The problem with using "instanceof" as above is that "this" may actually
be a subclass of SudokuXY. For example, someone who wrote the above code
might also end up writing the following code:

<code>
class SudokuXYZ extends SudokuXY {
 private int z = 0;

 public void z(int newValue) {
   this.z = newValue;
 }

 public int z() {
   return z;
 }

 public boolean equals(Object o) {
   if (super.equals(o)) {
     if (other instanceof SudokuXYZ) {
       if (this.z == (SudokuXYZ).z) {
         return true;
       }
     }
   }
   return false;
 }
}
</code>

But now, imagine you have 3 objects:

<code>
SudokuXYZ s1, s2;
SudokuXY s3;

/*
[Assume the appropriate constructors are called so that:

s1.x = 100, s2.x = 100, s3.x = 100
s1.y = 200, s2.y = 200, s3.y = 200
s1.z = 400, s2.z = 800
*/
</code>

s3.equals(s1) will return true.
s3.equals(s2) will return true.
s1.equals(s2) will return false.

This breaks the contract of the equals() method which says that the equals
relationship should be transitive.

   - Oliver