Encyclopedia of Computer Science and Technology

JJavaJava is a computer language similar in structure to C++.Although Java is a general-purpose programming language,it is most often used for creating applications to run on theInternet, such as Web servers. A special type of Java programcalled an applet can be linked into Web pages and runon the user’s Web browser (see applet).As an object-oriented language, Java uses classes thatprovide commonly needed functions including the creationof user interface objects such as windows and buttons (seeclass and object-oriented programming). A variety ofsets of classes (“class frameworks”) are available, such asthe AWT (Abstract Windowing Toolkit).are built. The AWT (Abstract Windowing Toolkit) is a set ofclasses that provide a graphical user interface.The program then declares a new class called Hello-World and specifies that it is built on (extends) the appletclass.Program StructureA Java program begins by importing or defining classes andusing them to create the objects needed for the program’sfunctions. Code statements then create the desired output orinteraction from the objects, such as drawing a picture or puttingtext in a window. Here is a simple Java applet program:import java.applet.Applet;import java.awt.Graphics;public class HelloWorld extends Applet {public void paint(Graphics g) {g.drawString(“Hello world!”, 50, 25);}}The first two lines import (bring in) standard classes.The applet class is the foundation on which applet programsAfter an embedded Java program (called an applet) is compiled, itsexecutable file (Javacode) is stored on the Web server, together withthe HTML file for the Web page to which the program is linked.254