWT components: `TextField`, `Button`, `Checkbox`, `Choice`. - Add components in `init()` with `add()`; handle with event listeners (`ActionListener`, `actionPerformed(ActionEvent e)`). - Example: read two numbers from TextFields, convert with `Integer.parseInt(tf.getText())`, add and `repaint()`. - Also `showStatus()` displays text in the status bar. ## Likely Exam Questions 1. What is an applet? Differentiate between applet and application. 2. Explain the life cycle of an applet. 3. Differentiate between local and remote applets. 4. Write the applet tag and explain its attributes. 5. How are parameters passed to an applet? Give an example. 6. Write an applet that accepts input from the user and displays the sum.