Faster HashMap implementation

[Doug Lea]

Ismael Juma wrote:
> Out of curiosity, do you know if any tests have been done with an open
> addressing scheme similar to Python dictionaries[1] in Java? Near the top,
> there's a comment explaining how it works and the motivation.
> 

More or less. This is roughly similar to the schemes I mentioned
a few days ago, of first direct-addressing. The python
approach (as noted in its comments) has most trouble with
cases that occur too often in Java to ignore: Poor entropy
in low bits (the hashCodes for commonly used Floats and Doubles are
especially problematic (Aside: you wouldn't think these would
ever be used keys, but they are)) and much-greater-than-expected
duplicate codes (mainly the particular value zero). However,
these do seem amenable to one of the hybrid approaches
I mentioned; I'll try to get a full version together soon.

-Doug