Package cern.colt.map

Class PrimeFinder

java.lang.Object
cern.colt.map.PrimeFinder
public class PrimeFinder extends Object
Not of interest for users; only for implementors of hashtables. Used to keep hash table capacities prime numbers.

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 int's). Memory requirements: 1 KB static memory.

Version:
1.0, 09/24/99
Author:
wolfgang.hoschek@cern.ch

  • Field Summary

    Fields
    Modifier and Type
    Field
    Description
    static final int
    largestPrime
    The largest prime this class can generate; currently equal to Integer.MAX_VALUE.

  • Constructor Summary

    Constructors
    Modifier
    Constructor
    Description
    protected
    PrimeFinder()
    Makes this class non instantiable, but still let's others inherit from it.

  • Method Summary

    Modifier and Type
    Method
    Description
    protected static void
    main(String[] args)
    Tests correctness.
    static int
    nextPrime(int desiredCapacity)
    Returns a prime number which is >= desiredCapacity and very close to desiredCapacity (within 11% if desiredCapacity >= 1000).
    protected static void
    statistics(int from, int to)
    Tests correctness.

    Methods inherited from class java.lang.Object

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

  • Field Details

    • largestPrime

      public static final int largestPrime
      The largest prime this class can generate; currently equal to Integer.MAX_VALUE.
      See Also:

  • Constructor Details

    • PrimeFinder

      protected PrimeFinder()
      Makes this class non instantiable, but still let's others inherit from it.

  • Method Details

    • main

      protected static void main(String[] args)
      Tests correctness. Try from=1000, to=10000 from=200, to=1000 from=16, to=1000 from=1000, to=Integer.MAX_VALUE

    • 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.

    • statistics

      protected static void statistics(int from, int to)
      Tests correctness.