Do Set implementations waste memory?

[Dimitris Andreou]

2010/3/18 Rémi Forax <forax at univ-mlv.fr>

> Le 18/03/2010 00:59, Paulo Levi a écrit :
>
>  My understanding is that set implementations are implemented by using Maps
>> internally + a marker object, and that since Maps are implemented using
>> arrays of entries this is at least n*3 references more that what is needed,
>> since there are never multiple values.
>>
>> Any plans to change this? I suspect it would be a boon for programs that
>> use the correct data structure.
>>
>>
> You have to test it.
> My guess is that there will be no difference.
> As far as I remember, an object needs to be aligned on a valid 64bits
> address even in 32bits mode,
> Hotspot uses a 64bits header and the internal hash map entry contains 4
> ints,
> if you remove the reference corresponding to the value, the empty place
> will be
> considered as garbage and not used.
>


> Else, you can try to remove the internal entry object but in that case
> the hashcode of the element will be not stored anymore and you will
> have a slowdown for all objects that doesn't cache their hashcode by
> itself.
>
> Rémi
>
>
See my second-to-last post in this thread:
http://groups.google.com/group/guava-discuss/browse_thread/thread/23bc8fa5ae479698

In short, I tested removing the "value" field of a HashMap's entry object,
and indeed (through Instrumentation#getObjectSize) I observed no reduction
in memory. I had to remove one further field (e.g. "hash") to make a
reduction (of 8 bytes per entry).

Dimitris
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