Re: Overflowing HashMap

From:

Patricia Shanahan <pats@acm.org>

Newsgroups:

comp.lang.java.help

Date:

Sun, 03 Jun 2012 20:59:48 -0700

Message-ID:

<OOWdnaKU06A9r1HSnZ2dnUVZ_oudnZ2d@earthlink.com>

On 6/3/2012 8:51 PM, Roedy Green wrote:

On Sun, 03 Jun 2012 08:40:22 -0700, Knute Johnson
<nospam@knutejohnson.com> wrote, quoted or indirectly quoted someone
who said :

One of the problems is HashMaps, even in 64-bit JVMs, can still only
hold 1<< 30 elements.

What does that mean?

see http://www.mathe-online.at/JavaCalc/jcintro.html
it will evaluate it for you to :
1,073,741,824
Note it is smaller than
Integer.MAX_VALUE
+2,147,483,647 [2^31-1] aka 2 gig, roughly 2 billion

I "cheated" and looked inside HashMap source. The docs lie. The max
value is controlled by a constant
MAX_CAPACITY = 1<< 30;

Internally it needs an array sized a power of 2 with some breathing
room to track when the various hash chains start. The power of two
restriction allows it to convert a hashCode to an array index with
just a shift rather than a divide/modulus.

I thought that was a restriction on the number of buckets, not on the
number of entries.

Patricia

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