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From: Eric Sosman <esosman@comcast-dot-net.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: Help with Deck of cards assignment and getting it to deal...
Date: Sun, 03 Aug 2014 16:49:27 -0400
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On 8/3/2014 3:26 PM, Arne Vajhøj wrote:
>[...]
> I must admit that I can not follow you.
>
> The fact that different games may use a different ordering is
> not an argument against Comparable<>.
>
> Comparable<> is not the one and only ordering. Comparable<> is
> just the natural ordering. It is perfectly valid to have a
> Comparable<> with the natural ordering and a bunch of
> Comparator<> to handle the context specific orderings.

     Although one could select different Comparators for different
games -- one for Ace-low, say, and another for Ace-high -- there
is no way a Comparator could work for "Ace-either" games like Poker
(and certainly not for "Ace-both" games like high-low Poker!).  Like
Comparable, Comparator reflects a _total ordering_ on the objects it
compares, and a total ordering must be transitive:

	If X < Y and Y < Z then X < Z, for all X,Y,Z.

In Poker, one can have

	Ace < Five and Five < Ten but Ten < Ace

... violating transitivity, so the ranks do not follow a total order.

     One could, of course, implement a Comparator (or a Comparable)
that broke the rules and violated its "contractual obligation" to
impose a total order.  But a Comparable/Comparator that didn't behave
as it should would make mockery of the claim

 > Your Card class will be more easily used if it implement the
 > java.lang.Comparable<T> interface

... because none of the Comparable- or Comparator-based utility
classes and methods would behave properly: Collections.sort(),
Arrays.binarySearch(), SortedMap(), ...  Something that simply
doesn't work cannot be "more easily used."

-- 
esosman@comcast-dot-net.invalid