Why do I need a class here?

Question

class Myclass{
    int x;
    Myclass(int i){
      x = i;
   }
}

    class UseMyclass { //Why do I need another class?
     public static void main (String args[]){
     Myclass y = new Myclass(10);
     System.out.println(y.x);
   }
}

Why can't I run main() out of Myclass? From the book it says I would be running the UseMyclass so I guess that would be my file name. Why couldn't I just use the Myclass though as the file name and run main() in there? I'm new to programming so I'm just trying to figure stuff out.

Answer 1

You don't actually need another class. If you just put the main method into the class, it will work. For example, this code will work just fine:

class Myclass{
   int x;
   Myclass(int i){
      x = i;
   }
   public static void main (String args[]){
     Myclass y = new Myclass(10);
     System.out.println(y.x);
   }
}

However, separating the main class is a good idea when you are dealing with large programs with many classes. Then, you can sneak unit tests into main methods in other classes.