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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
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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: • <html><body><applet code=“name.class” width=value height=value”></applet></body></html> • 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 www.nku.edu/~username/app.html • 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!
