Re: Casting an object in equals Java 5
On Wed, 1 Oct 2008, Andreas Leitgeb wrote:
Eric Sosman <esosman@ieee-dot-org.invalid> wrote:
Andreas Leitgeb wrote:
In my Java classes they told me not to do that at all.
Either make these classes (or equals() itself, along with
hashCode()) final or abstract.
Maybe I've misunderstood, but this seems crazy.
In that class we had our "hands-on" example, some class
describing a (3D-)Point, and it had an equals() that compared
all three coordinates, and a hashCode() that was consistent
with equals().
Then the instructor raised the point of deriving a "ColoredPoint"
and showed us the bad effect of the naive approach for overriding
equals (and hashCode()) in ColoredPoint.
My take on this is that subclassing, and overriding equals, is okay, as
long as the symmetry of equals is preserved. So, ColouredPoint can
override equals and compare colour as well as position when it's comparing
itself to another ColouredPoint, but when comparing itself to a normal
Point, it should just compare coordinates, as a Point would. Like this:
public boolean equals(Object that) {
if (that instanceof Point) return equals((Point)that) ;
else return false ;
}
public boolean equals(Point that) {
if (that instanceof ColouredPoint) return equals((ColouredPoint)that) ;
else return (this.x == that.x) && (this.y == that.y) ;
}
public boolean equals(ColouredPoint that) {
return (this.x == that.x) && (this.y == that.y) && (this.colour.equals(that.colour)) ;
}
An equals method implemented like this is reflexive, and symmetric even in
the presence of mixed types. What it isn't is transitive:
Point p = new Point(5, 5) ;
Point q = new ColouredPoint(5, 5, Color.RED) ;
Point r = new ColouredPoint(5, 5, Color.BLUE) ;
p.equals(q) ; // true
p.equals(r) ; // true
q.equals(r) ; // false!
Which could be a problem. Or might not be. It's definitely not pretty, and
feels like an accident waiting to happen.
As for hashCode, if you want to to be consistent with equals, which is
generally considered a must, then you can't override the version in Point,
since the version of equals in Point is serving as the benchmark for
equality. I doubt this will be a huge problem in practice.
tom
--
We must perform a quirkafleeg