Basic Java Commands

Getting Started

Java is in /usr/local/java/bin; make sure that this directory is in your load path. Use javac to compile java programs as in

        prompt% javac Hello.java

The compiler produces files with the extension .class, which is the object code for each of the classes defined within the file.

If you've written an application, you can run it using java; as in:

        prompt% java Hello
        Hello World!

for the classic first program.

If you've written an applet to be run through a browser, you can run it using a viewer, appletviewer as in:

        prompt% appletviewer Hello.html

which will bring up a separate window.

Simple Example Programs

Hello World as Application

All the code is in a single file called Hello.java:

        // standard first program
        // runs via terminal i/o

        class Hello {
          public static void main (String[] args)
          {
            System.out.println("Hello World!");
          }
        }

Hello World as Applet

For this we need two files: one for the java code and one to define the html page from which to run it. The java code looks like:

        // standard first program
        // run as an applet
        import java.applet.Applet;
        import java.awt.*;
        
        public class HelloWorld extends Applet {
          public void paint (Graphics g)
          {
             g.drawString("Hello World!", 25, 25);
          }
        }

In this case, we need to inherit the Applet class for our class definition. Additionally, we overwrite a method from the class, paint, to print what we want.

We also need the companion html file, as shown in your notes to define the size of the window to open and designates the java code to be run within it.