(For those of you who missed javanlp last time, we decided to have a
biweekly "Did you know...?" email, where someone introduces a
non-general-knowledge fact about javanlp (or just java) that they've found
useful.)

nlp.util.Index is a very simple idea that is incredibly useful.  It allows
you to easily make a one-to-one mapping of Objects to ints in the range
0..n.  This can represent enormous time savings in intense computations by
letting you use arrays indexed by ints instead of Maps indexed by Objects.

For example, during PCFG parsing, for each span (x,y), it is necessary to
repeatedly look up scores associated with each String-valued category
label.  The naive way to do this would be to have a 2-D array of Maps from
String to Double.  But if you've compiled an Index of the category labels,
you can perform the whole parse using a 3-D array of doubles, never having
to do a single hashCode evaluation!

Chris (Cox) has recently made Index *even better* by genericizing it.

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