Java - Calculate Area Of Rectangle And Area Of Circle Using Interface

interface area
      {
          double pi = 3.14;
          double calc(double x,double y);
       }

class rect implements area
   {
      public double calc(double x,double y)
        {
            return(x*y);
         }
   }

class cir implements area
   {
      public double calc(double x,double y)
        {
            return(pi*x*x);
         }
   }

class test7
  {
     public static void main(String arg[])
       {
          rect r = new rect();
          cir c = new cir();
          area a;

          a = r;
          System.out.println("\nArea of Rectangle is : " +a.calc(10,20));

          a = c;
          System.out.println("\nArea of Circle is : " +a.calc(15,15));
        }
  }