Hello, World in Java 1.5 on Mac OS X

This document instructs you on how to setup a Java programming environment for your Mac OS X computer and provides a step-by-step guide for creating, compiling, and executing a Java program. You must be using Mac OS X v 10.4.5 Tiger or later. (Previous versions do not support Java 1.5.) All of the software is freely available on the Web.

Java

To check if Apple's implementation of Java 2 Standard Edition (J2SE 5.0) is already installed:

Run Software Update.
Run Applications/Utilities/Java/J2SE 5.0/Java Preferences and verify that the Java Version is J2SE 5.0.

If the J2SE 5.0 folder does not exist:

Download and install J2SE 5.0 Release 6 from Apple.

DrJava

You will develop your Java programs in an application called DrJava. DrJava features many specialized programming tools including syntax highlighting, bracket matching, auto indenting, indent shifting, and line numbering. To install DrJava:

Download the most current stable release from drjava.org by clicking the Download Mac OS X App button.
Double-click the .tar.gz file to extract the DrJava application.
Drag DrJava.app to your Applications folder for permanent storage; drag the .tar.gz files to the trash.
Add the DrJava application to your dock for quick access.

Launch DrJava and make the following customizations.

Display line numbers by selecting Preferences -> Display Options -> Show All Line Numbers.
Set the indentation level to 4 by selecting Preferences -> Miscellaneous -> Indent Level -> 4.

Create the program

Now you are ready to write your first Java program.
- Create a directory, say /Users/yourname/introcs/hello, to store your code. We recommend a separate subdirectory for each programming assignment.
- In the main DrJava window, type the Java program HelloWorld.java exactly as it appears below. If you omit even a semicolon, the program won't work.
```
public class HelloWorld { 
    public static void main(String[] args) { 
        System.out.println("Hello, World");
    }
}
```
- Finally, save the file with Cmd-s. Store in the subdirectory /Users/yourname/introcs/hello and name the file HelloWorld.java. The file name is case sensitive and must exactly match the name of the class in the Java program.
Compile the program

It is now time to convert your Java program into a form more amenable for execution on a computer. To do this, click the Compile button.
If all goes well, you should see the following message in the Compiler Output window at the bottom:
```
Compilation completed.
```
If DrJava complains in some way, you mistyped something, and you should check your program carefully. Use the error messages in the Compiler Output window to guide your search.

Execute the program

Now it is time to run your program. This is the fun part.
- Click the Run button in DrJava. If all goes well, you should see the following message in the Interactions window at the bottom:
```
Welcome to DrJava.  Working directory is /Users/wayne/introcs/hello
> java HelloWorld
Hello, World
```
- You may need to repeat this edit-compile-execute cycle a few times before it all goes smoothly. Ask for help if you can't see the mistake.
Congratulations, you are now a Java programmer!

Troubleshooting

How do I break out of an infinite loop in DrJava? Click the Reset button in the menubar or select the menu option Tools -> Reset Interactions.
How do I use Java from the command-line. Here are instructions for compiling and executing Java from the command-line.
I have Mac OS X v 10.5. What should I do? By default, you should have Java 2 Standard Edition (J2SE 5.0) installed. You can run Applications/Utilities/Java/Java Preferences to check.