Applets Java code is compiled into byte code instead of machine language –Languages like C, C++, Pascal and others are compiled into machine language so that the programs can be directly executed –In Java, byte code cannot be directly executed so instead, it iis interpreted by a program called the Java Virtual Machine The difference is that the JVM is part of every web browser –This is important because a C++ program compiled for one machine (say Windows) will not run on another (Mac or Linux) – you have to compile the program for each machine and environment –Since the JVM is built into every web browser, we could compile a Java program into byte code which then can run on any computer To run a Java program in a web browser, you have to write an applet instead of a normal program

JApplet Class In order to write an applet you extend the JApplet class (similar to how we extended JPanel) –a program that extends a JApplet does not use a main method, instead the JApplet class uses an init method you move all of the code from main into the class’ constructor and/or init and add init( ); into the constructor Some commands will not work now, for instance, addActionListener(this); or new Timer(time, this); –because “this” refers to this program but a JApplet is not capable of being a listener –to get around this problem, we create extra JPanels for the things that need listeners –this will cause us to change where and how we specify our GUI components

Simple Applet import java.awt.*; import javax.swing.*; public class SimpleApplet extends JApplet { private GraphicsPanel gp; public SimpleApplet( ) { gp = new GraphicsPanel(); gp.setPreferredSize(new Dimension(400,200)); add(gp); init( ); } public void init( ) { gp.repaint( ); } // continued on next slide }

Continued public class GraphicsPanel extends JPanel { public void paintComponent(Graphics g) { super.paintComponent(g); g.setColor(Color.black); g.fillRect(0,0,400,200); g.setFont(new Font("Ariel", Font.BOLD, 24)); g.setColor(Color.red); g.drawString("Here's a simple applet", 50, 75); }

Applet with Timer To add a Timer to control action, such as having the text on the previous JApplet slide across the screen –you need to add: private Timer t; // to the JApplet class private int x; // to the GraphicsPanel class t = new Timer(value,gp); // makes gp the ActionListener and start the timer in init implements ActionListener // to the GraphicsPanel header and an actionPerformed method to GraphicsPanel –For this example, we would use x to control where the text should be displayed, we increment x in the actionPerformed method and we change g.drawString(“text”, 50, 75); to be (“text”, x, 75); so that we control where the String is to start

To Display an Applet The Applet will not run in JCreator –write and compile your JApplet in Jcreator –create an html file called app.html that consists of the following: where name is the name of the JApplet’s class such as SimpleApplet, and width and height are numeric values equal to the size of your JPanel –in your web browers, type in the URL of your html file, such as The applet will load and run in your browers –NOTE: if you need to fix your JApplet, things get kind of complicated – first, fix your code and recompile the JApplet, next click on reload in your browser – however, this may not actually load the new version of the class since the old class has been cached – instead, you may have to change the name of the JApplet and/or change the name of the html file!