Java Classes

Many new Java programmer seem to encounter difficulty when they first start using packages. It's actually quite straightforward once you understand the basics. Let's begin with a discussion of environment variables.

Environment variables

JAVA_HOME

The JAVA_HOME environment variable should point to the directory in which Java has been installed. Depending on the version of Java you're using, this variable is used to locate the run-time environment and standard libraries. You'll sometimes hear (especially on the comp.lang.java.programmer newsgroup) that you shouldn't need to set this environment variable. While that might be true in production environments, it's not the case when you're developing and testing your code.

Here's the typical HelloWorld application.

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

Assuming that you've successfully installed the JDK (note that the JRE doesn't include the Java compiler) then your session will look something like this:

$ javac HelloWorld.java
$ java HelloWorld
Hello world!
$

There are a couple of things worth noting here. First of all, the example assumes that the current directory is included in the CLASSPATH. That's the essence of "visibility".