Re: Subclasses and the equals method

From:

Mark Space <markspace@sbc.global.net>

Newsgroups:

comp.lang.java.programmer

Date:

Sun, 10 Aug 2008 15:44:13 -0700

Message-ID:

<9bKnk.21142$N87.3936@nlpi068.nbdc.sbc.com>

pek wrote:

Which is darn similar to what Scott Meyers and Bjarne Stroustrup have
been saying for a while: classes for designed inheritance should be
abstract. Concrete classes should be leaf classes (not inherited from).

I must admit, I really liked this. Will try to follow it from now on.
Nice advise, thank you..

See my idea above about defining an interface and having both classes
subclass from that. This is exactly the pattern Scott Myers recommends
in Effective C++ (which I just remembered and used.)

This is broken:

   (---------)
  ( Class A )
   (_________)
       \
         \
     (---------)
    ( Class B )
     (_________)

This is the correct way to switch this around

       (---------)
      ( Abstract )
       (_________)
        / \
       / \
  (---------) (---------)
( Class A ) ( Class B )
  (_________) (_________)

Now you don't have problems, and A and B both can be the same type
("Abstract") and not inherit from each other. A and B should be leaf
classes too. This is exactly what interfaces do for you in Java. I
can't think off the top of my head where too classes inherit and are
directly comparable. Something in collections might be a worthy
candidate, where you subclass a HashMap for example, to add some
function. I think Block is showing that this has to be done with great
care...