com.jaxfront.core.util
Class DoubleKeyHashtable

java.lang.Object
  extended by com.jaxfront.core.util.DoubleKeyHashtable
All Implemented Interfaces:
java.io.Serializable, java.lang.Cloneable
Direct Known Subclasses:
DoubleKeyHashtableUnsynced

public class DoubleKeyHashtable
extends java.lang.Object
implements java.lang.Cloneable, java.io.Serializable

This class implements a hashtable, which maps keys to values. Any non-null object can be used as a key or as a value.

To successfully store and retrieve objects from a hashtable, the objects used as keys must implement the hashCode method and the equals method.

An instance of DoubleKeyHashtable has two parameters that affect its performance: initial capacity and load factor. The capacity is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created. Note that the hash table is open: in the case of a "hash collision", a single bucket stores multiple entries, which must be searched sequentially. The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. When the number of entries in the hashtable exceeds the product of the load factor and the current capacity, the capacity is increased by calling the rehash method.

Generally, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the time cost to look up an entry (which is reflected in most DoubleKeyHashtable operations, including get and put).

The initial capacity controls a tradeoff between wasted space and the need for rehash operations, which are time-consuming. No rehash operations will ever occur if the initial capacity is greater than the maximum number of entries the DoubleKeyHashtable will contain divided by its load factor. However, setting the initial capacity too high can waste space.

If many entries are to be made into a DoubleKeyHashtable, creating it with a sufficiently large capacity may allow the entries to be inserted more efficiently than letting it perform automatic rehashing as needed to grow the table.

This example creates a hashtable of numbers. It uses the names of the numbers as keys:

 DoubleKeyHashtable numbers = new DoubleKeyHashtable();
 numbers.put("one", new Integer(1));
 numbers.put("two", new Integer(2));
 numbers.put("three", new Integer(3));
 

To retrieve a number, use the following code:

 Integer n = (Integer) numbers.get("two");
 if (n != null) {
        System.out.println("two = " + n);
 }
 

As of the Java 2 platform v1.2, this class has been retrofitted to implement Map, so that it becomes a part of Java's collection framework. Unlike the new collection implementations, DoubleKeyHashtable is synchronized.

The Iterators returned by the iterator and listIterator methods of the Collections returned by all of DoubleKeyHashtable's "collection view methods" are fail-fast: if the DoubleKeyHashtable is structurally modified at any time after the Iterator is created, in any way except through the Iterator's own remove or add methods, the Iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the Iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. The Enumerations returned by DoubleKeyHashtable's keys and values methods are not fail-fast.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

This class is a member of the Java Collections Framework.

Since:
JDK1.0
Version:
1.95, 01/23/03
Author:
Arthur van Hoff, Josh Bloch
See Also:
Object.equals(java.lang.Object), Object.hashCode(), rehash(), Collection, Map, HashMap, TreeMap, Serialized Form

Nested Class Summary
static class DoubleKeyHashtable.Entry
          DoubleKeyHashtable collision list.
 
Constructor Summary
DoubleKeyHashtable()
          Constructs a new, empty hashtable with a default initial capacity (11) and load factor, which is 0.75.
DoubleKeyHashtable(int initialCapacity)
          Constructs a new, empty hashtable with the specified initial capacity and default load factor, which is 0.75.
DoubleKeyHashtable(int initialCapacity, float loadFactor)
          Constructs a new, empty hashtable with the specified initial capacity and the specified load factor.
 
Method Summary
 void clear()
          Clears this hashtable so that it contains no keys.
 java.lang.Object clone()
          Creates a shallow copy of this hashtable.
 boolean contains(java.lang.Object value)
          Tests if some key maps into the specified value in this hashtable.
 boolean containsKey(java.lang.Object key1, java.lang.Object key2)
          Tests if the specified object is a key in this hashtable.
 boolean containsValue(java.lang.Object value)
          Returns true if this DoubleKeyHashtable maps one or more keys to this value.
 java.lang.String createTestStringKey(java.lang.String a, java.lang.String b)
           
 java.util.Enumeration elements()
          Returns an enumeration of the values in this hashtable.
 java.util.Set entrySet()
          Returns a Set view of the entries contained in this DoubleKeyHashtable.
 boolean equals(java.lang.Object o)
          Compares the specified Object with this Map for equality, as per the definition in the Map interface.
 java.lang.Object get(DoubleKeyHashtable.Entry keyEntry)
           
 java.lang.Object get(java.lang.Object key, java.lang.Object key2)
          Returns the value to which the specified key is mapped in this hashtable.
 int hashCode()
          Returns the hash code value for this Map as per the definition in the Map interface.
 boolean isEmpty()
          Tests if this hashtable maps no keys to values.
 java.util.Enumeration keys()
          Returns an enumeration of the keys in this hashtable.
 java.util.Set keySet()
          Returns a Set view of the keys contained in this DoubleKeyHashtable.
static void main(java.lang.String[] args)
           
 java.lang.Object put(java.lang.Object key1, java.lang.Object key2, java.lang.Object value)
          Maps the specified key to the specified value in this hashtable.
 java.lang.Object remove(DoubleKeyHashtable.Entry aEntry)
           
 java.lang.Object remove(java.lang.Object key1, java.lang.Object key2)
          Removes the key (and its corresponding value) from this hashtable.
 int removePrimary(java.lang.Object key)
           
 int removeSecondary(java.lang.Object key)
           
 java.lang.Object removeValue(java.lang.Object value)
           
 int size()
          Returns the number of keys in this hashtable.
 void test()
           
 java.lang.String toString()
          Returns a string representation of this DoubleKeyHashtable object in the form of a set of entries, enclosed in braces and separated by the ASCII characters "" (comma and space).
 java.util.Collection values()
          Returns a Collection view of the values contained in this DoubleKeyHashtable.
 
Methods inherited from class java.lang.Object
getClass, notify, notifyAll, wait, wait, wait
 

Constructor Detail

DoubleKeyHashtable

public DoubleKeyHashtable(int initialCapacity,
                          float loadFactor)
Constructs a new, empty hashtable with the specified initial capacity and the specified load factor.

Parameters:
initialCapacity - the initial capacity of the hashtable.
loadFactor - the load factor of the hashtable.
Throws:
java.lang.IllegalArgumentException - if the initial capacity is less than zero, or if the load factor is nonpositive.

DoubleKeyHashtable

public DoubleKeyHashtable(int initialCapacity)
Constructs a new, empty hashtable with the specified initial capacity and default load factor, which is 0.75.

Parameters:
initialCapacity - the initial capacity of the hashtable.
Throws:
java.lang.IllegalArgumentException - if the initial capacity is less than zero.

DoubleKeyHashtable

public DoubleKeyHashtable()
Constructs a new, empty hashtable with a default initial capacity (11) and load factor, which is 0.75.

Method Detail

size

public int size()
Returns the number of keys in this hashtable.

Returns:
the number of keys in this hashtable.

isEmpty

public boolean isEmpty()
Tests if this hashtable maps no keys to values.

Returns:
true if this hashtable maps no keys to values; false otherwise.

keys

public java.util.Enumeration keys()
Returns an enumeration of the keys in this hashtable.

Returns:
an enumeration of the keys in this hashtable.
See Also:
Enumeration, elements(), keySet(), Map

elements

public java.util.Enumeration elements()
Returns an enumeration of the values in this hashtable. Use the Enumeration methods on the returned object to fetch the elements sequentially.

Returns:
an enumeration of the values in this hashtable.
See Also:
Enumeration, keys(), values(), Map

contains

public boolean contains(java.lang.Object value)
Tests if some key maps into the specified value in this hashtable. This operation is more expensive than the containsKey method.

Note that this method is identical in functionality to containsValue, (which is part of the Map interface in the collections framework).

Parameters:
value - a value to search for.
Returns:
true if and only if some key maps to the value argument in this hashtable as determined by the equals method; false otherwise.
Throws:
java.lang.NullPointerException - if the value is null.
See Also:
#containsKey(Object), containsValue(Object), Map

containsValue

public boolean containsValue(java.lang.Object value)
Returns true if this DoubleKeyHashtable maps one or more keys to this value.

Note that this method is identical in functionality to contains (which predates the Map interface).

Parameters:
value - value whose presence in this DoubleKeyHashtable is to be tested.
Returns:
true if this map maps one or more keys to the specified value.
Throws:
java.lang.NullPointerException - if the value is null.
Since:
1.2
See Also:
Map

containsKey

public boolean containsKey(java.lang.Object key1,
                           java.lang.Object key2)
Tests if the specified object is a key in this hashtable.

Parameters:
key - possible key.
Returns:
true if and only if the specified object is a key in this hashtable, as determined by the equals method; false otherwise.
Throws:
java.lang.NullPointerException - if the key is null.
See Also:
contains(Object)

get

public java.lang.Object get(java.lang.Object key,
                            java.lang.Object key2)
Returns the value to which the specified key is mapped in this hashtable.

Parameters:
key - a key in the hashtable.
Returns:
the value to which the key is mapped in this hashtable; null if the key is not mapped to any value in this hashtable.
Throws:
java.lang.NullPointerException - if the key is null.
See Also:
#put(Object, Object)

get

public java.lang.Object get(DoubleKeyHashtable.Entry keyEntry)

put

public java.lang.Object put(java.lang.Object key1,
                            java.lang.Object key2,
                            java.lang.Object value)
Maps the specified key to the specified value in this hashtable. Neither the key nor the value can be null.

The value can be retrieved by calling the get method with a key that is equal to the original key.

Parameters:
key - the hashtable key.
value - the value.
Returns:
the previous value of the specified key in this hashtable, or null if it did not have one.
Throws:
java.lang.NullPointerException - if the key or value is null.
See Also:
Object.equals(Object), #get(Object)

removeValue

public java.lang.Object removeValue(java.lang.Object value)

remove

public java.lang.Object remove(java.lang.Object key1,
                               java.lang.Object key2)
Removes the key (and its corresponding value) from this hashtable. This method does nothing if the key is not in the hashtable.

Parameters:
key - the key that needs to be removed.
Returns:
the value to which the key had been mapped in this hashtable, or null if the key did not have a mapping.
Throws:
java.lang.NullPointerException - if the key is null.

remove

public java.lang.Object remove(DoubleKeyHashtable.Entry aEntry)

removePrimary

public int removePrimary(java.lang.Object key)

removeSecondary

public int removeSecondary(java.lang.Object key)

clear

public void clear()
Clears this hashtable so that it contains no keys.


clone

public java.lang.Object clone()
Creates a shallow copy of this hashtable. All the structure of the hashtable itself is copied, but the keys and values are not cloned. This is a relatively expensive operation.

Overrides:
clone in class java.lang.Object
Returns:
a clone of the hashtable.

toString

public java.lang.String toString()
Returns a string representation of this DoubleKeyHashtable object in the form of a set of entries, enclosed in braces and separated by the ASCII characters "" (comma and space). Each entry is rendered as the key, an equals sign =, and the associated element, where the toString method is used to convert the key and element to strings.

Overrides to toString method of Object.

Overrides:
toString in class java.lang.Object
Returns:
a string representation of this hashtable.

keySet

public java.util.Set keySet()
Returns a Set view of the keys contained in this DoubleKeyHashtable. The Set is backed by the DoubleKeyHashtable, so changes to the DoubleKeyHashtable are reflected in the Set, and vice-versa. The Set supports element removal (which removes the corresponding entry from the DoubleKeyHashtable), but not element addition.

Returns:
a set view of the keys contained in this map.
Since:
1.2

entrySet

public java.util.Set entrySet()
Returns a Set view of the entries contained in this DoubleKeyHashtable. Each element in this collection is a Map.Entry. The Set is backed by the DoubleKeyHashtable, so changes to the DoubleKeyHashtable are reflected in the Set, and vice-versa. The Set supports element removal (which removes the corresponding entry from the DoubleKeyHashtable), but not element addition.

Returns:
a set view of the mappings contained in this map.
Since:
1.2
See Also:
Map.Entry

values

public java.util.Collection values()
Returns a Collection view of the values contained in this DoubleKeyHashtable. The Collection is backed by the DoubleKeyHashtable, so changes to the DoubleKeyHashtable are reflected in the Collection, and vice-versa. The Collection supports element removal (which removes the corresponding entry from the DoubleKeyHashtable), but not element addition.

Returns:
a collection view of the values contained in this map.
Since:
1.2

equals

public boolean equals(java.lang.Object o)
Compares the specified Object with this Map for equality, as per the definition in the Map interface.

Overrides:
equals in class java.lang.Object
Parameters:
o - object to be compared for equality with this DoubleKeyHashtable
Returns:
true if the specified Object is equal to this Map.
Since:
1.2
See Also:
Map.equals(Object)

hashCode

public int hashCode()
Returns the hash code value for this Map as per the definition in the Map interface.

Overrides:
hashCode in class java.lang.Object
Since:
1.2
See Also:
Map.hashCode()

test

public void test()

createTestStringKey

public java.lang.String createTestStringKey(java.lang.String a,
                                            java.lang.String b)

main

public static void main(java.lang.String[] args)