java.lang.Object java.util.Dictionary java.util.Hashtable
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 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.
The initial capacity and load factor parameters are merely hints to the implementation. The exact details as to when and whether the rehash method is invoked are implementation-dependent.
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 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, Hashtable 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 Enumerations returned by Hashtable'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 .
Constructor Summary | |
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Hashtable
() Constructs a new, empty hashtable with a default initial capacity (11) and load factor, which is 0.75 . |
|
Hashtable
(int initialCapacity) Constructs a new, empty hashtable with the specified initial capacity and default load factor, which is 0.75 . |
|
Hashtable
(int initialCapacity, float loadFactor) Constructs a new, empty hashtable with the specified initial capacity and the specified load factor. |
|
Hashtable
(
Map
<? extends
K
,? extends
V
Constructs a new hashtable with the same mappings as the given Map. |
Method Summary | |
---|---|
void |
clear
() Clears this hashtable so that it contains no keys. |
Object |
clone
() Creates a shallow copy of this hashtable. |
boolean |
contains
(
Object
value) Tests if some key maps into the specified value in this hashtable. |
boolean |
containsKey
(
Object
key) Tests if the specified object is a key in this hashtable. |
boolean |
containsValue
(
Object
value) Returns true if this Hashtable maps one or more keys to this value. |
Enumeration < V |
elements
() Returns an enumeration of the values in this hashtable. |
Set < Map.Entry < K , V |
entrySet
() Returns a Set view of the entries contained in this Hashtable. |
boolean |
equals
(
Object
o) Compares the specified Object with this Map for equality, as per the definition in the Map interface. |
V
|
get
(
Object
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. |
Enumeration < K |
keys
() Returns an enumeration of the keys in this hashtable. |
Set < K |
keySet
() Returns a Set view of the keys contained in this Hashtable. |
V
|
put
Maps the specified key to the specified value in this hashtable. |
void |
putAll
(
Map
<? extends
K
,? extends
V
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. |
protected void |
rehash
() Increases the capacity of and internally reorganizes this hashtable, in order to accommodate and access its entries more efficiently. |
V
|
remove
(
Object
Removes the key (and its corresponding value) from this hashtable. |
int |
size
() Returns the number of keys in this hashtable. |
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). |
Collection < V |
values
() Returns a Collection view of the values contained in this Hashtable. |
Methods inherited from class java.lang. Object |
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finalize , getClass , notify , notifyAll , wait , wait , wait |
Constructor Detail |
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public Hashtable(int initialCapacity, float loadFactor)
public Hashtable(int initialCapacity)
public Hashtable()
public Hashtable(Map<? extends K,? extends V> t)t)
Method Detail |
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public int size()
public boolean isEmpty()
public Enumeration< K> keys()
public Enumeration< V> elements()
public boolean contains(Object value)
Note that this method is identical in functionality to containsValue, (which is part of the Map interface in the collections framework).
public boolean containsValue(Object value)
Note that this method is identical in functionality to contains (which predates the Map interface).
public boolean containsKey(Object key)
public VObjectget(Object key)
protected void rehash()
public VObjectput( KObjectkey, VObjectvalue)
The value can be retrieved by calling the get method with a key that is equal to the original key.
public VObjectremove(Object key)
public void putAll(Map<? extends K,? extends V> t)t)
public void clear()
public Object clone()
public String toString()
Overrides to toString method of Object .
public Set< K> keySet()
public Set< Map.Entry< K, V>> entrySet()
public Collection< V> values()
public boolean equals(Object o)
public int hashCode()