- java.lang.Object
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- java.util.Dictionary<K,V>
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- java.util.Hashtable<K,V>
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- Type Parameters:
K- the type of keys maintained by this mapV- the type of mapped values
- All Implemented Interfaces:
Serializable,Cloneable,Map<K,V>
- Direct Known Subclasses:
Properties,UIDefaults
public class Hashtable<K,V> extends Dictionary<K,V> implements Map<K,V>, Cloneable, Serializable
This class implements a hash table, which maps keys to values. Any non-nullobject 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
hashCodemethod and theequalsmethod.An instance of
Hashtablehas 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
Hashtableoperations, includinggetandput).The initial capacity controls a tradeoff between wasted space and the need for
rehashoperations, which are time-consuming. Norehashoperations will ever occur if the initial capacity is greater than the maximum number of entries theHashtablewill 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
iteratormethod 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 ownremovemethod, 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'skeysandelementsmethods are not fail-fast; if the Hashtable is structurally modified at any time after the enumeration is created then the results of enumerating are undefined.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
ConcurrentModificationExceptionon 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
Mapinterface, making it a member of the Java Collections Framework. Unlike the new collection implementations,Hashtableis synchronized. If a thread-safe implementation is not needed, it is recommended to useHashMapin place ofHashtable. If a thread-safe highly-concurrent implementation is desired, then it is recommended to useConcurrentHashMapin place ofHashtable.- Since:
- 1.0
- See Also:
Object.equals(java.lang.Object),Object.hashCode(),rehash(),Collection,Map,HashMap,TreeMap, Serialized Form
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Constructor Summary
Constructors 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<? extends K,? extends V> t)Constructs a new hashtable with the same mappings as the given Map.
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Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description voidclear()Clears this hashtable so that it contains no keys.Objectclone()Creates a shallow copy of this hashtable.Vcompute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)Attempts to compute a mapping for the specified key and its current mapped value (ornullif there is no current mapping).VcomputeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)If the specified key is not already associated with a value (or is mapped tonull), attempts to compute its value using the given mapping function and enters it into this map unlessnull.VcomputeIfPresent(K key, BiFunction<? super K,? super V,? extends V> 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.booleancontains(Object value)Tests if some key maps into the specified value in this hashtable.booleancontainsKey(Object key)Tests if the specified object is a key in this hashtable.booleancontainsValue(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 aSetview of the mappings contained in this map.booleanequals(Object o)Compares the specified Object with this Map for equality, as per the definition in the Map interface.Vget(Object key)Returns the value to which the specified key is mapped, ornullif this map contains no mapping for the key.inthashCode()Returns the hash code value for this Map as per the definition in the Map interface.booleanisEmpty()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 aSetview of the keys contained in this map.Vmerge(K key, V value, BiFunction<? super V,? super V,? extends V> 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.Vput(K key, V value)Maps the specifiedkeyto the specifiedvaluein this hashtable.voidputAll(Map<? extends K,? extends V> t)Copies all of the mappings from the specified map to this hashtable.protected voidrehash()Increases the capacity of and internally reorganizes this hashtable, in order to accommodate and access its entries more efficiently.Vremove(Object key)Removes the key (and its corresponding value) from this hashtable.intsize()Returns the number of keys in this hashtable.StringtoString()Returns a string representation of thisHashtableobject in the form of a set of entries, enclosed in braces and separated by the ASCII characters "," (comma and space).Collection<V>values()Returns aCollectionview of the values contained in this map.-
Methods declared in interface java.util.Map
forEach, getOrDefault, putIfAbsent, remove, replace, replace, replaceAll
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Constructor Detail
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Hashtable
public Hashtable(int initialCapacity, float loadFactor)Constructs a new, empty hashtable with the specified initial capacity and the specified load factor.- Parameters:
initialCapacity- the initial capacity of the hashtable.loadFactor- the load factor of the hashtable.- Throws:
IllegalArgumentException- if the initial capacity is less than zero, or if the load factor is nonpositive.
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Hashtable
public Hashtable(int initialCapacity)
Constructs a new, empty hashtable with the specified initial capacity and default load factor (0.75).- Parameters:
initialCapacity- the initial capacity of the hashtable.- Throws:
IllegalArgumentException- if the initial capacity is less than zero.
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Hashtable
public Hashtable()
Constructs a new, empty hashtable with a default initial capacity (11) and load factor (0.75).
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Hashtable
public Hashtable(Map<? extends K,? extends V> t)
Constructs a new hashtable with the same mappings as the given Map. The hashtable is created with an initial capacity sufficient to hold the mappings in the given Map and a default load factor (0.75).- Parameters:
t- the map whose mappings are to be placed in this map.- Throws:
NullPointerException- if the specified map is null.- Since:
- 1.2
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Method Detail
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size
public int size()
Returns the number of keys in this hashtable.
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isEmpty
public boolean isEmpty()
Tests if this hashtable maps no keys to values.
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keys
public Enumeration<K> keys()
Returns an enumeration of the keys in this hashtable. Use the Enumeration methods on the returned object to fetch the keys sequentially. If the hashtable is structurally modified while enumerating over the keys then the results of enumerating are undefined.- Specified by:
keysin classDictionary<K,V>- Returns:
- an enumeration of the keys in this hashtable.
- See Also:
Enumeration,elements(),keySet(),Map
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elements
public Enumeration<V> elements()
Returns an enumeration of the values in this hashtable. Use the Enumeration methods on the returned object to fetch the elements sequentially. If the hashtable is structurally modified while enumerating over the values then the results of enumerating are undefined.- Specified by:
elementsin classDictionary<K,V>- Returns:
- an enumeration of the values in this hashtable.
- See Also:
Enumeration,keys(),values(),Map
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contains
public boolean contains(Object value)
Tests if some key maps into the specified value in this hashtable. This operation is more expensive than thecontainsKeymethod.Note that this method is identical in functionality to
containsValue, (which is part of theMapinterface in the collections framework).- Parameters:
value- a value to search for- Returns:
trueif and only if some key maps to thevalueargument in this hashtable as determined by theequalsmethod;falseotherwise.- Throws:
NullPointerException- if the value isnull
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containsValue
public boolean containsValue(Object value)
Returns true if this hashtable maps one or more keys to this value.Note that this method is identical in functionality to
contains(which predates theMapinterface).- Specified by:
containsValuein interfaceMap<K,V>- Parameters:
value- value whose presence in this hashtable is to be tested- Returns:
trueif this map maps one or more keys to the specified value- Throws:
NullPointerException- if the value isnull- Since:
- 1.2
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containsKey
public boolean containsKey(Object key)
Tests if the specified object is a key in this hashtable.- Specified by:
containsKeyin interfaceMap<K,V>- Parameters:
key- possible key- Returns:
trueif and only if the specified object is a key in this hashtable, as determined by theequalsmethod;falseotherwise.- Throws:
NullPointerException- if the key isnull- See Also:
contains(Object)
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get
public V get(Object key)
Returns the value to which the specified key is mapped, ornullif this map contains no mapping for the key.More formally, if this map contains a mapping from a key
kto a valuevsuch that(key.equals(k)), then this method returnsv; otherwise it returnsnull. (There can be at most one such mapping.)- Specified by:
getin interfaceMap<K,V>- Specified by:
getin classDictionary<K,V>- Parameters:
key- the key whose associated value is to be returned- Returns:
- the value to which the specified key is mapped, or
nullif this map contains no mapping for the key - Throws:
NullPointerException- if the specified key is null- See Also:
put(Object, Object)
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rehash
protected void rehash()
Increases the capacity of and internally reorganizes this hashtable, in order to accommodate and access its entries more efficiently. This method is called automatically when the number of keys in the hashtable exceeds this hashtable's capacity and load factor.
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put
public V put(K key, V value)
Maps the specifiedkeyto the specifiedvaluein this hashtable. Neither the key nor the value can benull.The value can be retrieved by calling the
getmethod with a key that is equal to the original key.- Specified by:
putin interfaceMap<K,V>- Specified by:
putin classDictionary<K,V>- Parameters:
key- the hashtable keyvalue- the value- Returns:
- the previous value of the specified key in this hashtable,
or
nullif it did not have one - Throws:
NullPointerException- if the key or value isnull- See Also:
Object.equals(Object),get(Object)
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remove
public V remove(Object key)
Removes the key (and its corresponding value) from this hashtable. This method does nothing if the key is not in the hashtable.- Specified by:
removein interfaceMap<K,V>- Specified by:
removein classDictionary<K,V>- Parameters:
key- the key that needs to be removed- Returns:
- the value to which the key had been mapped in this hashtable,
or
nullif the key did not have a mapping - Throws:
NullPointerException- if the key isnull
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putAll
public void putAll(Map<? extends K,? extends V> t)
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.- Specified by:
putAllin interfaceMap<K,V>- Parameters:
t- mappings to be stored in this map- Throws:
NullPointerException- if the specified map is null- Since:
- 1.2
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clear
public void clear()
Clears this hashtable so that it contains no keys.
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clone
public Object clone()
Creates a shallow copy of this hashtable. All the structure of the hashtable itself is copied, but the keys and values are not cloned. This is a relatively expensive operation.
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toString
public String toString()
Returns a string representation of thisHashtableobject in the form of a set of entries, enclosed in braces and separated by the ASCII characters "," (comma and space). Each entry is rendered as the key, an equals sign=, and the associated element, where thetoStringmethod is used to convert the key and element to strings.
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keySet
public Set<K> keySet()
Returns aSetview 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 ownremoveoperation), 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, andclearoperations. It does not support theaddoraddAlloperations.
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entrySet
public Set<Map.Entry<K,V>> entrySet()
Returns aSetview 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 ownremoveoperation, or through thesetValueoperation 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,retainAllandclearoperations. It does not support theaddoraddAlloperations.
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values
public Collection<V> values()
Returns aCollectionview 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 ownremoveoperation), 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,retainAllandclearoperations. It does not support theaddoraddAlloperations.
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equals
public boolean equals(Object o)
Compares the specified Object with this Map for equality, as per the definition in the Map interface.
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hashCode
public int hashCode()
Returns the hash code value for this Map as per the definition in the Map interface.
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computeIfAbsent
public V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)
If the specified key is not already associated with a value (or is mapped tonull), attempts to compute its value using the given mapping function and enters it into this map unlessnull.If the mapping function returns
null, no mapping is recorded. If the mapping 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);The mapping function should not modify this map during computation.
This method will, on a best-effort basis, throw a
ConcurrentModificationExceptionif the mapping function modified this map during computation.- Specified by:
computeIfAbsentin interfaceMap<K,V>- Parameters:
key- key with which the specified value is to be associatedmappingFunction- the mapping function to compute a value- Returns:
- the current (existing or computed) value associated with the specified key, or null if the computed value is null
- Throws:
ConcurrentModificationException- if it is detected that the mapping function modified this map
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computeIfPresent
public V computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> 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.If the remapping function returns
null, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.The remapping function should not modify this map during computation.
This method will, on a best-effort basis, throw a
ConcurrentModificationExceptionif the remapping function modified this map during computation.- Specified by:
computeIfPresentin interfaceMap<K,V>- Parameters:
key- key with which the specified value is to be associatedremappingFunction- the remapping function to compute a value- Returns:
- the new value associated with the specified key, or null if none
- Throws:
ConcurrentModificationException- if it is detected that the remapping function modified this map
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compute
public V compute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
Attempts to compute a mapping for the specified key and its current mapped value (ornullif there is no current mapping). For example, to either create or append aStringmsg to a value mapping:
(Methodmap.compute(key, (k, v) -> (v == null) ? msg : v.concat(msg))merge()is often simpler to use for such purposes.)If the remapping function returns
null, the mapping is removed (or remains absent if initially absent). If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.The remapping function should not modify this map during computation.
This method will, on a best-effort basis, throw a
ConcurrentModificationExceptionif the remapping function modified this map during computation.- Specified by:
computein interfaceMap<K,V>- Parameters:
key- key with which the specified value is to be associatedremappingFunction- the remapping function to compute a value- Returns:
- the new value associated with the specified key, or null if none
- Throws:
ConcurrentModificationException- if it is detected that the remapping function modified this map
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merge
public V merge(K key, V value, BiFunction<? super V,? super V,? extends V> 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. Otherwise, replaces the associated value with the results of the given remapping function, or removes if the result isnull. This method may be of use when combining multiple mapped values for a key. For example, to either create or append aString msgto a value mapping:map.merge(key, msg, String::concat)If the remapping function returns
null, the mapping is removed. If the remapping function itself throws an (unchecked) exception, the exception is rethrown, and the current mapping is left unchanged.The remapping function should not modify this map during computation.
This method will, on a best-effort basis, throw a
ConcurrentModificationExceptionif the remapping function modified this map during computation.- Specified by:
mergein interfaceMap<K,V>- Parameters:
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 remapping function to recompute a value if present- Returns:
- the new value associated with the specified key, or null if no value is associated with the key
- Throws:
ConcurrentModificationException- if it is detected that the remapping function modified this map
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