Java Hello World Example: How To Write and Execute Java Program on Unix OS

http://www.thegeekstuff.com/2010/02/java-hello-world-example-how-to-write-and-execute-java-program-on-unix-os/

Question: I would like to understand the basics of how to write, compile and execute a Java program on UNIX / Linux OS. Can you explain it with a simple example?

Answer: In this article, let us review very quickly how to write a basic Hello World Java program and how to compile *.java program on Linux or Unix OS.

1. Write a Hello World Java Program

Create the helloworld.java program using a Vim editor as shown below.

$ vim helloworld.java
/* Hello World Java Program */
class helloworld  {
    public static void main(String[] args) {
        System.out.println("Hello World!");
    }
}

2. Make sure Java Compiler (javac) is installed on your system

Make sure javac is installed on your system as shown below.

$ dpkg -l | grep java
ii  java-common                                0.30ubuntu5                                Base of all Java packages
ii  sun-java6-bin                              6-16-0ubuntu1.9.04                         Sun Java(TM) Runtime Environment (JRE) 6 (ar
ii  sun-java6-jdk                              6-16-0ubuntu1.9.04                         Sun Java(TM) Development Kit (JDK) 6
ii  sun-java6-jre                              6-16-0ubuntu1.9.04                         Sun Java(TM) Runtime Environment (JRE) 6 (ar
ii  sun-java6-plugin                           6-16-0ubuntu1.9.04                         The Java(TM) Plug-in, Java SE 6

$ whereis javac
javac: /usr/bin/javac /usr/share/man/man1/javac.1.gz

$ which javac
/usr/bin/javac

3. Compile the helloworld.java Program

Compile the helloworld.java using javac command as shown below. This will create the helloworld.class file.

$ javac helloworld.java

$ ls -ltr
total 1760
-rw-r--r-- 1 ramesh ramesh     149 2010-01-23 09:51 helloworld.java
-rw-r--r-- 1 ramesh ramesh     426 2010-01-23 09:52 helloworld.class

4. Execute the Java Class Program (helloworld.class)

Execute helloworld.class as shown below.

$ java helloworld
Hello World!

5. If you code give something like the following, it means your $CLASSPATH may be assigned to some other value.  Then just run: export CLASSPATH=$CLASSPATH:`pwd`; re-run, it should be fixed.


Exception in thread "main" java.lang.NoClassDefFoundError: helloworld
Caused by: java.lang.ClassNotFoundException: helloworld
at java.net.URLClassLoader$1.run(URLClassLoader.java:217)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:294)
at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
Could not find the main class: helloworld. Program will exit.