Java and the Windows Command Prompt

This document instructs you on how to use the Windows XP Command Prompt with Java.

You will use the Java compiler javac to compile your Java programs and the Java interpreter java to run them. You should skip the first step if Java is already installed on your machine.

• Download the Java 2 Platform Standard Edition 5.0 to the Desktop (i.e., Click on Download JDK 5.0 Update 16, check the Accept License Agreementbox, and click Windows Offline Installation, Multi-language). To install, double click the file jdk-1_5_0_16-windows-i586-p.exe. We recommend all of the default options. Note the installation directory for later - probably something like C:\Program Files\Java\jdk1.5.0_16\bin.

   

• To make sure that Windows can find the Java compiler and interpreter:

   

  • Select Start -> Control Panel -> System -> Advanced -> Environment Variables -> System Variables -> PATH.

    [ In Vista, select Start -> Control Panel -> System and Maintenance -> System -> Advanced system settings -> Advanced -> Environment Variables -> PATH. ]

     

  • Prepend C:\Program Files\Java\jdk1.5.0_16\bin; to the beginning of the PATH variable.

     

  • Click OK three times.

  You might need to switch to Classic view instead of Category view to discover all of the options.

You will type commands in an application called the Command Prompt.

• Launch the command prompt via Start -> Run and type cmd in the box. You should see the command prompt; it will look something like:

  Microsoft Windows XP [Version 5.1.2600]
  (C) Copyright 1985-2001 Microsoft Corp.

   

• To check that you have the right version of Java installed, type the text that is shown below. You should see something similar to the information printed below. (It's important that you see the number 1.5, but the rest is not critical.)

  C:\Documents and Settings\wayne>java -version
  java version "1.5.0_16"
  Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_16-b05)
  Java HotSpot(TM) Client VM (build 1.5.0_16-b05, mixed mode, sharing)
  

  Then type:

  C:\Documents and Settings\wayne>javac -version
  javac 1.5.0_16
  javac: no source files
  ...

   

• Since you will be using the Command Prompt frequently, we recommend customizing the default settings. Right-click the title bar of an open Command Prompt window, select Properties and then:

   

  • Set Layout -> Screen Buffer Size to 80 x 500.
     
  • Select Options -> Edit Options -> QuickEdit Mode.
     
  • Select Options -> Edit Options -> Insert Mode.

  Click OK and check Save properties for future windows with same title to make the changes permanent.

   

Now you are ready to write your first Java program.  

• Create a directory, say C:\pi1\hello, to store your code. We recommend a separate subdirectory for each programming assignment.

   

• Using the ASCII editor of your choice, 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 to the directory C:\pi1\hello and name it HelloWorld.java. The file name is case sensitive and must exactly match the name of the class in the Java program.

You will use the javac command to convert your Java program into a form more amenable for execution on a computer.

• From the Command Prompt, navigate to the directory containing your .java files, say C:\introcs\hello, by typing the cd command below.

  C:\Documents and Settings\pi1> cd c:\pi1\hello
  C:\pi1\hello\>
  

• Assuming the file, say HelloWorld.java, is in the current working directory, type the javac command in boldface below to compile it.

  C:\pi1\hello\> javac HelloWorld.java
  C:\pi1\hello\>
  

  If everything went well, you should see no error messages.

You will use the java command to execute your program.

• From the Command Prompt, type the java command below.

  C:\pi1\hello\>java HelloWorld
  Hello, World

  If all goes well, you should see the output of the program - Hello, World.

Here are a few suggestions that might help correct any installation woes you are experiencing. If you need assistance, don't hesitate to contact a staff member.

When I type, "java -version" I get an error. Check that you edited your PATH environment variable as indicated. A missing ; or an added % is enough to screw things up. Close and re-open a command prompt. Type path at the command prompt and look for an entry that includes C:\Program Files\Java\jdk1.5.0_16\bin;. Check that the version number 1.5.0_16 matches the one you installed since Sun updates Java periodically and you might have a more recent version. If this doesn't fix the problem, check if you have any old versions of Java on your system. If so, un-install them and re-install Java.

The command "java -version" works, but not "javac -version". Any thoughts? It's likely a path issue. Try the suggestions from the previous question. Also check that you installed the JDK properly by checking that the folder Program Files\Java\jdk1.5.0_16\bin exists.

How can I check the values of my PATH variable? Type the following at the command prompt.

C:\pi1\hello\> echo %PATH%

I can compile with javac, but I get the error message "Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld" when I try to execute it with java. First, be sure that HelloWorld.class is now in the current directory. Be sure to type java HelloWorld without a trailing .class or .java. Check that the command "java -version" works. Now try to execute with "java -cp . HelloWorld". If this works, you need to edit your classpath. (iTunes has a proclivity for changing the classpath, so if you recently upgraded iTunes, this is likely the source of the problem.). If you have problems, ask the teaching staff for help.

Where can I learn more about the Windows XP command line? Here is a short tutorial on the Windows command prompt. Microsoft maintains a complete command line reference.