Hello, World in Java on Mac OS X

This documents instructs you on how to setup a Java programming environment for your Mac OS X computer and provides a step-by-step guide to creating, compiling, and executing a Java program. We assume you are running at least OS X 10.2.3. 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 implementation of Java 2 Standard Edition (J2SE) 1.4.1, so there is nothing to do in this step.

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 command prompt that looks something like:

  [username:] ~> 

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

  [username:] ~> java -version

  If Java is installed, you should see something like:

  java version "1.4.1_01"
  Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.1_01-69.1)
  Java HotSpot(TM) Client VM (build 1.4.1_01-24, mixed mode)
  

  Now, check that the Java compiler is accessible.

  [username:] ~> which javac

  If it is installed, you should see something like

  /usr/bin/javac

  If not, install the Developers Tools CD that came with your Mac and use Software Update to make sure you have the latest version.

• Now, create a directory to store your Java programs. In the window containing the Command Prompt, type the bold commands below:

  [username:] ~> mkdir introcs
  [username:] ~> cd introcs
  [username:] ~/introcs> mkdir hello
  [username:] ~/introcs> cd hello
  

  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 ~username/introcs/hello/. All of your files for Assignment 0 will go here. Don't be scared 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, we recommend customizing We recommend customizing the default settings....

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. It's even written in Java.

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

• We recommend the following JEdit customizations.

  • Enable Mac OS style menubar in Utilities -> Global Options -> Plugins -> Mac OS Settings

  • Change the default indentation to four spaces via Utilities -> Global Options -> Editing -> Indent Width and Utilities -> Global Options -> Editing -> Tab Width.

  • 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 -> Loading & Saving -> 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 (or click then shift-right click it if using Windows NT)

  • 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.

• In the JEdit window, type the Java program 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");
     }
  }
  

• When you're done, save the program using File -> Save As, chose the directory C:\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 thefile name.

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

• From the Command Prompt, type

  [username:] ~/introcs/hello> javac HelloWorld.java

  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.

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

  [username:] ~/introcs/hello> javac HelloWorld.java
  [username:] ~/introcs/hello> 
  

  Silence is golden in computer science.

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

• At the Command Prompt, type

  [username:] ~/introcs/hello> java HelloWorld

• If all goes well, you should see

  [username:] ~/introcs/hello> 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 Your CLASSPATH may have been set by some other program so that it no longer includes the current working directory in the search path. Try running your program with the command line

[username:] ~/introcs/hello> java -classpath ./ HelloWorld