Why below code gives compiler error :"main method not found in class a"?

Question

import java.io.*;
class a{}
class b{
    public static void main (String[] args) {
        System.out.println("Geeks..");
    }
}

If in above snippet,

1. I make class b as public, it gives compiler error:

   class b is public, should be declared in a file named b.java

   Why it then doesn't expect main in class a after making b as public?

2. I make class a as public,it again gives : main not found in class A.

3. If I make both classes public, it gives: class b is public, should be declared in a file named b.java

4. If I change it to below:

   public class b{
       public static void main (String[] args) {
           System.out.println("Geeks..");
       }
   }
   class a{}
   

It gives proper output: Geeks..

Please clarify the relation between order of classes, public and non public class, main method and file name.