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From "Michael A. Smith" <...@apache.org>
Subject Re: [collections] ClassMap and others
Date Wed, 23 Oct 2002 04:35:17 GMT
Henri Yandell wrote:
> I just pushed a ClassMap class in. Uses inheritence in the get, so you can
> do:
> 
> map.put(Number.class, new NumberConverter());
> 
> Converter c = (Converter)map.get(Float.class);
> 
> assuming a Converter class.

I like, but I see some issues with the implementation with regards to 
interfaces.  I just commented on your cvs commit (did that before I saw 
this message).

> It doesn't do anything fancy with Integer.TYPE. ie) claiming that that
> extends Number.class etc. Any views?

these would be special cases, right?  dunno...  I'll think about it 
though.

> Clearing out my collections package...
> Other things:
> 
> I still need to add an isSorted to CollectionUtils, will go ahead and do
> that at some point. Making myself hold back until I have a unit-test is
> almost like hard work :)

I'm not sure that an isSorted will work for CollectionUtils.  Collection 
implementations have no restrictions on maintaining iterator order from 
one iterator to the next, so while the first time you call isSorted it 
may return true, the second time it may not.  In fact, the 
Collections.iterator() documentation explicitly states that:  "There are 
no guarantees concerning the order in which the elements are returned 
(unless this collection is an instance of some class that provides a 
guarantee)."

Now, if you add that to ListUtils and perform on lists, then that seems 
reasonable.

> I have a SortedIterator class. Basic usage being:
> ..
> Iterator iterator = ...
> SortedIterator si = new SortedIterator(iterator, new SomeComparator() );
> ..
> Then it reads that iterator out in sorted order. Obviously has to suck it
> into memory internally. Anyone think this is of use?

This same thing can be accomplished using the three-step:

   List l = IteratorUtils.toList(iterator);
   Collections.sort(list, new SomeComparator());
   Iterator sorted = list.iterator();

to get the desired effect.  Why?  Because the implementation cannot do 
magic and turn an interator into a sorted iterator without reading the 
entire iterator, storing each element into a temporary store, then 
sorting it.  Leaving it up to the user to do this three-step process 
(create List to store all elements, sort list, get iterator) ensures 
that a developer understands the memory implications (i.e. that the 
entire iterator is read into a list).

In other words, I don't see this as adding much functionality.

but, maybe that's just me.  :)

> OrderedSet -> Basically a List which doesn't allow duplicates.

ok.

> LimitedList -> A List which maintains a fixed max length.

usefulness?  Maybe an LRUList?

> SortedLimitedList -> A List which uses a Comparator to maintain a sorted
>                      max length. Useful to do a quick sort to find the
>                      first N elements of a list of length L.
>                      <Insertion Sort>

huh?

> ProxySet/List. Any reason not to have these? [or even ProxyCollection]?

not that I can think of.

> typed.*  a Map/List/Set wrapper which enforces the Type of the value or
> key. So you'd do:
> ..
> List list = new TypedList(Integer.class)
> ..
> and it will throw an Exception if u pass the wrong thing in.

isn't this just a ListUtils.predicatedList(p) with a predicate, p, that 
checks for the right class?

maybe provide the predicate to construct such a thing?

> That's all that leaps out as mature for now. Apologies if I'm repeating
> myself with any of these, am attempting to clean out my collections
> package so I can force myself to use Commons Collections more.
> 
> Will slowly add things as I get time and unit tests together.

cool.

> Or at least the ones that aren't blown apart.

;)

michael
-- 
Michael A. Smith
mas@apache.org



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