public class Hashtable<K,V> extends Dictionary<K,V> implements Map<K,V>, Cloneable, Serializable
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:
{
@code
Hashtable<String, Integer> numbers = new Hashtable<String, Integer>();
numbers.put("one", 1);
numbers.put("two", 2);
numbers.put("three", 3);
}
To retrieve a number, use the following code:
{
@code
Integer n = numbers.get("two");
if (n != null) {
System.out.println("two = " + n);
}
}
The iterators returned by the iterator method of the collections returned by all of this
class'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 method, 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 elements 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.
| Constructor and Description |
|---|
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> t)
Constructs a new hashtable with the same mappings as the given Map.
|
| Modifier and Type | Method and Description |
|---|---|
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 mappings contained in this map. |
boolean |
equals(Object o)
Compares the specified Object with this Map for equality, as per the definition in the Map
interface.
|
V |
get(Object key)
Returns the value to which the specified key is mapped, or
null if this 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 view of the keys contained in this map. |
V |
put(K key,
V value)
Maps the specified
key to the specified value in this hashtable. |
void |
putAll(Map<? extends K,? extends V> t)
Copies all of the mappings from the specified map to this 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 key)
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 map. |
public Hashtable()
public Hashtable(int initialCapacity)
initialCapacity - the initial capacity of the hashtable.IllegalArgumentException - if the initial capacity is less than zero.public Hashtable(int initialCapacity,
float loadFactor)
initialCapacity - the initial capacity of the hashtable.loadFactor - the load factor of the hashtable.IllegalArgumentException - if the initial capacity is less than zero, or if the load factor is nonpositive.public Hashtable(Map<? extends K,? extends V> t)
t - the map whose mappings are to be placed in this map.NullPointerException - if the specified map is null.public void clear()
public Object clone()
public boolean contains(Object value)
containsKey method.
Note that this method is identical in functionality to containsValue,
(which is part of the Map interface in the collections framework).
value - a value to search fortrue if and only if some key maps to the value argument in
this hashtable as determined by the equals method; false
otherwise.NullPointerException - if the value is nullpublic boolean containsKey(Object key)
containsKey in interface Map<K,V>key - possible keytrue if and only if the specified object is a key in this hashtable, as
determined by the equals method; false otherwise.NullPointerException - if the key is nullcontains(Object)public boolean containsValue(Object value)
Note that this method is identical in functionality to contains (which
predates the Map interface).
containsValue in interface Map<K,V>value - value whose presence in this hashtable is to be testedNullPointerException - if the value is nullpublic Enumeration<V> elements()
elements in class Dictionary<K,V>Enumeration,
keys(),
values(),
Mappublic Set<Map.Entry<K,V>> entrySet()
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.public boolean equals(Object o)
public V get(Object key)
null if this 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.)
get in interface Map<K,V>get in class Dictionary<K,V>key - the key whose associated value is to be returnednull if this map contains
no mapping for the keyNullPointerException - if the specified key is nullput(Object, Object)public int hashCode()
public boolean isEmpty()
public Enumeration<K> keys()
keys in class Dictionary<K,V>Enumeration,
elements(),
keySet(),
Mappublic Set<K> keySet()
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.public V put(K key, V value)
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.
put in interface Map<K,V>put in class Dictionary<K,V>key - the hashtable keyvalue - the valuenull if it
did not have oneNullPointerException - if the key or value is nullObject.equals(Object),
get(Object)public void putAll(Map<? extends K,? extends V> t)
putAll in interface Map<K,V>t - mappings to be stored in this mapNullPointerException - if the specified map is nullprotected void rehash()
public V remove(Object key)
remove in interface Map<K,V>remove in class Dictionary<K,V>key - the key that needs to be removednull if
the key did not have a mappingNullPointerException - if the key is nullpublic int size()
public String toString()
public Collection<V> values()
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.