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Java Applets ! An applet is a Java program that can be embedded into Introduction to Java Development CS234 Week 1 Lecture 2 HTML much in the same way an image is included in a page ! The applet is sent over the internet and interpreted on a client machine ! In HTML5, the <object> tag is used to embed an applet in a web page ! Not all browsers current support the <object> tag, Firefox is HTML5 compliant in this way ! Even supported browsers must have the correct plug-ins and configuration to allow applets to run Example Our First Applet ! Embedding an applet using the <object> tag // We grab the awt and applet class libraries – these are documented in the Java API import java.awt.*; import java.applet.*; <object codetype="application/java" classid="java:HelloWorldApplet.class” width="500" height="500"> </object> This code assumes the applet is in the same directory as the HTML page. Additional details: http://www.w3.org/TR/REC-html40/struct/objects.html#edef-OBJECT // All applets must be public classes that extend the Applet class // Note that the name of the class must match the name of the file // All types of java files must also be named with the .java extension public class HelloWorldApplet extends Applet { // We override the paint method of java.applet.Applet to show our message public void paint(Graphics g) { g.drawString("Hello World!", 50, 25); } } Creating our Applet Compiling our Applet ! We will use Dr. Java to create our applet ! In Dr. Java, we use the compile button to compile our applet ! We may wish to configure Dr. Java under preferences ! Alternately, we may do this at the command line: ! Add Line Numbers ! javac HelloWorldApplet.java ! Either way, we rely on the Java SDK that is installed on your machine to compile the file ! The compiler reads class and interface definitions written in java and creates bytecode class files ! These mirror the names of the class but have a .class extension ! Ex: HelloWorldApplet.class JVM ! The class files produced are platform independent ! Unique to Java ! The JVM then reads the bytecode and executes the application i.e. the bytecode is a set of instructions for the JVM to execute ! Originally interpreted, now most often JIT compiled Compilation and execution Running our Applet ! 1. Click the run button in Dr. Java ! 2. At the command line: ! appletviewer HelloWorldApplet.java ! 3. View the applet as embedded in our webpage Java Applications ! Java applications are Java programs that are designed to stand alone ! They may interact with the user through the console or through a GUI ! Compilation is the same as for Java applets ! To run a Java program ! Select “run” in Dr. Java ! Enter “java HelloWorld” at the command line in the directory containing your HelloWorld.class file HelloWorld Java Application // This is the definition of the HelloWorld class // The class definition contains only the main method where // execution begins public class HelloWorld { public static void main(String[] args) { System.out.println("Hello Again"); } }