Hello, World in Java on Windows

This document instructs you on how to setup a Java programming environment for your Windows XP computer and provides a step-by-step guide for creating, compiling, and executing a Java program. (Other flavors of Windows are similar, but but you might need to make some minor adjustments.) All of the software is freely available on the Web.

Java

You will use the Java compiler javac to compile your Java programs and the Java interpreter java to run them.

Download the Java 2 Platform Standard Edition 5.0 to the Desktop (i.e., Click on Download JDK 5.0 Update, check the Accept License Agreement box, and click Windows Offline Installation, Multi-language). Be sure to choose the JDK, not the JRE or J2EE. You don't need the NetBeans IDE cobundle. To install, double click the file jdk-1_5_0_06-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_06\bin. While it's installing, you might enjoy reading Neal Stephenson's light-hearted essay In the Beginning was the Command Line.
To make sure that Windows can find the Java compiler and interpreter,
- Select Start -> Control Panel -> System -> Advanced -> Environment Variables -> System Variables -> Path.
- Prepend C:\Program Files\Java\jdk1.5.0_06\bin; to the beginning of the Path variable.
- Cllick OK three times.
In Windows XP, you might need to switch to classic view instead of category view to discover all of the options.

Command Line Interface

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 which looks 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 in boldface 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_06"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_06-b05)
Java HotSpot(TM) Client VM (build 1.5.0_06-b05, mixed mode, sharing)

Then type Then type

C:\Documents and Settings\wayne>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 window containing the Command Prompt, type the bold commands below:
```
C:\Documents and Settings\wayne>cd c:\
C:\>mkdir introcs
C:\>cd introcs
C:\introcs>mkdir hello
C:\introcs>cd hello
C:\introcs\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 C:\introcs\hello. All of your files for Assignment 0 will go here. Don't be intimidated by the Command Prompt - 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 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.

Text Editor

You will type and edit your programs in a text editor called JEdit. JEdit is similar to conventional word processors like MS Word and Notepad, 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 Windows installer, and install it using the default options.
We strongly recommend the following JEdit customizations.
- 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

Create the Program

Now you are ready to write your first Java program.

Launch JEdit Start -> jEdit.
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");
   }
}
```
When you're done, save the program using File -> Save As, choose 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.
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

Compile the Program

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

From the Command Prompt, type
```
C:\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:
```
C:\introcs\hello\>javac HelloWorld.java
C:\introcs\hello\>
```
Silence is golden in computer science.

Execute the Program

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

At the Command Prompt, type
```
C:\introcs\hello\>java HelloWorld
```

If all goes well, you should see

C:\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.

Congratulations, you are now a Java programmer! To continue, return to to the Introduction to Computer Science booksite.

Troubleshooting

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.

I can't install JEdit. When I double-click the .jar file, it's not associated with any programs. What can I do? First, make sure that Java 1.5 is installed. Then, when it prompts you for an application to associate to .jar files, select "Java". Alternatively, from the command prompt, cd to the directory containing .jar file and execute the program as follows:

cd C:\Downloads
java -jar jedit42install.jar

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_06\bin;. Check that the version number 1.5.0_06 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_06\bin exists.

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

C:\introcs\hello\> echo %PATH%
C:\introcs\hello\> echo %CLASSPATH%

The PATH variable should begin with C:\Program Files\Java\jdk1.5.0_06\bin;. The CLASSPATH variable should either be empty (%CLASSPATH%) or it should begin with .;.

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 have an environment variable (system or user) set for the CLASSPATH. If so, prepend .; to the beginning of your CLASSPATH variable using the same method that you used to edit your PATH variable. (iTunes has a proclivity for changing the CLASSPATH variable, so if you recently upgraded iTunes, this is likely the source of the problem.)

I followed the instructions above for associating .java files with JEdit, but it doesn't work on my system. How can I fix it? Do the following steps.

Open an Explorer window and click Tools -> Folder Options
Go to the File Types tab and select the JAVA extension.
Click Advanced.
Select open from the Actions selection box, then click edit.
In the box labeled Application used to perform action type the following, adapted for your appropriate path names:
```
"C:\Program Files\Java\jdk1.5.0_06\bin\javaw.exe" -jar "C:\Program Files\jEdit\jedit.jar" %1
```

If you get a Restore button instead of Advanced, click it and it becomes Advanced.