Re: lookup by EnumSet

Robert Klemme <>
Tue, 28 Feb 2012 23:08:47 +0100
On 28.02.2012 14:55, Roedy Green wrote:

I had a long and annoying dream that there was a Java Collection that
let you look up by EnumSet. It was not a simple Map.

It worked something like this: You could assign a set of binary
attributes to a Person, e.g. male/female, fat, thin, average, atheist,
Christian, Moslem, Jew, Buddhist. Asian, European, African, North
American, South American..

Then you could ask for all the fat or average females, Buddhist but
not Asian.

You might specify an EnumSet for what you want and one for what you
don't want. Anything not specified in either does not matter.

You need a multi dimensional index which can efficiently select from any
subset of dimensions given. Whether properties are boolean or need more
than one bit to represent is just a minor challenge here.

In the dream I was trying to write example code and an entry in the
Java glossary. When I woke, I could not think of such a class, and
further it was not obvious how one could be implemented.

I wondered how you would do it.

The most straightforward solution in memory would probably be something
like Map<${PropertyType},Set<Person>> per property. Depending on
application logic you would build these just once when reading the data
or update indexes whenever you update fields. Querying would use set
operations to reduce the set of results.

I thought you might extract the attributes into an array of longs and
check each one for compliance with your masks.

If the sets were stable, you might extract a BitSet for each
attribute, and do logical operations on giant bit strings of the
relevant bits.

There would be another use for BitSets: you create a BitSet per instance
which represents key state. And you store these keys along with the
data in a Map<BitSet,Person>. Downside: this only works good for static
data and exact matches.

I vaguely recall SQL databases optimising queries of this type by
transparently building inverse look up indexed.

There are two ways with RDBMS:

1. Create an index for every subset of properties that you want to use
as filter in a query. Note: indexes whose columns are a subset of
another index can be left out if you manage to make those columns
leading columns.

2. Create a bitmap index (in Oracle) for example. Bitmap indexes in
Oracle need coarse grained locks and thusly are not suited for OLTP
applications - they are usually used in DWH applications.

In memory you could do the same although the bitmap type index would
probably contain references to objects instead of bits identifying
rowids. If memory is tight using a BitSet to point into a List<Person>
or Person[] might be worthwhile.

Kind regards


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