Object reference or Copy Constructor using Java

A class that
contains a method which will have parameter of type class itself (object
reference), How to pass value to that method from main.

Program:

class Rectangle

 {

  double l,b;

  Rectangle(double x,double y)

   {

    l=x;

    b=y;

   }

  double RecArea(Rectangle ob)

    {

     return (ob.l*ob.b);

    }

 }

class Sk20

 {

 

  public static void main(String args[ ])

   {

     Rectangle 
ob1= new Rectangle(4,5);

     double ar = ob1.RecArea(ob1);

     System.out.println(+ar);

    }

 }

Compile:

D:\>javac
Sk20.java

Run:

D:\>java Sk20

Output:

20.0