Question

Running basic java programs from commands line is a 3 steps process:

1. Write code:

   public class HelloWorld {
   public static void main(String[] args) {
   System.out.println(“Hello, World”);
   }
   }

2. Compile by javac HellWorld.java which would check for errors & generate HelloWorld.class file.

3. run code by giving the class name –> java HelloWorld

Now,
I am curious to know why:

java HelloWorld works but when we give fullpath of the classfile, it throws an error

$ java HelloWorld.class 
Error: Could not find or load main class HelloWorld.class

What does it make a difference if we give just the classname Vs classname with file-extension?

Answer 1

What does it make a difference if we give just the classname Vs classname with file-extension?

The argument you give to the java binary isn’t meant to be a filename. It’s meant to be a class name. So in particular, if you’re trying to start a class called Baz in package foo.bar you would run:

java foo.bar.Baz

So similarly, if you try to run java HelloWorld.class it’s as if you’re trying to run a class called class in a package HelloWorld, which is incorrect.

Basically, you shouldn’t view the argument as a filename – you should view it as a fully-qualified class name. Heck there may not even be a simple Baz.class file on the file system – it may be hidden away within a jar file.