Class 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 Description
      static int largestPrime
      The largest prime this class can generate; currently equal to 2,004,663,929.
    • Constructor Summary

      Constructors 
      Constructor Description
      PrimeFinder()  
    • Method Summary

      All Methods Static Methods Concrete Methods 
      Modifier and Type Method 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.