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From "Stephen Colebourne" <>
Subject Re: An improvement to AbstractHashedMap, and a new HashedSet class
Date Tue, 10 Feb 2004 17:57:48 GMT
Unfortunately, as version 3 of [collections] has now been released with
HashEntry as a class, that cannot be changed to an interface.

I would however, support the addition of a similar AbstractHashedSet class
to [collections], preferably with a default HashedSet implementation.


----- Original Message -----
From: "Henry Story" <>
To: <>
Sent: Monday, February 09, 2004 9:50 PM
Subject: An improvement to AbstractHashedMap, and a new HashedSet class

> Hi,
> A little background to this contribution is in order. I was looking at
> an interesting framework for Value Objects written by Dirk Riehle,
> available under the lgpl at, when I found a
> small bug in his code. It was obvious that what was needed was a
> WeakHashedSet - a HashSet that would wrap its contents in
> WeakReference's. Given the difficulty of extending java.util classes I
> opted to look at the apache collections package instead.
> I first decided to create a HashedSet class, an equivalent of
> The best way to do this I realised, would be to
> extend the AbstractHashedMap. The JavaDoc for this class says:
>  > This class implements all the features necessary for a subclass
> hash-based map.
>  > Key-value entries are stored in instances of the HashEntry class,
> which can be
>  > overridden and replaced. The iterators can similarly be replaced,
> without the
>  > need to replace the KeySet, EntrySet and Values view classes.
> But in fact it is not quite as extensible as it could be because the
> HashEntry is currently a class, not an interface. Let me explain by
> taking the simpler example of rewriting the java.util.HashSet class.
> HashSet
>      map -> HashMap
>               entries -> HashEntries[]
>                       key -> Object
>                            value -> Object
>                       next -> HashEntry
> This picture show that HashSet contains a pointer to a HashMap, which
> contains a pointer to an Array of HashEntries. Each HashEntry contains
> a key pointer, a value pointer and a pointer to the next HashEntry.
> A HashSet uses the HashMap because the keys in a HashMap are unique. As
> a result the value pointer in the HashEntry has really no role to play.
> The way AbstractHashedMap is written in the apache collection means
> that to simply follow the above pattern would waste one pointer for
> every HashEntry, that is 4 bytes wasted for every entry.
> The solution is simple.
>   1. I turned the HashEntry inner class into an inner interface of
> AbstractHashedMap.
>   2. I added a few methods to get/set the key and the value. These
> methods should be inlined by all modern compilers, so I don't think
> there is any efficiency problem here.
>   3. I then wrote a HashedSet which has an inner class that extends
> AbstractHashedMap, but which takes a modified HashEntry class where the
> key and the value are the same object. The resulting
> net.bblfish.util.HashedSet is thus more memory efficient than
> java.util.HashSet.
> HashedSet
>      map -> SpecialisedHashMap
>               entries -> HashEntries[]
>                       key -> Object
>                            next -> HashEntry
> Perhaps it would be worth adding these changes to the library.
> If the change to
> is
> accepted then it will make it relatively easy to write a WeakHashedSet.
> It will be just a matter of extending WeakReference by implementing the
> HashEntry methods.
> WeakHashedSet
>      map -> SpeacialWeakHashMap
>               entries -> WeakHashEntries[] extends WeakReference
>                                            implements HashEntry
>                       key -> Object
>                            next -> HashEntry
> Henry
> I have placed the changes in a tar file at:
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