Compile main at different class

[HanMing Low]

Hi,
I'd apprecitate if anyone can explain the following compilation rules.
I'm trying to create 2 classes in the same file with a main method in either class.
The class file will only run if the main is in the class or the superclass of the filename.
Is this observation correct? And why is it done this way?
Thanks for any advice.
Cheers.

Han Ming
//filename D.java - run OK

class B {
  public static void main(String args[]) {
    System.out.println("Main");
  }
}
class D extends B {
}

//filename D.java - run OK

class B {
}
class D extends B {
  public static void main(String args[]) {
    System.out.println("Main");
  }
}

//filename B.java - run OK

class B {
  public static void main(String args[]) {
    System.out.println("Main");
  }
}
class D extends B {
}

//filename B.java - run Error :- NoSuchMethodError

class B {
}
class D extends B {
  public static void main(String args[]) {
    System.out.println("Main");
  }
}

[Madan, Gopal]

The class file will only run if the main is in the class or the superclass of the filename.
Is this observation correct?

Yes - your observation is correct

And why is it done this way?

!!??? - You need to ask James Goslig.
You need to start somewhere. The class you give to java must contain a main() method - either in or inherited. - This was the case in your first 3 cases. This is just like in OLD "C" programs, where the execution starts in the main(argv, argc) method.
This was NOT the case in your 4th example. Hence "NoSuchMethod" Error

[HanMing Low]

Hi,
Thanks for the explanation.
I think your explanation is easier to remember.

!!??? - You need to ask James Goslig.

Thanks again.
Cheers.

Han Ming