public class HelloWorld
   {
   public static void main ( String [] args )
      {
      System.out.println( "Hello World" );
      } // end main
   } // end class HelloWorld

javac.exe -classpath . HelloWorld.java

java.exe -classpath . HelloWorld

1. The name of the source file must be HelloWorld.java, precisely, and the name of the public class in your source must be HelloWorld, precisely, including case. If you use notepad, you may have inadvertently named your file hellowworld.txt or helloworld.java.txt.
2. You specify the .java extension to compile, but leave the .class extension off to execute. How logical!
3. You may not specify the fully qualified drive and path: e.g.

   java.exe -classpath . C:\TEMP\HelloWorld

   is not permitted.
4. As a corollary, make sure the current directory is where the HelloWorld.class file is.
5. You have to get the case exactly correct. e.g.

   java.exe -classpath . Helloworld

   is not permitted.
6. If your HelloWorld is an Applet rather than an application, you need some HTML commands to invoke it. You don’t just load it in your browser as if it were a web page.
   Applet for more details on composing that HTML
7. Did you remember to make your class public?
8. Did you remember to make your main method public and static? It must have a signature exactly like this:

   public static void main ( String [] args )

9. If you used the abbreviation javac instead of the fully qualified name such as javac.exe, check that the javac.exe you wanted is on the classpath and is the only javac.* on the classpath.