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Java Applets

Java Applets. What is Java Applet. An applet is a Java program that is referenced by a Web page and runs inside a Java-enabled Web browser.. An applet begins execution when an HTML page that "contains" it is loaded..

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Java Applets

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  1. Java Applets

  2. What is Java Applet • An applet is a Java program that is referenced by a Web page and runs inside a Java-enabled Web browser.. • An applet begins execution when an HTML page that "contains" it is loaded.. • Either a Java-enabled Web browser or the appletviewer is required to run an applet..

  3. Applet Disadvantage • Do not read from the local (client) disk • Do not write to the local (client) disk • Do not open network connections other than to the server from which the applet was loaded • Do not link to client-side C code or call programs installed on the browser machine • Cannot discover private information about the user

  4. Applet Life Cycle • public void init() - Called when applet is first initialized • public void start() • Called immediately after init and again revisited after browser left page containing applet • Used to start animation threads • public void paint(Graphics g) • Called by the browser after init and start, and again whenever the browser redraws the screen, (typically when part of the screen has been obscured and then re exposed) • This method is where user-level drawing is placed • Inherited from java.awt.Container • public void stop() • Called when the browser leaves the page • Used to stop animation threads • public void destroy() • Called when applet is killed (rarely used)

  5. Web browser-to-applet interface

  6. Applet Template import java.applet.Applet; import java.awt.*; public class AppletTemplateextends Applet { // Variable declarations. public void init() { // Variable initializations, image loading, etc. } public void paint(Graphics g) { // Drawing operations. } }

  7. Applet HTML Template <html><head> <title>A Template for Loading Applets</title> </head> <body> <applet code="AppletTemplate.class" width=120 height=60> <b>Error! You must use a Java-enabled browser.</b> </applet> </body> </html>

  8. Applet Inheritance Hierarchy • Applet class derives from the Abstract Window Toolkit (AWT) hierarchy.

  9. AWT Pre-Made UI Components • AWT supplies the following UI components: • Buttons (java.awt.Button) • Checkboxes (java.awt.Checkbox) • Single-line text fields (java.awt.TextField) • Menus (java.awt.MenuItem) • Containers (java.awt.Panel) • Lists (java.awt.List)

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