I received the following email from a friend of mine, which documents some
backward-compatibility problems with the java.util.BitSet class. The behavior
of BitSet.size() appears to have changed between 1.1.x and 1.2.

fred.ecks@Eng 1999-02-22

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Date: Fri, 19 Feb 1999 17:06:16 -0500
From: Russ Teasdale <###@###.###>
To: Jeff Dunn <###@###.###>
CC: fred.ecks@Eng

> Wasn't there some sort of Java backwards-compatibility
> bug that you were seeing? Can you send me a test case?

    Sure. Without belaboring the point, let me just snip from
my thesis coding log. I used the BitSet class to store long binary
"chromosomes" for use by the genetic algorithm that I employ in my
thesis engine. See below.

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12_31_98:

The engine was experimentally migrated to the JDK 1.2 libraries and JVM.

Severe errors resulted. After a long manual debugging process, the source
of the error was traced to changes made to the BitSet utility class.

The changes in question were:

The BitSet.size() method previously returned the canonical number of bits
used in the set. Under JDK 1.2, it returns the number of bits used to store
the canonical number. The new method length() performs the function which
size() had previously performed.

Under the previous version of BitSet, clearing a bit above the highest range
in the set would result in the set growing to include that bit. It is not
clear whether or not this behavior persists (test it). It can be surmised
that this behavior does hold under 1.2 from runtime evidences. However, the
length() function now returns only the index of the highest -set- bit. This
can result in two chromosomes of supposedly identical canonical length being
reported as different by sanity checking methods.

Modifications have been made to Evolver and Processor classes to allow them to
mimic the functionality which they had under the 1.1 JDK. The key modifications
are to Processor.GetChromosome(). In particular, an artificial high-order bit
is set in order to ensure proper reporting of the length of the chromosome.
There are also extensive chromosome sanity checks which have been added.

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