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CSCI/CMPE 3326 Object-Oriented Programming in Java 1. Applet Dongchul Kim Department of Computer Science University of Texas – Rio Grande Valley your name What Are Applets? • An applet is a special Java program that can be embedded in HTML documents. • It is automatically executed by (applet-enabled) web browsers. • In Java, non-applet programs are called applications. 2 your name Application vs. Applet • Application – Trusted (i.e., has full access to system resources) – Invoked by Java Virtual Machine (JVM, java), e.g., java HelloWorld – Should contain a main method, i.e., public static void main(String[]) • Applet – Not trusted (i.e., has limited access to system resource to prevent security breaches) – Invoked automatically by the web browser – Should be a subclass of class java.applet.Applet 3 your name Life Cycle of an Applet • Four methods in the Applet class give you the framework on which you build any serious applet: • init: This method is intended for whatever initialization is needed for your applet. It is called after the param tags inside the applet tag have been processed. • start: This method is automatically called after the browser calls the init method. It is also called whenever the user returns to the page containing the applet after having gone off to other pages. • stop: This method is automatically called when the user moves off the page on which the applet sits. It can, therefore, be called repeatedly in the same applet. • destroy: This method is only called when the browser shuts down normally. Because applets are meant to live on an HTML page, you should not normally leave resources behind after a user leaves the page that contains the applet. • paint: Invoked immediately after the start() method, and also any time the applet yourfrom name needs to repaint itself in the browser. The paint() method is actually inherited Examples • HelloWorld.java public class HelloWord { public static void main(String[] args) { System.out.println(“Hello, World!”); } } • HelloWorldApplet.java import java.applet.*; import java.awt.*; public class HelloWorldApplet extends Applet { public void paint (Graphics g) { g.drawString ("Hello World", 25, 50); } } 5 your name First Java Applet • Java source in HelloWorldApplet.java import java.awt.*; import java.applet.Applet; public class HelloWorldApplet extends Applet { public void paint(Graphics g) { Dimension d = getSize(); g.setColor(Color.BLACK); g.fillRect(0, 0, d.width, d.height); // paint background g.setFont(new Font("San-serif", Font.BOLD, 24)); g.setColor(new Color(255, 215,0)); g.drawString("Hello, world!", 60, 40); g.drawImage(getImage(getCodeBase(), “sample.jpg"), 20, 60, this); } } 6 your name import javax.swing.*; // Swing GUI classes are defined here. import java.awt.event.*; // Event handling class are defined here. public class HelloSwing extends JApplet implements ActionListener { public void init() { JButton button = new JButton("Click Me!"); button.addActionListener(this); getContentPane().add(button); } public void actionPerformed(ActionEvent evt) { String title = "Greetings"; // Shown in title bar of dialog box. String message = "Hello from the Swing User Interface Library."; JOptionPane.showMessageDialog(null, message, title,JOptionPane.INFORMATION_MESSAGE); } } your name Graphics Coordinate (0,0) x height y width 8 your name Embedding Applet into HTML • HTML source in HelloWorld.html <!--HelloWorld.html--> <html> <head> <title>HelloWord</title> </head> <body> <center> <applet code="HelloWorldApplet.class" width=300 height=350></applet> </center> <a href="HelloWorldApplet.java">The source.</a> </body> </html> 9 your name Compiling and Running • To compile javac HelloWorldApplet.java Produces HelloWorldApplet.class • To run – Open page HelloWorld.htmlfrom web browser or – Use appletviewer of JDK appletviewer HelloWorld.html 10 your name Elements of Applets • Superclass: java.applet.Applet • No main method • paint method to paint the picture • Applet tag: <applet> </applet> – code – width and height 11 your name