BigDecimal nextProbablePrime() method performance (to set initial capacity of HashMap to a prime num

[chris brat]

Hi,

Where can I find information about the performance of the
nextProbablePrime() method in the java.math.BigInteger class? The
relates to the initial capacity of a HashMap.

The HashMap that can hold data of at least size X and at most Y,
and I want to set the initial capacity to the next possible prime
between X and Y.

By the way Y = X * 2.

At the moment I am setting the capacity to X but I've read that
HashMaps perform better if the initial capacity is a prime number - I
want to test this.

Thanks,
Chris

 

[Thomas Hawtin]

At the moment I am setting the capacity to X but I've read that
HashMaps perform better if the initial capacity is a prime number - I
want to test this.

The current implementation of java.util.HashMap (in Sun JREs) always
rounds up to a power of two.

Tom Hawtin

 

[Jeffrey Schwab]

The current implementation of java.util.HashMap (in Sun JREs) always
rounds up to a power of two.

So for the best performance, start with a capacity of two.

 

[Roedy Green]

At the moment I am setting the capacity to X but I've read that
HashMaps perform better if the initial capacity is a prime number - I
want to test this.

Hashtables do, but HashMaps round up to a power of two.

See http://mindprod.com/jgloss/hashtable.html
http://mindprod.com/jgloss/hashmap.html