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Class com.sun.java.util.collections.Hashtable

java.lang.Object
   |
   +----java.util.Dictionary
           |
           +----com.sun.java.util.collections.Hashtable

public class Hashtable
extends Dictionary
implements Map, Cloneable, 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 Hashtable 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 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 Hashtable 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 Hashtable 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 Hashtable, 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:

     Hashtable numbers = new Hashtable();
     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 JDK1.2, this class has been retrofitted to implement Map, so that it becomes a part of Java's collection framework. Unlike the new collection implementations, Vector is synchronized.

The Iterators returned by the iterator and listIterator methods of the Collections returned by all of Hashtable's "collection view methods" are fail-fast: if the Hashtable 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 java.util.Enumerations returned by Hashtable's keys and values methods are not fail-fast.

See Also:
equals, hashCode, rehash, Collection, Map, HashMap, TreeMap

Constructor Index

 o Hashtable()
Constructs a new, empty hashtable with a default capacity and load factor, which is 0.75.
 o Hashtable(int)
Constructs a new, empty hashtable with the specified initial capacity and default load factor, which is 0.75.
 o Hashtable(int, float)
Constructs a new, empty hashtable with the specified initial capacity and the specified load factor.
 o Hashtable(Map)
Constructs a new hashtable with the same mappings as the given Map.

Method Index

 o clear()
Clears this hashtable so that it contains no keys.
 o clone()
Creates a shallow copy of this hashtable.
 o contains(Object)
Tests if some key maps into the specified value in this hashtable.
 o containsKey(Object)
Tests if the specified object is a key in this hashtable.
 o containsValue(Object)
Returns true if this Hashtable maps one or more keys to this value.
 o elements()
Returns an enumeration of the values in this hashtable.
 o entrySet()
Returns a Set view of the entries contained in this Hashtable.
 o equals(Object)
Compares the specified Object with this Map for equality, as per the definition in the Map interface.
 o get(Object)
Returns the value to which the specified key is mapped in this hashtable.
 o hashCode()
Returns the hash code value for this Map as per the definition in the Map interface.
 o isEmpty()
Tests if this hashtable maps no keys to values.
 o keys()
Returns an enumeration of the keys in this hashtable.
 o keySet()
Returns a Set view of the keys contained in this Hashtable.
 o put(Object, Object)
Maps the specified key to the specified value in this hashtable.
 o putAll(Map)
Copies all of the mappings from the specified Map to this Hashtable These mappings will replace any mappings that this Hashtable had for any of the keys currently in the specified Map.
 o rehash()
Increases the capacity of and internally reorganizes this hashtable, in order to accommodate and access its entries more efficiently.
 o remove(Object)
Removes the key (and its corresponding value) from this hashtable.
 o size()
Returns the number of keys in this hashtable.
 o toString()
Returns a string representation of this Hashtable object in the form of a set of entries, enclosed in braces and separated by the ASCII characters "" (comma and space).
 o values()
Returns a Collection view of the values contained in this Hashtable.

Constructors

 o Hashtable
 public Hashtable(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: IllegalArgumentException
if the initial capacity is less than zero, or if the load factor is nonpositive.
 o Hashtable
 public Hashtable(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: IllegalArgumentException
if the initial capacity is less than zero.
 o Hashtable
 public Hashtable()
Constructs a new, empty hashtable with a default capacity and load factor, which is 0.75.

 o Hashtable
 public Hashtable(Map t)
Constructs a new hashtable with the same mappings as the given Map. The hashtable is created with a capacity of twice the number of entries in the given Map or 11 (whichever is greater), and a default load factor, which is 0.75.

Methods

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

Returns:
the number of keys in this hashtable.
Overrides:
size in class Dictionary
 o 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.
Overrides:
isEmpty in class Dictionary
 o keys
 public synchronized Enumeration keys()
Returns an enumeration of the keys in this hashtable.

Returns:
an enumeration of the keys in this hashtable.
Overrides:
keys in class Dictionary
See Also:
Enumeration, elements, keySet, Map
 o elements
 public synchronized Enumeration elements()
Returns an enumeration of the values in this hashtable. Use the java.util.Enumeration methods on the returned object to fetch the elements sequentially.

Returns:
an enumeration of the values in this hashtable.
Overrides:
elements in class Dictionary
See Also:
Enumeration, keys, values, Map
 o contains
 public synchronized boolean contains(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: NullPointerException
if the value is null.
See Also:
containsKey, containsValue, Map
 o containsValue
 public boolean containsValue(Object value)
Returns true if this Hashtable 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 Hashtable is to be tested.
See Also:
Map
 o containsKey
 public synchronized boolean containsKey(Object key)
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.
See Also:
contains
 o get
 public synchronized Object get(Object key)
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.
Overrides:
get in class Dictionary
See Also:
put
 o rehash
 protected void rehash()
Increases the capacity of and internally reorganizes this hashtable, in order to accommodate and access its entries more efficiently. This method is called automatically when the number of keys in the hashtable exceeds this hashtable's capacity and load factor.

 o put
 public synchronized Object put(Object key,
                                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: NullPointerException
if the key or value is null.
Overrides:
put in class Dictionary
See Also:
equals, get
 o remove
 public synchronized Object remove(Object key)
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.
Overrides:
remove in class Dictionary
 o putAll
 public synchronized void putAll(Map t)
Copies all of the mappings from the specified Map to this Hashtable These mappings will replace any mappings that this Hashtable had for any of the keys currently in the specified Map.

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

 o clone
 public synchronized 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.

Returns:
a clone of the hashtable.
Overrides:
clone in class Object
 o toString
 public synchronized String toString()
Returns a string representation of this Hashtable 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.

Returns:
a string representation of this hashtable.
Overrides:
toString in class Object
 o keySet
 public Set keySet()
Returns a Set view of the keys contained in this Hashtable. The Set is backed by the Hashtable, so changes to the Hashtable are reflected in the Set, and vice-versa. The Set supports element removal (which removes the corresponding entry from the Hashtable), but not element addition.

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

See Also:
Entry
 o values
 public Collection values()
Returns a Collection view of the values contained in this Hashtable. The Collection is backed by the Hashtable, so changes to the Hashtable are reflected in the Collection, and vice-versa. The Collection supports element removal (which removes the corresponding entry from the Hashtable) via its Iterator, but not element addition or "direct" element removal.

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

Returns:
true if the specified Object is equal to this Map.
Overrides:
equals in class Object
See Also:
equals
 o hashCode
 public synchronized int hashCode()
Returns the hash code value for this Map as per the definition in the Map interface.

Overrides:
hashCode in class Object
See Also:
hashCode

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