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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)
