Help on polymorphism in my code

Question

Hi, I'd be glad if anyone could explain why java compiler gives errors
compiling a class of a simple application.
This is the scenario: class Point is meant to simplicistically abstract the
concept of 2D point (x and y are the coordinates). Class Point3D extends Point,
adding z coordinate. Attributes x, y and z are encapsulated (private and each
one with setter and getter methods).
There is a third class, called Ruler, its aim is to set the distance between
two 2D points or two 3D points using the polymorphic method
computeDistance(Point p1, Point p2).
The compiler fails compiling the class Ruler with this message:
Ruler.java:21: p1 is already defined in computeDistance(Point,Point)
Point3D p1 = (Point3D) p1;
^
Ruler.java:22: p2 is already defined in computeDistance(Point,Point)
Point3D p2 = (Point3D) p2;
^
2 errors
This is the relevant part of the class:
public class Ruler
    private double distance;
    public void computeDistance(Point p1, Point p2)
        if (p1 instanceof Point && p2 instanceof Point)
            // This is a 2D points case.
            // Obtain x1, x2, y1, y2 via getX() and getY() then  compute the
            // distance.
        else if (p1 instanceof Point3D && p2 instanceof Point3D)
            // This is a 3D points case.
            // Obtain x1, x2, y1, y2, z1, z2 using getX() and getY(), getZ().
            // BUT, in order to obtain z1, z2 it is needed a casting between
            // objects, because Point objects don't provide getZ(), while
            // Point3D do.
            Point3D p1 = (Point3D) p1;
            Point3D p2 = (Point3D) p2;
        else
            // This case is not valid.
}If this piece of code is not enough I'll write in another post all the three
classes exactly as they are.
Thanks for any help!

Answer 1

In Point, declare a computeDistance:
public double computeDistance(Point other)
   // compute and return distance from this point to other point;
}Override it in Point3D to account for the Z values:
public double computeDistance(Point other)
   if (other instanceof Point3D)
      // Cast to Point3D
     // compute and return distance from this point to other point;
   else
       // decide how to handle (throw IllegalArgumentException, use Z=0, etc.)
}Or, just try the cast, and allow the ClassCastException to fail [instead of explicitly throwing IllegalArgumentException]. Just document whatever it should do.
In Ruler, do this, and it will do the polymorphism without instanceof:
public void computeDistance(Point p1, Point p2)
   distance = p1.computeDistance(p2);
}