Question

Why Class B can’t become public?
How can I use class be in other classes? Is it better to define it inside Cons?!

   //    public class B { why not?

   class B {
        int x;  
        B (int n) {
            x=n;
            System.out.println("constructor 'B (int n)' called!");
        }
    }

    public class Cons { 
        public static void main(String[] args) {B b = new B();}
    }

Answer 1

As per java language specification, there can be only one public class in a file (.java) and file name should be same as public class name.

If you want class B accessible in other placs, you may create a separate B.java file and move your Class B code to that file.

This thread may give you some more information.