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From "Stephen Colebourne" <>
Subject Re: LinkedFastHashMap?
Date Sun, 19 Jan 2003 12:36:18 GMT
From: "Henri Yandell" <>
> I've not looked at the code in question yet, but it feels like the wrong
> solution. Is it not possible for the existing SequencedHashMap to be able
> to wrap a FastHashMap?
> ie) SequencedHashMap.wrap(new FastHashMap()) which would override the map
> used internally by sequenced hash map.
Essentially, that is what the code submitted tries to do. However, it often
causes difficulties, because of the multitude of views that collections

> 1) java.util.Map
> 2) org.apache.commons.collections.ProxyMap
> 3) java.util.HashMap
> 4) org.apache.commons.collections.FastHashMap  [The hash denoting that it
>    uses hashing for its mapping internally, and not some other structure]
> 5) org.apache.commons.collections.SequencedMap
> SequencedMap decorates any other map, so doesn''t matter if it's HashMap
> or FastHashMap or TreeMap. Same for any other 5)s.
To some degree, I agree with what you're saying. However, the main problem
is with backwards compatability in [collections].

FastHashMap currently extends HashMap
FastTreeMap currently extends TreeMap
SequencedHashMap currently implements Map directly, and stores data in a
HashMap and LinkedList

Both FastHashMap and FastTreeMap are essentially the same class, and
_should_ be implemented as minimal subclasses extending an abstract
AbstractFastMap. FastSequencedHashMap would then extend the abstract class
too and take 5 seconds to implement.

Unfortunately, the original designers of these classes chose to make them
extend HashMap and TreeMap even though they make no use of the
implementation. This is bad for memory use (unused variables) and bad for
code reuse. Presumeably, it was deemed important to have the classes as drop
in replacements that would function if someone used a cast to a

So, we could fix the design to have an AbstractFastMap (I did this
yesterday, only to realise the problem...). But this breaks backwards
compatability for some users. My comments on FastSequencedHashMap were thus
based on consistency with the place we are in.


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