public classHashtable<K,V>extendsDictionary<K,V>implementsMap<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 thehashCode method and theequals method.
An instance ofHashtable has two parameters that affect its performance:initial capacity andload factor. Thecapacity is the number ofbuckets in the hash table, and theinitial capacity is simply the capacity at the time the hash table is created. Note that the hash table isopen: in the case of a "hash collision", a single bucket stores multiple entries, which must be searched sequentially. Theload 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 mostHashtable operations, includingget andput).
The initial capacity controls a tradeoff between wasted space and the need forrehash operations, which are time-consuming. Norehash operations willever occur if the initial capacity is greater than the maximum number of entries theHashtable 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 aHashtable, 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 theiterator method of the collections returned by all of this class's "collection view methods" arefail-fast: if the Hashtable is structurally modified at any time after the iterator is created, in any way except through the iterator's ownremove method, the iterator will throw aConcurrentModificationException. 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 arenot 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 throwConcurrentModificationException 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 theMap 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 useHashMap in place ofHashtable. If a thread-safe highly-concurrent implementation is desired, then it is recommended to useConcurrentHashMap in place ofHashtable.
Object.equals(java.lang.Object),Object.hashCode(),rehash(),Collection,Map,HashMap,TreeMap,Serialized Form| Constructor | 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<? extendsK,? extendsV> t) | Constructs a new hashtable with the same mappings as the given Map. |
| Modifier and Type | Method | Description |
|---|---|---|
void | clear() | Clears this hashtable so that it contains no keys. |
Object | clone() | Creates a shallow copy of this hashtable. |
V | compute(K key,BiFunction<? superK,? superV,? extendsV> remappingFunction) | Attempts to compute a mapping for the specified key and its current mapped value (or null if there is no current mapping). |
V | computeIfAbsent(K key,Function<? superK,? extendsV> mappingFunction) | If the specified key is not already associated with a value (or is mapped to null), attempts to compute its value using the given mapping function and enters it into this map unlessnull. |
V | computeIfPresent(K key,BiFunction<? superK,? superV,? extendsV> remappingFunction) | If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value. |
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. |
void | forEach(BiConsumer<? superK,? superV> action) | Performs the given action for each entry in this map until all entries have been processed or the action throws an exception. |
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. |
V | getOrDefault(Object key,V defaultValue) | Returns the value to which the specified key is mapped, or defaultValue 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 | merge(K key,V value,BiFunction<? superV,? superV,? extendsV> remappingFunction) | If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value. |
V | put(K key,V value) | Maps the specified key to the specifiedvalue in this hashtable. |
void | putAll(Map<? extendsK,? extendsV> t) | Copies all of the mappings from the specified map to this hashtable. |
V | putIfAbsent(K key,V value) | If the specified key is not already associated with a value (or is mapped to null) associates it with the given value and returnsnull, else returns the current value. |
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. |
boolean | remove(Object key,Object value) | Removes the entry for the specified key only if it is currently mapped to the specified value. |
V | replace(K key,V value) | Replaces the entry for the specified key only if it is currently mapped to some value. |
boolean | replace(K key,V oldValue,V newValue) | Replaces the entry for the specified key only if currently mapped to the specified value. |
void | replaceAll(BiFunction<? superK,? superV,? extendsV> function) | Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception. |
int | size() | Returns the number of keys in this hashtable. |
String | toString() | Returns a string representation of thisHashtable 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(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<? extendsK,? extendsV> 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 tocontainsValue, (which is part of theMap interface in the collections framework).
value - a value to search fortrue if and only if some key maps to thevalue argument in this hashtable as determined by theequals method;false otherwise.NullPointerException - if the value isnullpublic boolean containsValue(Object value)
Note that this method is identical in functionality tocontains (which predates theMap interface).
containsValue in interface Map<K,V>value - value whose presence in this hashtable is to be testedNullPointerException - if the value isnullpublic 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 theequals method;false otherwise.NullPointerException - if the key isnullcontains(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 keyk to a valuev such that(key.equals(k)), then this method returnsv; otherwise it returnsnull. (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 specifiedvalue in this hashtable. Neither the key nor the value can benull. The value can be retrieved by calling theget 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 isnullObject.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 isnullpublic void putAll(Map<? extendsK,? extendsV> 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 ownremove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via theIterator.remove,Set.remove,removeAll,retainAll, andclear operations. It does not support theadd oraddAll 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 ownremove operation, or through thesetValue 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 theIterator.remove,Set.remove,removeAll,retainAll andclear operations. It does not support theadd oraddAll 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 ownremove operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via theIterator.remove,Collection.remove,removeAll,retainAll andclear operations. It does not support theadd oraddAll operations.public boolean equals(Object o)
public int hashCode()
public V getOrDefault(Object key,V defaultValue)
MapdefaultValue if this map contains no mapping for the key.getOrDefault in interface Map<K,V>key - the key whose associated value is to be returneddefaultValue - the default mapping of the keydefaultValue if this map contains no mapping for the keypublic void forEach(BiConsumer<? superK,? superV> action)
Mappublic void replaceAll(BiFunction<? superK,? superV,? extendsV> function)
MapreplaceAll in interface Map<K,V>function - the function to apply to each entrypublic V putIfAbsent(K key,V value)
Mapnull) associates it with the given value and returnsnull, else returns the current value.putIfAbsent in interface Map<K,V>key - key with which the specified value is to be associatedvalue - value to be associated with the specified keynull if there was no mapping for the key. (Anull return can also indicate that the map previously associatednull with the key, if the implementation supports null values.)public boolean remove(Object key,Object value)
Mappublic boolean replace(K key,V oldValue,V newValue)
Mappublic V replace(K key,V value)
Mapreplace in interface Map<K,V>key - key with which the specified value is associatedvalue - value to be associated with the specified keynull if there was no mapping for the key. (Anull return can also indicate that the map previously associatednull with the key, if the implementation supports null values.)public V computeIfAbsent(K key,Function<? superK,? extendsV> mappingFunction)
Mapnull), attempts to compute its value using the given mapping function and enters it into this map unlessnull.If the function returnsnull no mapping is recorded. If the function itself throws an (unchecked) exception, the exception is rethrown, and no mapping is recorded. The most common usage is to construct a new object serving as an initial mapped value or memoized result, as in:
map.computeIfAbsent(key, k -> new Value(f(k)));Or to implement a multi-value map,Map<K,Collection<V>>, supporting multiple values per key:
map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);computeIfAbsent in interface Map<K,V>key - key with which the specified value is to be associatedmappingFunction - the function to compute a valuepublic V computeIfPresent(K key,BiFunction<? superK,? superV,? extendsV> remappingFunction)
MapIf the function returnsnull, the mapping is removed. If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.
computeIfPresent in interface Map<K,V>key - key with which the specified value is to be associatedremappingFunction - the function to compute a valuepublic V compute(K key,BiFunction<? superK,? superV,? extendsV> remappingFunction)
Mapnull if there is no current mapping). For example, to either create or append aString msg to a value mapping: map.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))merge() is often simpler to use for such purposes.)If the function returnsnull, the mapping is removed (or remains absent if initially absent). If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.
public V merge(K key,V value,BiFunction<? superV,? superV,? extendsV> remappingFunction)
Mapnull. This method may be of use when combining multiple mapped values for a key. For example, to either create or append aString msg to a value mapping: map.merge(key, msg, String::concat)If the function returnsnull the mapping is removed. If the function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.
merge in interface Map<K,V>key - key with which the resulting value is to be associatedvalue - the non-null value to be merged with the existing value associated with the key or, if no existing value or a null value is associated with the key, to be associated with the keyremappingFunction - the function to recompute a value if present