gnu.trove.impl

Class PrimeFinder

java.lang.Object

gnu.trove.impl.PrimeFinder

public final class PrimeFinder
extends java.lang.Object

Used to keep hash table capacities prime numbers. Not of interest for users; only for implementors of hashtables.

Choosing prime numbers as hash table capacities is a good idea to keep them working fast, particularly under hash table expansions.

However, JDK 1.2, JGL 3.1 and many other toolkits do nothing to keep capacities prime. This class provides efficient means to choose prime capacities.

Choosing a prime is O(log 300) (binary search in a list of 300 ints). Memory requirements: 1 KB static memory.

Field Summary

Fields

Modifier and Type	Field and Description
static int	largestPrime The largest prime this class can generate; currently equal to 2,004,663,929.

Constructor Summary

Constructors

Constructor and Description
PrimeFinder()

Method Summary

Modifier and Type	Method and Description
static int	nextPrime(int desiredCapacity) Returns a prime number which is >= desiredCapacity and very close to desiredCapacity (within 11% if desiredCapacity >= 1000).

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

largestPrime

public static final int largestPrime

The largest prime this class can generate; currently equal to 2,004,663,929. While Integer.MAX_VALUE is in fact the largest representable prime in the integer space, consumers of this class are intended to create arrays of size returned from nextPrime(int). Since the VM needs to reserve a few bytes for internal overhead, new int[Integer.MAX_VALUE] fails with an "exceeds VM limits" exception. So, we pick the second-largest prime as the practical largest.

Constructor Detail

PrimeFinder

public PrimeFinder()

Method Detail

nextPrime

public static int nextPrime(int desiredCapacity)

Returns a prime number which is >= desiredCapacity and very close to desiredCapacity (within 11% if desiredCapacity >= 1000).

Parameters:

desiredCapacity - the capacity desired by the user.

Returns:

the capacity which should be used for a hashtable.