- Type Parameters:
- K- the type of keys maintained by this map
- V- the type of mapped values
- All Implemented Interfaces:
- Serializable,- Cloneable,- Map<K,- V> 
- Direct Known Subclasses:
- Properties,- UIDefaults
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; 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 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.
- Since:
- 1.0
- See Also:
- 
Constructor SummaryConstructorsConstructorDescriptionConstructs 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.Constructs a new hashtable with the same mappings as the given Map.
- 
Method SummaryModifier and TypeMethodDescriptionvoidclear()Clears this hashtable so that it contains no keys.clone()Creates a shallow copy of this hashtable.Attempts to compute a mapping for the specified key and its current mapped value (ornullif there is no current mapping).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.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.booleanTests 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.elements()Returns an enumeration of the values in this hashtable.entrySet()Returns aSetview of the mappings contained in this map.booleanCompares the specified Object with this Map for equality, as per the definition in the Map interface.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.keys()Returns an enumeration of the keys in this hashtable.keySet()Returns aSetview of the keys contained in this map.If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value.Maps the specifiedkeyto the specifiedvaluein this hashtable.voidCopies 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.Removes the key (and its corresponding value) from this hashtable.intsize()Returns the number of keys in this hashtable.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).values()Returns aCollectionview of the values contained in this map.Methods declared in interface java.util.MapforEach, getOrDefault, putIfAbsent, remove, replace, replace, replaceAll
- 
Constructor Details- 
Hashtablepublic 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.
 
- 
Hashtablepublic 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.
 
- 
Hashtablepublic Hashtable()Constructs a new, empty hashtable with a default initial capacity (11) and load factor (0.75).
- 
HashtableConstructs 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
 
 
- 
- 
Method Details- 
sizepublic int size()Returns the number of keys in this hashtable.
- 
isEmptypublic boolean isEmpty()Tests if this hashtable maps no keys to values.
- 
keysReturns 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 class- Dictionary<K,- V> 
- Returns:
- an enumeration of the keys in this hashtable.
- See Also:
 
- 
elementsReturns 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 class- Dictionary<K,- V> 
- Returns:
- an enumeration of the values in this hashtable.
- See Also:
 
- 
containsTests 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 the- valueargument in this hashtable as determined by the- equalsmethod;- falseotherwise.
- Throws:
- NullPointerException- if the value is- null
 
- 
containsValueReturns 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 interface- Map<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 is- null
- Since:
- 1.2
 
- 
containsKeyTests if the specified object is a key in this hashtable.- Specified by:
- containsKeyin interface- Map<K,- V> 
- Parameters:
- key- possible key
- Returns:
- trueif and only if the specified object is a key in this hashtable, as determined by the- equalsmethod;- falseotherwise.
- Throws:
- NullPointerException- if the key is- null
- See Also:
 
- 
getReturns 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 interface- Map<K,- V> 
- Specified by:
- getin class- Dictionary<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:
 
- 
rehashprotected 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.
- 
putMaps 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 interface- Map<K,- V> 
- Specified by:
- putin class- Dictionary<K,- V> 
- Parameters:
- key- the hashtable key
- value- 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 is- null
- See Also:
 
- 
removeRemoves the key (and its corresponding value) from this hashtable. This method does nothing if the key is not in the hashtable.- Specified by:
- removein interface- Map<K,- V> 
- Specified by:
- removein class- Dictionary<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 is- null
 
- 
putAllCopies 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 interface- Map<K,- V> 
- Parameters:
- t- mappings to be stored in this map
- Throws:
- NullPointerException- if the specified map is null
- Since:
- 1.2
 
- 
clearpublic void clear()Clears this hashtable so that it contains no keys.
- 
cloneCreates 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.
- 
toStringReturns 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.
- 
keySetReturns 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.
- 
entrySetReturns 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.
- 
valuesReturns 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.
- 
equalsCompares the specified Object with this Map for equality, as per the definition in the Map interface.
- 
hashCodepublic int hashCode()Returns the hash code value for this Map as per the definition in the Map interface.
- 
computeIfAbsentIf 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 interface- Map<K,- V> 
- Parameters:
- key- key with which the specified value is to be associated
- mappingFunction- 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
 
- 
computeIfPresentIf 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 interface- Map<K,- V> 
- Parameters:
- key- key with which the specified value is to be associated
- remappingFunction- 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
 
- 
computeAttempts 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 interface- Map<K,- V> 
- Parameters:
- key- key with which the specified value is to be associated
- remappingFunction- 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
 
- 
mergeIf 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 interface- Map<K,- V> 
- Parameters:
- key- key with which the resulting value is to be associated
- value- 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 key
- remappingFunction- 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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