Swapping numbers using Call by Value and Call by Reference

import java.util.Scanner;
class Numbers{
    int a, b;
}
class Swapping{
    public static void swap_by_ref(Numbers n){
        System.out.println("[swap_by_ref(), at start]n.a = "+n.a+" and n.b = "+n.b);
        int temp = n.a;
        n.a = n.b;
        n.b = temp;
        System.out.println("[swap_by_ref()], at end]n.a = "+n.a+" and n.b = "+n.b);
    }
    public static void swap_by_val(int a, int b){
        System.out.println("[swap_by_val()], at start]a = "+a+" and b = "+b);
        int temp = a;
        a = b;
        b = temp;
        System.out.println("[swap_by_val()], at start]a = "+a+" and b = "+b);
    }
    public static void main(String args[]){
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter two numbers.");
        Numbers n = new Numbers();
        n.a = sc.nextInt();
        n.b = sc.nextInt();
        int x = n.a;
        int y = n.b;

        System.out.println("\n\n[main(), before swap_by_val()]x = "+x+" and y = "+y);
        swap_by_val(x, y);
        System.out.println("[main(), after swap_by_val()]x = "+x+" and y = "+y+"\n\n");

        System.out.println("[main(), before swap_by_ref()]n.a = "+n.a+" and n.b = "+n.b);
        swap_by_ref(n);
        System.out.println("[main(), after swap_by_ref()]n.a = "+n.a+" and n.b = "+n.b);
    }
}