Inheritance and Interfaces - Building Java Programs: A Back to Basics Approach - page 586

9 super (x, y);
10 this .z = z;
11 }
12
13 // returns the z-coordinate of this Point3D
14 public int getZ() {

15 return z;

16 }
17 }

On the surface, this seems to be a reasonable implementation. However, consider
the equals method defined in the Point class. It compares the x - and y -coordinates
of two Point objects and returns true if they are the same:

// returns whether o refers to a Point with the same
// (x, y) coordinates as this Point
public boolean equals(Object o) {
if (o instanceof Point) {
Point other = (Point) o;
return x == other.x && y == other.y;
} else { // not a Point object
return false;

}

}

You might also want to write an equals method for the Point3D class. Two
Point3D objects are equal if they have the same x -, y -, and z -coordinates. The fol-
lowing is a working implementation of equals that is correct but stylistically
unsatisfactory:

public boolean equals(Object o) {
if (o instanceof Point3D) {
Point3D p = (Point3D) o;
return getX() == p.getX() && getY() == p.getY() && z == p.z;
} else {
return false;

}

}

The preceding code compiles and runs correctly in many cases, but it has a subtle
problem that occurs when you compare Point objects to Point3D objects. The
Point class's equals method tests whether the parameter is an instance of Point
and returns false if it is not. However, the instanceof operator will return true