Draft proposal: allow the use of relational operators on Comparable classes

[Reinier Zwitserloot]

Replies inline.

  --Reinier Zwitserloot



On Mar 10, 2009, at 17:21, Jeremy Manson wrote:

> My major concerns with this is how consistent it is with the rest of  
> the system:
>
> 1) There is no guarantee that if a < b and b < c, then a < c.  This is
> not true in general in Java.

But Comparator's contract states you should adhere to this. HashMaps/ 
Sets randomly fail if you screw up the contract, too. The notion that  
stuff breaks if you don't adhere to contract so far hasn't been a  
powerful enough reason to hold java improvements back.

>
>
> 2) It is inconsistent to have a < b call compareTo(), but to have ==
> not call equals().
>

And yet that's exactly what happens now with Integer, Double, Float,  
Short, Byte, and Long.

> 3) If a < b means compareTo, and a == b means reference equality, then
> you can't specify reasonable behavior for <= and >=.
>

Sure you can. <= means: the compareTo job returns either 0 or a  
negative number.

Don't get me wrong, the notion that '==' doesn't mean equality in java  
is an unfortunate brainfart I'd love to see fixed, but I see  
absolutely no way for this to happen in project coin's scope. There's  
also nothing in this proposal that would stop a future fix; in fact,  
it sort of sets it up. That's good.

> To fix this, we'd have to go back in time and have == mean object
> equality instead of reference equality.  That would be nice, but we
> can't do it.
>

That's not an argument against this particular proposal.

> Jeremy
>
> On Tue, Mar 10, 2009 at 9:09 AM, Vilya Harvey  
> <vilya.harvey at gmail.com> wrote:
>> I've attached a draft of a proposal to allow classes which  
>> implement the
>> Comparable interface to be used as operands for the relational  
>> operators. So
>> for example if you had two Strings, a and b, you would be able to  
>> write
>>
>> if (a < b) {
>>    ...
>> }
>>
>> instead of
>>
>> if (a.compareTo(b) < 0) {
>>    ...
>> }
>>
>> and you could do the same with your own classes as well.
>>
>> Thanks in advance for any feedback,
>>
>> Vil.
>>
>>
>>
>>
>