Re: Subclasses and the equals method

Mark Space <>
Sun, 10 Aug 2008 15:31:43 -0700
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.

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