- 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
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 Summary
ConstructorDescriptionConstructs 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 Summary
Modifier and TypeMethodDescriptionvoid
clear()
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, ornull
if there is no current mapping (optional operation).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
(optional operation).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 (optional operation).boolean
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.elements()
Returns an enumeration of the values in this hashtable.entrySet()
Returns aSet
view of the mappings contained in this map.boolean
Compares 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, ornull
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.keys()
Returns an enumeration of the keys in this hashtable.keySet()
Returns aSet
view 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 (optional operation).Maps the specifiedkey
to the specifiedvalue
in this hashtable.void
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.Removes the key (and its corresponding value) from this hashtable.int
size()
Returns the number of keys in this hashtable.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).values()
Returns aCollection
view of the values contained in this map.Methods declared in interface java.util.Map
forEach, getOrDefault, putIfAbsent, remove, replace, replace, replaceAll
-
Constructor Details
-
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.
-
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.
-
Hashtable
public Hashtable()Constructs a new, empty hashtable with a default initial capacity (11) and load factor (0.75). -
Hashtable
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
-
-
Method Details
-
size
-
isEmpty
-
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:
keys
in classDictionary<K,
V> - Returns:
- an enumeration of the keys in this hashtable.
- See Also:
-
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:
elements
in classDictionary<K,
V> - Returns:
- an enumeration of the values in this hashtable.
- See Also:
-
contains
Tests if some key maps into the specified value in this hashtable. This operation is more expensive than thecontainsKey
method.Note that this method is identical in functionality to
containsValue
, (which is part of theMap
interface in the collections framework).- Parameters:
value
- a value to search for- Returns:
true
if and only if some key maps to thevalue
argument in this hashtable as determined by theequals
method;false
otherwise.- Throws:
NullPointerException
- if the value isnull
-
containsValue
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 theMap
interface).- Specified by:
containsValue
in interfaceMap<K,
V> - Parameters:
value
- value whose presence in this hashtable is to be tested- Returns:
true
if this map maps one or more keys to the specified value- Throws:
NullPointerException
- if the value isnull
- Since:
- 1.2
-
containsKey
Tests if the specified object is a key in this hashtable.- Specified by:
containsKey
in interfaceMap<K,
V> - Parameters:
key
- possible key- Returns:
true
if and only if the specified object is a key in this hashtable, as determined by theequals
method;false
otherwise.- Throws:
NullPointerException
- if the key isnull
- See Also:
-
get
Returns the value to which the specified key is mapped, ornull
if this map contains no mapping for the key.More formally, if this map contains a mapping from a key
k
to a valuev
such that(key.equals(k))
, then this method returnsv
; otherwise it returnsnull
. (There can be at most one such mapping.)- Specified by:
get
in interfaceMap<K,
V> - Specified by:
get
in 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
null
if this map contains no mapping for the key - Throws:
NullPointerException
- if the specified key is null- See Also:
-
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. -
put
Maps the specifiedkey
to the specifiedvalue
in this hashtable. Neither the key nor the value can benull
.The value can be retrieved by calling the
get
method with a key that is equal to the original key.- Specified by:
put
in interfaceMap<K,
V> - Specified by:
put
in classDictionary<K,
V> - Parameters:
key
- the hashtable keyvalue
- the value- Returns:
- the previous value of the specified key in this hashtable,
or
null
if it did not have one - Throws:
NullPointerException
- if the key or value isnull
- See Also:
-
remove
Removes the key (and its corresponding value) from this hashtable. This method does nothing if the key is not in the hashtable.- Specified by:
remove
in interfaceMap<K,
V> - Specified by:
remove
in 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
null
if the key did not have a mapping - Throws:
NullPointerException
- if the key isnull
-
putAll
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:
putAll
in interfaceMap<K,
V> - Parameters:
t
- mappings to be stored in this map- Throws:
NullPointerException
- if the specified map is null- Since:
- 1.2
-
clear
-
clone
-
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). Each entry is rendered as the key, an equals sign=
, and the associated element, where thetoString
method is used to convert the key and element to strings. -
keySet
Returns aSet
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. -
entrySet
Returns aSet
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. -
values
Returns aCollection
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. -
equals
Compares the specified Object with this Map for equality, as per the definition in the Map interface. -
hashCode
-
computeIfAbsent
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
(optional operation).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
ConcurrentModificationException
if the mapping function modified this map during computation.- Specified by:
computeIfAbsent
in 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
-
computeIfPresent
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 (optional operation).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
ConcurrentModificationException
if the remapping function modified this map during computation.- Specified by:
computeIfPresent
in 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
-
compute
Attempts to compute a mapping for the specified key and its current mapped value, ornull
if there is no current mapping (optional operation). 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 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
ConcurrentModificationException
if the remapping function modified this map during computation.- Specified by:
compute
in 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
-
merge
If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value (optional operation). 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 msg
to 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
ConcurrentModificationException
if the remapping function modified this map during computation.- Specified by:
merge
in 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
-