Applets
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An applet is a Java program that is run within a web browser and allows for user interaction, it is defined by extending the Applet class and overriding lifecycle methods like init(), start(), stop(), and paint(), and applets are run in a sandbox for security reasons and have restrictions on access to the client system.
Applets
- 2. An applet is a Panel that allows interaction with a Java program A applet is typically embedded in a Web page and can be run from a browser You need special HTML in the Web page to tell the browser about the applet For security reasons, applets run in a sandbox: they have no access to the clients file system
- 3. You write an applet by extending the class Applet Applet is just a class like any other; you can even use it in applications if you want When you write an applet, you are only writing part of a program The browser supplies the main method
- 4. java.lang.Object | +----java.awt.Component | +----java.awt.Container | +----java.awt.Panel | +----java.applet.Applet
- 5. public void init () public void start () public void stop () public void destroy () public void paint (Graphics) Also: public void repaint() public void update (Graphics) public void showStatus(String) public String getParameter(String)
- 6. You write an applet by extending the class Applet Applet defines methods init( ), start( ), stop( ), paint(Graphics), destroy( ) These methods do nothing--they are stubs You make the applet do something by overriding these methods
- 7. This is the first method to execute It is an ideal place to initialize variables It is the best place to define the GUI Components (buttons, text fields, scrollbars, etc.), lay them out, and add listeners to them Almost every applet you ever write will have an init( ) method
- 8. Not always needed Called after init( ) Called each time the page is loaded and restarted Used mostly in conjunction with stop( ) start() and stop( ) are used when the Applet is doing time-consuming calculations that you dont want to continue when the page is not in front
- 9. Not always needed Called when the browser leaves the page Called just before destroy( ) Use stop( ) if the applet is doing heavy computation that you dont want to continue when the browser is on some other page Used mostly in conjunction with start()
- 10. Seldom needed Called after stop( ) Use to explicitly release system resources (like threads) System resources are usually released automatically
- 11. init and destroy are only init() called once each start and stop are called start() whenever the browser enters and leaves the page do some work do some work is code called by your listeners stop() paint is called when the applet needs to be repainted destroy()
- 12. Needed if you do any drawing or painting other than just using standard GUI Components Any painting you want to do should be done here, or in a method you call from here Painting that you do in other methods may or may not happen Never call paint(Graphics), call repaint( )
- 13. Call repaint( ) when you have changed something and want your changes to show up on the screen repaint( ) is a request--it might not happen When you call repaint( ), Java schedules a call to update(Graphics g)
- 14. When you call repaint( ), Java schedules a call to update(Graphics g) Here's what update does: public void update(Graphics g) { // Fills applet with background color, then paint(g); }
- 15. A Graphics is something you can paint on g.drawString(Hello, 20, 20); Hello g.drawRect(x, y, width, height); g.fillRect(x, y, width, height); g.drawOval(x, y, width, height); g.fillOval(x, y, width, height); g.setColor(Color.red);