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From: Jim Janney <jjanney@shell.xmission.com>
Newsgroups: comp.lang.java.programmer
Subject: Re: "Program to an interface" - When to break a design pattern
Date: Thu, 05 May 2011 15:58:12 -0600
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Lew <noone@lewscanon.com> writes:

> On 05/05/2011 04:14 PM, Jim Janney wrote:
>> Zapanaz<http://joecosby.com/code/mail.pl@foo.com>  writes:
>>
>>> I've seen this design pattern before
>>>
>>> http://witte-consulting.com/documents/design-principles/
>>>
>>> and, in general, I see the point of it.
>>>
>>> But say we've got something like this
>>>
>>> LinkedHashMap<String, Integer>  sortedMap = this.getSortedMap();
>>>
>>> So you have the method
>>>
>>> public LinkedHashMap<String, Integer>  getSortedMap() {
>>>    //do stuff
>>> }
>>>
>>> (not necessarily public)
>>>
>>> Now the design principle says, the method signature should instead be
>>>
>>> public Map<String, Integer>  getSortedMap() {
>>>    //do stuff
>>> }
>>>
>>> The problem is, where I'm creating sortedMap above, I need the map to
>>> retain the insertion order. If what's returned actually is a Map,
>>> rather than a LinkedHashMap, then the results the user actually sees
>>> are going to be in the wrong order. Making things worse, in this case,
>>> nothing would actually break, only the end user would notice anything
>>> was actually wrong.
>>>
>>> So in this case, it seems to me, that using LinkedHashMap in the
>>> method signature makes sense. The fact that the return retains the
>>> insertion order is an integral part of what the method does.
>>>
>>> If nothing else, it's going to save Fred Developer down the line from
>>> looking at the code around this
>>>
>>> Map<String, Integer>  sortedMap = this.getSortedMap();
>>>
>>> and thinking "wait, how do I know getSortedMap() is going to return a
>>> result with the right ordering?", and having to waste time digging
>>> into that method.
>
> 'SortedMap', as so many have said.  Apparently you need to practice
> study of the API docs much, much more.  We all should know the
> essential collections types.  Get studying!

But that would be wrong, because a SortedMap does something entirely
different from a LinkedHashMap.  Take your own advice before dispensing
it to others.

-- 
Jim Janney