Re: Subclasses and the equals method

From:

Daniele Futtorovic <da.futt.news@laposte.invalid>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 11 Aug 2008 00:25:21 +0200

Message-ID:

<g7npth$cp1$1@registered.motzarella.org>

On 10/08/2008 23:17, jolz allegedly wrote:

There's no way to extend the Point class and still get it's equals
method to work right, because when

Point p1 = ... anything
ColorPoint cp1 = ... anything

p1.equals( cp1 );

is invoked, p1 doesn't know to check for a subclass.

It can be done:

public boolean equals( Object o ) {
if (o == null || !getClass().equals(o.getClass()))
return false;
....

However, that way you lose super.equals as a rump equality
implementation. Therefore it might be better to to it in two steps:

class X {
   //yes, bad method name
   protected boolean attributiveEquality(X x){
     ...
   }

   public boolean equals(Object o){
     return o == this || o != null && getClass().equals(o.getClass()) &&
attributiveEquality( (X) o );
   }
}

As for making equals final... I don't think it's a good idea. Only if
you strictly use composition over inheritance -- but even then. It's a
bit of throwing the baby out with the bath water.

--
DF.