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.

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

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.

Now you are ready to write your first Java program.

 

• 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 to the subdirectory ~username/introcs/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.

It is now time to convert your Java program into a form more amenable for execution on a computer. To do this, enter the following command at your shell prompt:

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

If the Java compiler complains in some way, you mistyped something, and you should check your program carefully. Use the error messages to guide your search.

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

 

• To execute, type:

  [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. Ask for help if you can't see the mistake.

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

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.