cern.colt.map
Class PrimeFinder

java.lang.Object
  extended by cern.colt.map.PrimeFinder

public class PrimeFinder
extends java.lang.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.

Author:
wolfgang.hoschek@cern.ch

Field Summary
static int largestPrime
          The largest prime this class can generate; currently equal to Integer.MAX_VALUE.
 
Constructor Summary
protected PrimeFinder()
          Makes this class non instantiable, but still let's others inherit from it.
 
Method Summary
protected static void main(java.lang.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 Detail

largestPrime

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

See Also:
Constant Field Values
Constructor Detail

PrimeFinder

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

Method Detail

main

protected static void main(java.lang.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.