Java application fails to start

I am currently using Ubuntu 11.10 and java SE 1.6.0_26. I am trying to run a very simple "Hello World" app. I placed the the java file `HelloWorld.java` on the Home folder. I compiled it using the command `javac HelloWorld.java`. I think its working because it doesn't show any compilation error and a `HelloWorld.class` is created. When I typed the command `java HelloWorld` I have this error: ``` Exception in thread "main" java.lang.NoClassDefFoundError: HelloWorld Caused by: java.lang.ClassNotFoundException: HelloWorld at java.net.URLClassLoader$1.run(URLClassLoader.java:202) at java.security.AccessController.doPrivileged(Native Method) at java.net.URLClassLoader.findClass(URLClassLoader.java:190) at java.lang.ClassLoader.loadClass(ClassLoader.java:306) at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301) at java.lang.ClassLoader.loadClass(ClassLoader.java:247) Could not find the main class: HelloWorld. Program will exit. ``` BTW, here is my java code: ``` public class HelloWorld { public static void main (String args[]) { System.out.println("Hello World!!!"); } } ```