java.util.Dictionary<K,V>
java.util.Hashtable<K,V>
is new.
java.lang.Object
public class Hashtable<K,V>
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.
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<String, Integer> numbers
= new Hashtable<String, Integer>();
numbers.put("one", 1);
numbers.put("two", 2);
numbers.put("three", 3);
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 = numbers.get("two");
if (n != null) {
System.out.println("two = " + n);
}
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
Iterators
returned by the
iterator
method
and listIterator methods
of the
collections
Collections
returned by all of
this class's
Hashtable's
"collection view methods" are fail-fast: if the Hashtable is structurally modified at any time after the
iterator
Iterator
is created, in any way except through the
iterator's
Iterator's
own
remove
method,
or add methods,
the
iterator
Iterator
will throw a
ConcurrentModificationException
.
ConcurrentModificationException.
Thus, in the face of concurrent modification, the
iterator
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
elements
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 | |
|---|---|
|
Hashtable
() Constructs a new, empty hashtable with a default initial capacity (11) and load
factor (0.75).
|
|
|
Hashtable
(int initialCapacity) Constructs a new, empty hashtable with the specified initial capacity and default load
factor (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
Returns true if this
hashtable
|
| Enumeration < V > |
elements
() Returns an enumeration of the values in this hashtable. |
| Set < Map.Entry < K , V |
entrySet
() Returns a
Set
|
| 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, or null if
map contains no mapping for the key.
|
| 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
|
| V |
put
(
K
key,
V
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
hashtable.
|
| 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
|
| Methods inherited from class java.lang. Object |
|---|
| finalize , getClass , notify , notifyAll , wait , wait , wait |
| Constructor Detail |
|---|
public Hashtable(int initialCapacity,
float loadFactor)
public Hashtable(int initialCapacity)
factor (0.75).
public Hashtable()
factor (0.75).
public Hashtable(Map<? extends K,? extends V> t)
factor (0.75).
| Method Detail |
|---|
public int size()
public boolean isEmpty()
public Enumeration<K> keys()
public Enumeration<V> elements()
public boolean contains(Object value)
containsKey
Note that this method is identical in functionality to
containsValue
,
containsValue,
(which is part of the
Map
Map
interface in the collections framework).
for
- if the value is null
public boolean containsValue(Object value)
hashtable
Note that this method is identical in functionality to
contains
contains
(which predates the
Map
Map
interface).
hashtable
tested
value
null
public boolean containsKey(Object key)
key
null
public V get(Object key)
mapped, or null if
map contains no mapping for the key.
More formally, if this map contains a mapping from a key k to a value v such that (key.equals(k)), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)
key - the key whose associated value is to be returned
specified
key is
mapped, or
this map contains no mapping for
the key
specified
key is
null
protected void rehash()
public V put(K key,
V value)
The value can be retrieved by calling the get method with a key that is equal to the original key.
key
value
one
null
public V remove(Object key)
removed
mapping
null
public void putAll(Map<? extends K,? extends V> t)
map
hashtable.
hashtable
map.
mappings
map
null
public void clear()
public Object clone()
hashtable
public String toString()
Overrides to
toString
method of
Object
.
hashtable
public Set<K> keySet()
Returns a
Set
view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's own
remove
operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the
Iterator.remove
,
Set.remove
,
removeAll
,
retainAll
, and
clear
operations. It does not support the
add
or
addAll
operations.
map
public Set<Map.Entry<K,V>> entrySet()
Returns a
Set
view of the mappings contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's own
remove
operation, or through the
setValue
operation on a map entry returned by the iterator) the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the
Iterator.remove
,
Set.remove
,
removeAll
,
retainAll
and
clear
operations. It does not support the
add
or
addAll
operations.
map
public Collection<V> values()
Returns a
Collection
view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. If the map is modified while an iteration over the collection is in progress (except through the iterator's own
remove
operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via the
Iterator.remove
,
Collection.remove
,
removeAll
,
retainAll
and
clear
operations. It does not support the
add
or
addAll
operations.
map
public boolean equals(Object o)
hashtable
Map
public int hashCode()