JAVA • Developed at Sun Microsystems by James Gosling and Bill Joy. • Designed As an Object-Oriented, Machine-Independent Pr
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JAVA · Developed at Sun Microsystems by James Gosling and Bill Joy. · Designed as an object-oriented, machine-independent programming language. · Java is both a compiled and an interpreted language. Source Code Java Runtime Byte Code UNIX PC Mac Salient Java Features · Java source code is compiled into binary instructions (byte-code) for a virtual machine (universal format) whereas C/C++ source code is compiled into native binary code. · Compiled Java byte-code (J-code) is executed by a Java run-time interpreter. · Java is portable. The same Java application can run on any platform that provides a Java run-time environment. · In general, interpreters are slow, but because Java interpreter runs compiled byte-code, Java is a fast interpreted language. · Fundamental unit of Java code is the class. · Class is an application component that holds both code and data. · Java supports only single-inheritance class hierarchy. · Java is a statically-typed (like C/C++), late-binding (like Smalltalk) language. · No preprocessor in Java, so no macros, #define's or conditional compilation in Java. · Java eliminates ad-hoc pointers and adds garbage collection and true arrays to the language. · Java supports threads. · Java has a powerful exception-handling mechanism. · Java provides dynamic memory management. Java Safety of Implementation System Resources Security Manager Class Loader Verifier Java Binary Byte-code Verifier Verifies byte-code before they are run and ensures that it obeys basic rules of Java language. Class Loader Loads binary classes that contain byte-code. Security Manager Restricts access to system resources and is the single point of control to system resources. Instructions to compile and run Java code Create a file called helloworld.java in a sub-dir called java. /* My first Java program */ class helloworld { public static void main(String [] args) { System.out.println("Hello World!"); } } To compile: javac helloworld.java (This will create a file called helloworld.class) To run: java helloworld · Website: http://www.javasoft.com/ APPLET · Applet is a small, subordinate or embeddable application confined within the walls of a Web Browser (eg. Netscape Navigator or Hot Java) or Applet Viewer provided by the JDK. · An applet can interact only with the user and the host from which it originates. Hello World Applet · Using a text editor, create a file named HelloWorld.java in the public_html directory with the Java code shown here: import java.applet.*; import java.awt.*; public class HelloWorld extends Applet { public void paint(Graphics g) { g.drawString("Hello World!", 50, 25); } } · Compile the Java source file: javac HelloWorld.java · Create an HTML file called Hello.html that includes the applet. The HTML file should be in the same directory that contains the HelloWorld.class file. <HTML> <HEAD> <TITLE> A Simple Program </TITLE> </HEAD> <BODY> Here is the output of my program: <APPLET CODE="HelloWorld.class" WIDTH=150 HEIGHT=25> </APPLET> </BODY> </HTML> To run the applet, you need to load the HTML file into an application (eg. Netscape Navigator) that can run Java applets. In the web browser specify the URL: http://www.csis.gvsu.edu/~userid /Hello.html You can also run the applet in an applet viewing program provided by the JDK. In this case the command is: appletviewer file:/home/userid/java/Hello.html (This assumes you have saved the Hello.html and HelloWorld.class files in the java sub-dir) .