GNU Trove

gnu.trove
Class THash

java.lang.Object
  |
  +--gnu.trove.THash
All Implemented Interfaces:
java.lang.Cloneable
Direct Known Subclasses:
TObjectHash, TPrimitiveHash

public abstract class THash
extends java.lang.Object
implements java.lang.Cloneable

Base class for hashtables that use open addressing to resolve collisions. Created: Wed Nov 28 21:11:16 2001

Version:
$Id: THash.java,v 1.7 2003/11/19 15:45:41 ericdf Exp $
Author:
Eric D. Friedman

Field Summary
protected  int _free
          the current number of free slots in the hash.
protected  float _loadFactor
          Determines how full the internal table can become before rehashing is required.
protected  int _maxSize
          The maximum number of elements allowed without allocating more space.
protected  int _size
          the current number of occupied slots in the hash.
protected static int DEFAULT_INITIAL_CAPACITY
          the default initial capacity for the hash table.
protected static float DEFAULT_LOAD_FACTOR
          the load above which rehashing occurs.
 
Constructor Summary
THash()
          Creates a new THash instance with the default capacity and load factor.
THash(int initialCapacity)
          Creates a new THash instance with a prime capacity at or near the specified capacity and with the default load factor.
THash(int initialCapacity, float loadFactor)
          Creates a new THash instance with a prime capacity at or near the minimum needed to hold initialCapacity elements with load factor loadFactor without triggering a rehash.
 
Method Summary
protected abstract  int capacity()
           
 void clear()
          Empties the collection.
 java.lang.Object clone()
           
 void compact()
          Compresses the hashtable to the minimum prime size (as defined by PrimeFinder) that will hold all of the elements currently in the table.
 void ensureCapacity(int desiredCapacity)
          Ensure that this hashtable has sufficient capacity to hold desiredCapacity additional elements without requiring a rehash.
 boolean isEmpty()
          Tells whether this set is currently holding any elements.
protected  void postInsertHook(boolean usedFreeSlot)
          After an insert, this hook is called to adjust the size/free values of the set and to perform rehashing if necessary.
protected abstract  void rehash(int newCapacity)
          Rehashes the set.
protected  void removeAt(int index)
          Delete the record at index.
protected  int setUp(int initialCapacity)
          initializes the hashtable to a prime capacity which is at least initialCapacity + 1.
 int size()
          Returns the number of distinct elements in this collection.
 void trimToSize()
          This simply calls compact.
 
Methods inherited from class java.lang.Object
equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Field Detail

_size

protected transient int _size
the current number of occupied slots in the hash.


_free

protected transient int _free
the current number of free slots in the hash.


DEFAULT_LOAD_FACTOR

protected static final float DEFAULT_LOAD_FACTOR
the load above which rehashing occurs.

See Also:
Constant Field Values

DEFAULT_INITIAL_CAPACITY

protected static final int DEFAULT_INITIAL_CAPACITY
the default initial capacity for the hash table. This is one less than a prime value because one is added to it when searching for a prime capacity to account for the free slot required by open addressing. Thus, the real default capacity is 11.

See Also:
Constant Field Values

_loadFactor

protected float _loadFactor
Determines how full the internal table can become before rehashing is required. This must be a value in the range: 0.0 < loadFactor < 1.0. The default value is 0.5, which is about as large as you can get in open addressing without hurting performance. Cf. Knuth, Volume 3., Chapter 6.


_maxSize

protected int _maxSize
The maximum number of elements allowed without allocating more space.

Constructor Detail

THash

public THash()
Creates a new THash instance with the default capacity and load factor.


THash

public THash(int initialCapacity)
Creates a new THash instance with a prime capacity at or near the specified capacity and with the default load factor.

Parameters:
initialCapacity - an int value

THash

public THash(int initialCapacity,
             float loadFactor)
Creates a new THash instance with a prime capacity at or near the minimum needed to hold initialCapacity elements with load factor loadFactor without triggering a rehash.

Parameters:
initialCapacity - an int value
loadFactor - a float value
Method Detail

clone

public java.lang.Object clone()
Overrides:
clone in class java.lang.Object

isEmpty

public boolean isEmpty()
Tells whether this set is currently holding any elements.

Returns:
a boolean value

size

public int size()
Returns the number of distinct elements in this collection.

Returns:
an int value

capacity

protected abstract int capacity()
Returns:
the current physical capacity of the hash table.

ensureCapacity

public void ensureCapacity(int desiredCapacity)
Ensure that this hashtable has sufficient capacity to hold desiredCapacity additional elements without requiring a rehash. This is a tuning method you can call before doing a large insert.

Parameters:
desiredCapacity - an int value

compact

public void compact()
Compresses the hashtable to the minimum prime size (as defined by PrimeFinder) that will hold all of the elements currently in the table. If you have done a lot of remove operations and plan to do a lot of queries or insertions or iteration, it is a good idea to invoke this method. Doing so will accomplish two things:
  1. You'll free memory allocated to the table but no longer needed because of the remove()s.
  2. You'll get better query/insert/iterator performance because there won't be any REMOVED slots to skip over when probing for indices in the table.


trimToSize

public final void trimToSize()
This simply calls compact. It is included for symmetry with other collection classes. Note that the name of this method is somewhat misleading (which is why we prefer compact) as the load factor may require capacity above and beyond the size of this collection.

See Also:
compact()

removeAt

protected void removeAt(int index)
Delete the record at index. Reduces the size of the collection by one.

Parameters:
index - an int value

clear

public void clear()
Empties the collection.


setUp

protected int setUp(int initialCapacity)
initializes the hashtable to a prime capacity which is at least initialCapacity + 1.

Parameters:
initialCapacity - an int value
Returns:
the actual capacity chosen

rehash

protected abstract void rehash(int newCapacity)
Rehashes the set.

Parameters:
newCapacity - an int value

postInsertHook

protected final void postInsertHook(boolean usedFreeSlot)
After an insert, this hook is called to adjust the size/free values of the set and to perform rehashing if necessary.


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GNU Trove is copyright 2001-2003 Eric D. Friedman. All Rights Reserved.