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:
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:
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.
As of the Java 2 platform v1.2, this class was retrofitted to
implement the Map interface, making it a member of the
Java Collections Framework. Unlike the new collection
implementations, Hashtable is synchronized. If a
thread-safe implementation is not needed, it is recommended to use
HashMap in place of Hashtable. If a thread-safe
highly-concurrent implementation is desired, then it is recommended
to use ConcurrentHashMap in place of
Hashtable.
Object.equals(java.lang.Object),
Object.hashCode(),
rehash(),
Collection,
Map,
HashMap,
TreeMap,
Serialized Form| 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. |
finalize, getClass, notify, notifyAll, wait, wait, waitcompute, computeIfAbsent, computeIfPresent, forEach, getOrDefault, merge, putIfAbsent, remove, replace, replace, replaceAllpublic 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(int initialCapacity)
initialCapacity - the initial capacity of the hashtable.IllegalArgumentException - if the initial capacity is less
than zero.public Hashtable()
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 int size()
public boolean isEmpty()
public Enumeration<K> keys()
keys in class Dictionary<K,V>Enumeration,
elements(),
keySet(),
Mappublic Enumeration<V> elements()
elements in class Dictionary<K,V>Enumeration,
keys(),
values(),
Mappublic 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 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 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 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)protected void rehash()
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 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 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 nullpublic void clear()
public Object clone()
public String toString()
public 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 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 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.public boolean equals(Object o)
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