Hello World in Java on Linux

Beta version of instructions.

This document instructs you on how to setup our Java programming environment under Linux. It also provides a step-by-step guide for creating, compiling, and executing your first Java program using either DrJava or the command line. We assume some familiarity with the command line. All of the software is freely available on the web.

0. Install the Programming Environment

You will use the Java Platform, Standard Edition Development Kit (JDK 6) and DrJava.

Log in to the user account in which you will be programming. Your account must have Administrator privileges and you must be connected to the Internet.
Launch your shell. We'll assume that the command prompt looks like the following (those yours will likely differ):
[username:~/]
The ~/ is shorthand for your home directory.

Create a directory ~/introcs and a subdirectory ~/introcs/bin.

[username:~/] mkdir introcs
[username:~/] cd introcs
[username:~/introcs/] mkdir bin

You will use Oracle's implementation of the Java Platform, Standard Edition Development Kit (JDK 6). Most Linux distributions provide their own mechanism for installing software. For example, on Gentoo, type:
[username:~/introcs/] sudo emerge --sync [username:~/introcs/] sudo emerge sun-jdk
If you use another distribution, use that distribution's package manager. (see the first Q+A under Troubleshooting). If it doesn't come with a package manager, install Java manually (see the second Q+A).

Download DrJava from drjava.jar to ~/introcs.

[username:~/introcs/] wget http://introcs.cs.princeton.edu/java/linux/drjava.jar
[username:~/introcs/] wget http://introcs.cs.princeton.edu/java/linux/drjava
[username:~/introcs/] chmod 700 drjava
[username:~/introcs/] mv drjava bin

The command wget downloads files from the web; if you don't have wget, use curl -O instead.

Download the textbook libraries stdlib.jar and algs4.jar to ~/introcs.

[username:~/introcs/] wget http://introcs.cs.princeton.edu/java/stdlib/stdlib.jar
[username:~/introcs/] wget http://introcs.cs.princeton.edu/java/stdlib/algs4.jar

Download Checkstyle 5.4 and Findbugs 1.3.9 from checkstyle.zip and findbugs.zip to ~/introcs. Unzip checkstyle.zip and findbugs.zip. Downloads our checkstyle and findbugs configuration files from checkstyle.xml and findbugs.xml to ~/introcs. Download the checkstyle and findbugs execution scripts from checkstyle.sh and findbugs.sh to ~/introcs/bin and set the two files to be executable.

[username:~/introcs/] wget http://introcs.cs.princeton.edu/java/linux/checkstyle.zip
[username:~/introcs/] wget http://introcs.cs.princeton.edu/java/linux/findbugs.zip
[username:~/introcs/] unzip checkstyle.zip
[username:~/introcs/] unzip findbugs.zip
[username:~/introcs/] wget http://introcs.cs.princeton.edu/java/linux/checkstyle.xml
[username:~/introcs/] wget http://introcs.cs.princeton.edu/java/linux/findbugs.xml
[username:~/introcs/] wget http://introcs.cs.princeton.edu/java/linux/checkstyle
[username:~/introcs/] wget http://introcs.cs.princeton.edu/java/linux/findbugs
[username:~/introcs/] chmod 700 checkstyle findbugs
[username:~/introcs/] mv checkstyle findbugs bin
[username:~/introcs/] mv checkstyle.xml checkstyle-5.4
[username:~/introcs/] mv findbugs.xml findbugs-1.3.9

Add the textbook standard libraries to the CLASSPATH environment variable and add checkstyle and findbugs to your PATH environment variable. The instructions depend on which shell you use.
- Bourne shell (sh), Bourne-again shell (bash), K-shell (ksh), or Z-shell (zsh). Download config.sh to ~/introcs/bin.
  [username:~/introcs/] wget http://introcs.cs.princeton.edu/java/linux/config.sh [username:~/introcs/] mv config.sh bin
  Add the following line to the end of either ~/.bash_profile or ~/.profile, depending on which one exists:
  test -r ~/introcs/bin/config.sh && source ~/introcs/bin/config.sh
- C-shell (csh) or T-shell (tcsh). Download config.csh to ~/introcs/bin.
  [username:~/introcs/] wget http://introcs.cs.princeton.edu/java/linux/config.csh [username:~/introcs/] mv config.csh bin
  Add the following line to the end of either ~/.tcshrc or ~/.cshrc, depending on which one exists:
  test -r ~/introcs/bin/config.csh && source ~/introcs/bin/config.csh

1. Create the Program in DrJava

Now you are ready to write your first Java program. You will develop your Java programs in an application called DrJava. DrJava features many specialized programming tools including syntax highlighting, bracket matching, auto indenting, and line numbering.

If you use a file manager such as Konqueror or Nautilus, you can launch DrJava by double-clicking the drjava.jar file. Otherwise, launch DrJava from the command line by typing:
[username:~/introcs/] drjava
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.
- Set the Java classpath by selecting Edit -> Preferences -> Resources -> Extra Classpath -> Add and adding the following two entries:
  ~/introcs/stdlib.jar
  ~/introcs/algs4.jar
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"); } }
As you type, DrJava does the indenting for you.
Finally, click the Save button to save the file. Use DrJava to create the directory ~/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.

2. Compile the Program from DrJava

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 pane at the bottom:

Compilation completed.

If DrJava complains in some way, you mistyped something. Check your program carefully, using the error messages in the Compiler Output pane as a guide.

3. Execute the Program from DrJava

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

Type the following in the Interactions pane at the bottom. By convention, we highlight the text you type in bold.
> java HelloWorld
If all goes well, you should see the following message:
Welcome to DrJava. Working directory is /Users/username/introcs/hello > java HelloWorld Hello, World
You may need to repeat this edit-compile-execute cycle a few times before it works.

4. Command-Line Interface

The command-line provides capabilities beyond those available in DrJava, including redirection and piping. You will type commands in an application called the shell.

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 Bourne-Again shell (bash), Bourne shell (sh), and C shell (csh).

To confirm that the Java compiler is installed, type the command in boldface below and check that the results match:
[username:~/] javac -version javac 1.6.0_26
It's important that you see the number 1.6 or 1.5 for the Java version number, but the rest is not critical.
To confirm that the Java interpreter is installed, type the command in boldface below and check that the results match:
[username:~/] java -version java version "1.6.0_26" Java(TM) SE Runtime Environment (build 1.6.0_26-b03-383-11A511) Java HotSpot(TM) 64-Bit Server VM (build 20.1-b02-383, mixed mode)
Again, it's important that you see the number 1.6 or 1.5 for the Java version number, but the rest is not critical.

5. Compile the Program from the Shell

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

From the shell, navigate to the directory containing HelloWorld.java, say ~/introcs/hello, by typing the cd (change directory) commands below:
[username:~/] cd introcs [username:~/introcs/] cd hello [username:~/introcs/hello/]
Compile it by typing the javac command below:
[username:~/introcs/hello/] javac HelloWorld.java [username:~/introcs/hello/]
Assuming the file HelloWorld.java is in the current working directory, you should see no error messages.

6. Execute the Program from the Shell

You will use the java command to execute your program.

From the shell, type the java command below.
[username:~/introcs/hello/] java HelloWorld Hello, World
You should see the output of the program.

7. Checkstyle and Findbugs

You can use Checkstyle and Findbugs to check the style of your programs and identify common bugs.

To run Checkstyle, type the following command in the Terminal:
machine:~/introcs/hello username$ checkstyle HelloWorld.java Running checkstyle on HelloWorld.java: Starting audit... Audit done.
Here is a list of available checks. You can customize the settings by editing the file /Users/username/introcs/checkstyle-5.4/checkstyle.xml.
To run Findbugs, type the following command in the Terminal:
machine:~/introcs/hello username$ findbugs HelloWorld.class Running findbugs on HelloWorld.class:
Here is a list of bug descriptions. You can customize the settings by editing the file /Users/username/introcs/findbugs-1.3.9/findbugs.xml.

Troubleshooting

My distribution of Linux is { Gentoo, Debian, Ubuntu, Fedora, Red Hat, SuSE, Mandriva, or Slackware }. How should I modify the instructions? We haven't tested out these instructions on all flavors of Linux, but the instructions should be identical except for installing Java. We recommend using your distribution's pacakge manager (such as portage, apt, or yum) to install Java.

How do I manually install Java?

Download Java SE 6 Update 27.
- Click on Download JDK button next to Java SE 6 Update 27. Be sure to choose the JDK, not the JRE. Also, be sure to choose Java SE 6, not Java SE 7. (You don't need the Java EE or NetBeans bundles.)
- Accept the Oracle Binary Code License Agreemment for Java SE.
- If you have root access, choose the Linux x64 - RPM Installer product by clicking the link jdk-6u27-linux-x64-rpm.bin; if not, choose the Linux x64 - Self Extracting Installer product by clicking the link jdk-6u27-linux-x64.bin.
Linux RPM installation. Run the installation program by typing
[username:~/] sh jdk-1_6_0_18-linux-i586-rpm.bin [username:~/] rpm -iv jdk-1_6_0_18-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_18-linux-i586.bin

Why Java 6 instead of Java 7? DrJava does not currently support Java 7.

Can I use a different version of Java? Yes, any version of Java 5 or Java 6 should work fine.

Can I update DrJava to use a newer version? Yes, but you do not need to do so.

I had to manually enter the location of tools.jar in DrJava, but it doesn't seem to have any effect. Any suggestions? This setting doesn't take effect until you restart DrJava.

Can I use a different IDE? Yes you can use another IDE (such as Eclipse) but you will have to configure the IDE properties yourself (such as the classpath).

How do I determine which shell I'm running? Type the following command:

[username:~/] echo $SHELL
bash

You shell will likey be bash, tcsh, sh, ksh or zsh.

When I compile or execute a program from the shell that uses one of the textbook libraries, I get an error. How can I fix this? First, verify that you have the files ~/introcs/stdlib.jar and ~/introcs/algs4.jar. If so, it is an issue with the CLASSPATH environment variable. From the shell, type the following command to display it:

[username:~/] echo $CLASSPATH

The CLASSPATH environment variable should include entries for ~/introcs/stdlib.jar and ~/introcs/algs4.jar.

How do I break out of an infinite loop? From DrJava, click the Reset button in the menubar or select the menu option Tools -> Reset Interactions; from the shell, type Ctrl-c.

When using standard input, how do I signify that there is no more data? If you are entering input from the keyboard, type Ctrl-d for EOF (end of file) from either DrJava or the shell.