Re: idea for more efficient HashMap

Robert Klemme <>
Sun, 13 Jan 2013 19:44:50 +0100
On 12.01.2013 10:55, Roedy Green wrote:

Inside HashMap are little glue Entry objects that point to the key and

What if you could implement an interface on your objects so that
HashMap could use them directly without separate key or Entry glue?.

e.g. getKey()

One drawback would be your objects could live on only one such
space-efficient HashMap.

I've once implemented a hash map which uses double hashingand uses a
single Object[] for storage of keys and values. It creates Entry
instances on the fly while iterating. We did this to get rid of a few
hundred thousand Entry instances and improve GC behavior of the
application. Works pretty good.

Side benefit of that implementation was that you get
ConcurrentModificationException only if the map needed to be resized as
part of an insert operation.

I cannot share this implementation here as it is proprietary code. But
you should pretty much have everything to implement it yourself. If you
do it do not forget to create meaningful unit tests.

Kind regards


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