Re: Subclasses and the equals method

From:

Mark Space <markspace@sbc.global.net>

Newsgroups:

comp.lang.java.programmer

Date:

Sun, 10 Aug 2008 15:31:43 -0700

Message-ID:

<r%Jnk.21141$N87.12785@nlpi068.nbdc.sbc.com>

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.