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 10.4.2 Tiger or later. (Previous versions do not support Java 1.5.) All of the software is freely available on the Web.

You will use the Java compiler javac to compile your Java programs and the Java interpreter java to run them. Mac OS X includes Apple's implementation of Java 2 Standard Edition (J2SE) 1.5.0. so there is nothing to do in this step except to run Software Update if you're not up-to-date.

You will type commands in an application called the Terminal. You might enjoy reading Neal Stephenson's light-hearted essay In the Beginning was the Command Line.

• Open a terminal window. You can find this under Go -> Applications -> Utilities. Drag the Terminal to your dock since you will be using it frequently.

• You should now have a Terminal window somewhere on the screen. It will have a prompt that looks something like:

  machine:~ username$

• To check that you have the right version of Java installed, type the commands in boldface below.

  machine:~ username$ java -version

  If Java is installed, you should see something like:

  java version "1.5.0_06"
  Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_06-112)
  Java HotSpot(TM) Client VM (build 1.5.0_06-64, mixed mode, sharing)
  

  Then type

  machine:~ username$ javac -version

  You should see something like:

  javac 1.5.0_06
  javac: no source files
  ...
  

• Now, create a directory to store your Java programs. In the Terminal, type the commands in boldface below:

  machine:~ username$ mkdir introcs
  machine:~ username$ cd introcs
  machine:~/introcs username$ mkdir hello
  machine:~/introcs username$ cd hello
  machine:~/introcs/hello username$
  

  The mkdir command creates a new directory; the cd command changes the current working directory. After executing these commands, your working directory is the newly created ~/introcs/hello. All of your files for Assignment 0 will go here. Don't be intimidated by the Terminal - you will only need to use a few basic commands. Keep this window open since you will need it later in the assignment.

• Since you will be using the Terminal frequently, you may want to customize the default window settings (e.g., Monaco 13pt font with antialiasing).

You will type and edit your programs in a text editor called JEdit. JEdit is similar to conventional word processors like MS Word, but it features many specialized programming tools including syntax highlighting, bracket matching, auto indenting, indent shifting, line numbering, and commenting out code.

• Download the latest stable version of JEdit using the MacOSX package link.

• We strongly recommend the following JEdit customizations.

  • Enable Mac OS style menubar in Plugins -> Plugin Options -> Enable Mac OS Menubar

  • Change the default indentation to 4 spaces via Utilities -> Global Options -> Editing -> Indent Width, Utilities -> Global Options -> Editing -> Tab Width, and Utilities -> Global Options -> Editing -> Soft (emulated with spaces) tabs.

  • Change the default edit mode to Java using Utilities -> Global Options -> Editing -> Default edit mode -> Java.

  • Change the default tab width when printing to four spaces via Utilities -> Global Options -> Printing -> Tab width when printing

  • Add line numbers by checking Utilities -> Global Options -> Gutter -> Line Numbering

  • Allow at most 80 characters per line by Utilities -> Global Options -> Editing -> Word wrap -> hard and Utilities -> Global Options -> Editing -> Wrap margin -> 80.

  • Change the default line separator to Unix via Utilities -> Global Options -> General -> Default Line Separator

  • Remove the annoying . that marks the end of a line by unchecking Utilities -> Global Options -> Text Area -> End of Line Markers

• To associate .java files with JEdit so that when you double-click a .java file, it opens it in JEdit:

  • Right click a .java file.

  • Choose Open With...

  • Select JEdit from the list of programs or locate JEdit by selecting Other if necessary.

  • Check the Always use this program to open this file type box

Now you are ready to write your first Java program.

• Launch JEdit Start -> jEdit. Begin by saving the empty document: Select File -> Save As, choose the directory /Users/username/introcs/hello and type in the file name HelloWorld.java. The file name is case sensitive and must exactly match the name of the class in the Java program. Don't forget to include the .java extension in the file name. By doing this, JEdit will know you are working on a Java file, and it will format it appropriately.

• Now, in the JEdit 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 using File -> Save.

It is now time to convert your Java program into a form more amenable for execution on a computer.

• From the Terminal, type

  machine:~/introcs/hello username$ javac HelloWorld.java

• If everything went well, you should see the following in the Terminal:

  machine:~/introcs/hello username$

  Silence is golden in computer science. If javac complains in some way, you mistyped something, and you should check your program carefully. Ask for help if you can't see the mistake.

Now it is time to run your program. This is the fun part.

• At the Terminal, type

  machine:~/introcs/hello username$ java HelloWorld

• If all goes well, you should see

  machine:~/introcs/hello username$ java HelloWorld
  Hello, World

• You may need to repeat this edit-compile-execute cycle a few times before it all goes smoothly. Use the up and down arrow keys to repeat previous commands and avoid extra typing.

When I try to run java I get: Exception in thread "main" java.lang.NoClassDefFoundError. First, be sure that HelloWorld.class is in the current directory. Be sure to type java HelloWorld without a trailing .class or .java. If this was not your problem, it's possible that your CLASSPATH was set by some other program so that it no longer includes the current working directory. Try running your program with the command line

machine:~/introcs/hello username$ java -cp ./ HelloWorld