java.lang.Object | ||
↳ | java.util.Dictionary<K, V> | |
↳ | java.util.Hashtable<K, V> |
Known Direct Subclasses |
Known Indirect Subclasses |
This class implements a hashtable, which maps keys to values. Any
non-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.
Public Constructors | |||||||||||
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Constructs a new, empty hashtable with the specified initial
capacity and the specified load factor.
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Constructs a new, empty hashtable with the specified initial capacity
and default load factor (0.75).
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Constructs a new, empty hashtable with a default initial capacity (11)
and load factor (0.75).
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Constructs a new hashtable with the same mappings as the given
Map.
|
Public Methods | |||||||||||
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Clears this hashtable so that it contains no keys.
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Creates a shallow copy of this hashtable.
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Tests if some key maps into the specified value in this hashtable.
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Tests if the specified object is a key in this hashtable.
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Returns true if this hashtable maps one or more keys to this value.
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Returns an enumeration of the values in this hashtable.
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Returns a
Set view of the mappings contained in this map. | |||||||||||
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,
or
null if this map contains no mapping for the key. | |||||||||||
Returns the hash code value for this Map as per the definition in the
Map interface.
| |||||||||||
Tests if this hashtable maps no keys to values.
| |||||||||||
Returns a
Set view of the keys contained in this map. | |||||||||||
Returns an enumeration of the keys in this hashtable.
| |||||||||||
Maps the specified
key to the specified
value in this hashtable. | |||||||||||
Copies all of the mappings from the specified map to this hashtable.
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Removes the key (and its corresponding value) from this
hashtable.
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Returns the number of keys in this hashtable.
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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).
| |||||||||||
Returns a
Collection view of the values contained in this map. |
Protected Methods | |||||||||||
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Increases the capacity of and internally reorganizes this
hashtable, in order to accommodate and access its entries more
efficiently.
|
[Expand]
Inherited Methods | |||||||||||
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From class
java.util.Dictionary
| |||||||||||
From class
java.lang.Object
| |||||||||||
From interface
java.util.Map
|
Constructs a new, empty hashtable with the specified initial capacity and the specified load factor.
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. |
---|
Constructs a new, empty hashtable with the specified initial capacity and default load factor (0.75).
initialCapacity | the initial capacity of the hashtable. |
---|
IllegalArgumentException | if the initial capacity is less than zero. |
---|
Constructs a new, empty hashtable with a default initial capacity (11) and load factor (0.75).
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).
t | the map whose mappings are to be placed in this map. |
---|
NullPointerException | if the specified map is null. |
---|
Clears this hashtable so that it contains no keys.
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.
Tests if some key maps into the specified value in this hashtable.
This operation is more expensive than the 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 for |
---|
true
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 null
|
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Tests if the specified object is a key in this hashtable.
key | possible key |
---|
true
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 null |
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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 the Map
interface).
value | value whose presence in this hashtable is to be tested |
---|
NullPointerException | if the value is null |
---|
Returns an enumeration of the values in this hashtable. Use the Enumeration methods on the returned object to fetch the elements sequentially.
Returns a 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.
Compares the specified Object with this Map for equality, as per the definition in the Map interface.
o | object to be compared for equality with this hashtable |
---|
Returns the value to which the specified key is mapped,
or 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.)
key | the key whose associated value is to be returned |
---|
null
if this map contains no mapping for the keyNullPointerException | if the specified key is null |
---|
Returns the hash code value for this Map as per the definition in the Map interface.
Tests if this hashtable maps no keys to values.
true
if this hashtable maps no keys to values;
false
otherwise.
Returns a 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.
Returns an enumeration of the keys in this hashtable.
Maps the specified 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.
key | the hashtable key |
---|---|
value | the value |
null
if it did not have oneNullPointerException | if the key or value is
null |
---|
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.
t | mappings to be stored in this map |
---|
NullPointerException | if the specified map is null |
---|
Removes the key (and its corresponding value) from this hashtable. This method does nothing if the key is not in the hashtable.
key | the key that needs to be removed |
---|
null
if the key did not have a mappingNullPointerException | if the key is null
|
---|
Returns the number of keys in this hashtable.
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). Each entry is rendered as the key, an equals sign =, and the associated element, where the toString method is used to convert the key and element to strings.
Returns a 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.
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.