Writing a Tester Class to test a compiled file

I am trying to write a tester class to test a file that has already been compiled. I am basically trying to see if the coordinates of are correct when rotated 90 degrees. I am not really sure how to write a tester class. ANy advice on how to do this?

   public static void testRotate90(){
     int fails = SUnit.testsFailed();
     System.out.println ("Testing Rotate 90...");

     CartesianPoint cp = new CartesianPoint();
     CartesianPoint cp2 = cp.rotate90();

     if (fails == SUnit.testsFailed())
         System.out.println("     PASS");
   }