Java programs are written in two forms: applications and applets.
Java applications are run by invoking the Java interpreter from the command line and specifying the file containing the compiled application.
Java applets are invoked from a browser. The HTML code interpreted by the browser names a file containing the compiled applet. This causes the browser to invoke the Java interpreter which loads and runs the applet.
Example 3-1 is the source of an application that displays "Hello World" on stdout. The method accepts arguments in the invocation, but does nothing with them.
// // HelloWorld Application // class HelloWorldApp{ public static void main (String args[]) { System.out.println ("Hello World"); } } |
Note that, as in C, the method or function to be initially executed is identified as main. The keyword public lets the method be run by anyone; static makes main refer to the class HelloWorldApp and no other instance of the class; void says that main returns nothing; and args[] declares an array of type String
To compile the application, enter
$ javac HelloWorldApp.java |
It is run by
$ java HelloWorldApp arg1 arg2 ... |
Example 3-2 is the source of the applet that is equivalent to the application in Example 3-1.
// // HelloWorld Applet // import java.awt.Graphics; import java.applet.Applet; public class HelloWorld extends Applet { public void paint (Graphics g) { g.drawstring ("Hello World", 25, 25); } } |
In an applet, all referenced classes must be explicitly imported. The keywords public and void mean the same as in the application; extend says that the class HelloWorld inherits from the class Applet.
To compile the applet, enter
$ javac HelloWorld.java |
The applet is invoked in a browser by HTML code. A minimum HTML page to run the applet is:
<title>Test</title> <hr> <applet code="HelloWorld.class" width=100 height=50> </applet> <hr> |