Hello, World in Java 1.5 on Linux

This document instructs you on how to setup a Java programming environment under the Gentoo flavor of Linux, and provides a step-by-step guide for creating, compiling, and executing a Java program. Other Linux distributions (e.g., Ubuntu, Fedora, Red Hat, SuSE, Mandriva, Gentoo, Slackware, etc.) should be similar. All of the software is freely available on the web.

Java /td>

You will use Sun's implementation of Java 2 Standard Edition (J2SE) 5.0. Most Linux distributions provide their own mechanism for installing software. For example, on Gentoo, type

[username:~/] sudo emerge --sync
[username:~/] sudo emerge sun-jdk

If you use another distribution, consult the documentation for instructions on how to install it. If it doesn't come with a package manager, see the first Q+A under troubleshooting.

Command Line Interface

You will type commands in an application known as the shell. Since you're using Linux, we assume you're somewhat familiar with it (as you used it in the previous step!).

To configure Java, you will need to know which shell you are running. In case you don't know, type the following command:
```
[username:~/] echo $SHELL
```
Your shell will likely be bash, tcsh, sh, or ksh.
To make sure Linux can find the Java compiler and interpreter, edit your shell login file. If you are using the bash shell, append the following line to your ~username/.bashrc file. If it doesn't exist, create a new file.
```
export PATH=$PATH:~/j2sdk1.5.0.14/bin
```
Then close your shell and open up a new one. If it does not work, in addition, try repeating the same instructions with the file ~username/.bash_profile.
Now, create a directory to store your Java programs. At the command line, type the bold commands below:
```
[username:~/] mkdir introcs
[username:~/] cd introcs
[username:~/] mkdir hello
[username:~/introcs] cd hello
[username:~/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 ~username/introcs/hello. All of your files for Assignment 0 will go here.

If you plan to take COS 217, you might want to buy the required book Programming with GNU Software by Loukides and Oram. It contains an overview of Unix from the user's point of view. It also describes shell fundamentals, with reference to the bash, Bourne, and C shells.

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 current stable release from drjava.org using the Download Jar File button.
If you use a file manager such as Konqueror or Nautilus, you can also launch DrJava by double-clicking the .jar file.
Otherwise, you can also launch DrJava from the command-line via
```
[username:~/] java -jar drjava-stable-20080106-0744.jar
```
Make the following customizations.
- Display line numbers by selecting Edit -> Preferences -> Display Options -> Show All Line Numbers.
- Set the indentation level to 4 by selecting Edit -> Preferences -> Miscellaneous -> Indent Level -> 4.

Create the program

Now you are ready to write your first Java program.

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 Ctrl-s. Store in the subdirectory ~username/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 ~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!

Command Line Alternative

You can also compile and execute your Java program directly from the shell. Set your working directory to the location of your .java file.

To compile, type:

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

To execute, type:

[username:~/introcs/hello] java HelloWorld

Troubleshooting

My Linux distribution doesn't come with a package manager. Can I install Java manually? Yes, download the Java 2 Platform Standard Edition 5.0 to the Desktop:

Click on Download button next to JDK 5.0 Update 14. Be sure to choose the JDK, not the JRE. (You don't need the NetBeans or Java EE cobundles.)
Check the Accept License Agreement box.
Click Linux RPM in self-extracting file link (if you have root access) or the Linux self-extracting file (if you don't) under Linux Platform - Java Development Kit 5.0 Update 14. (Don't use the Sun Download Manager.)
Once it is download, install it using the directions below.
- Linux RPM installation. Run the installation program by typing
```
[username:~/] sh jdk-1_5_0_14-linux-i586-rpm.bin
[username:~/] rpm -iv jdk-1_5_0_14-linux-i586.rpm
```
  If dependencies are missing and the RPM package cannot install, you either need to download the dependencies or use the single-user installation.
- Linux single user installation. Run the installation program by typing
```
[username:~/] sh jdk-1_5_0_14-linux-i586.bin
```

How can I check which version of Java I'm using? Type the following two commands.

[username:~/] java -version
java version "1.5.0_14"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_14-112)
Java HotSpot(TM) Client VM (build 1.5.0_14-64, mixed mode, sharing)

[username:~/] javac -version
javac 1.5.0_14
javac: no source files

When I try to run java I get: Exception in thread "main" java.lang.NoClassDefFoundError. First, be sure that HelloWorld.class is now 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

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

How do I permanently set the CLASSPATH? Put the following line in your .bashrc file (or an analogous line if you are using another shell).

export CLASSPATH=.:/path/to/look/in:$CLASSPATH

To check your CLASSPATH variable, type echo $CLASSPATH.

How do I set the path with the tcsh shell? Append the following line to your ~username/.tcshrc file:

setenv PATH ~/j2sdk1.5.0.14/bin:$PATH

How do I configure vi to replace tabs with four spaces? Edit your .exrc file and add the commands ":set ts=4" to display the tabs as four spaces and ":set expandtab" to translate the tabs into spaces.

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.