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java.util
Class Hashtable<K,V>
java.lang.Object
java.util.Dictionary<K,V>
java.util.Hashtable<K,V>
-
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 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);
}
As of the Java 2 platform v1.2, this class has been retrofitted to implement Map, so that it becomes a part of Java's collection framework. Unlike the new collection implementations, Hashtable is synchronized.
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
This class is a member of the
Java Collections Framework
interface, making it a member of the
Java Collections Framework
.
Unlike the new collection implementations, Hashtable is synchronized.
-
Since:
-
JDK1.0
-
See Also:
-
Object.equals(java.lang.Object)
,
Object.hashCode()
,
rehash()
,
Collection
,
Map
,
HashMap
,
TreeMap
,
Serialized Form
Constructor Summary
|
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. |
Method Summary
|
void |
clear
()
Clears this hashtable so that it contains no keys. |
Object
|
clone
()
Creates a shallow copy of this hashtable. |
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. |
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. |
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
|
put
(
K
key,
V
value)
Maps the specified key to the specified value in this hashtable. |
void |
putAll
(
Map
<? extends
K
,? extends
V
> t)
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. |
V
|
remove
(
Object
key)
Removes the key (and its corresponding value) from this hashtable. |
int |
size
()
Returns the number of keys in this hashtable. |
String
|
toString
()
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). |
Collection
<
V
> |
values
()
Returns a
Collection
view of the values contained in this map. |
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
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
size
public int size()
-
Returns the number of keys in this hashtable.
-
-
Specified by:
-
size
in interface
Map
<
K
,
V
>
-
Specified by:
-
size
in class
Dictionary
<
K
,
V
>
-
-
Returns:
-
the number of keys in this hashtable.
isEmpty
public boolean isEmpty()
-
Tests if this hashtable maps no keys to values.
-
-
Specified by:
-
isEmpty
in interface
Map
<
K
,
V
>
-
Specified by:
-
isEmpty
in class
Dictionary
<
K
,
V
>
-
-
Returns:
-
true if this hashtable maps no keys to values; false otherwise.
keys
public Enumeration<K> keys()
-
Returns an enumeration of the keys in this hashtable.
-
-
Specified by:
-
keys
in class
Dictionary
<
K
,
V
>
-
-
Returns:
-
an enumeration of the keys in this hashtable.
-
See Also:
-
Enumeration
,
elements()
,
keySet()
,
Map
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.
-
-
Specified by:
-
elements
in class
Dictionary
<
K
,
V
>
-
-
Returns:
-
an enumeration of the values in this hashtable.
-
See Also:
-
Enumeration
,
keys()
,
values()
,
Map
contains
public boolean contains(Object value)
-
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).
-
-
-
Parameters:
-
value - a value to search for
-
Returns:
-
true if and only if some key maps to the value argument in this hashtable as determined by the
equals
method; false otherwise.
-
Throws:
-
NullPointerException
- if the value is null
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 the
Map
interface).
-
-
Specified by:
-
containsValue
in interface
Map
<
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 is null
-
Since:
-
1.2
containsKey
public boolean containsKey(Object key)
-
Tests if the specified object is a key in this hashtable.
-
-
Specified by:
-
containsKey
in interface
Map
<
K
,
V
>
-
-
Parameters:
-
key - possible key
-
Returns:
-
true if and only if the specified object is a key in this hashtable, as determined by the
equals
method; false otherwise.
-
Throws:
-
NullPointerException
- if the key is null
-
See Also:
-
contains(Object)
get
public 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.
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.)
-
-
Specified by:
-
get
in interface
Map
<
K
,
V
>
-
Specified by:
-
get
in 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 null if this map contains no mapping for the key
-
Throws:
-
NullPointerException
- if the specified key is null
-
See Also:
-
put(Object, Object)
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
public V put(K key,
V value)
-
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.
-
-
Specified by:
-
put
in interface
Map
<
K
,
V
>
-
Specified by:
-
put
in class
Dictionary
<
K
,
V
>
-
-
Parameters:
-
key - the hashtable key
-
value - 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 is null
-
See Also:
-
Object.equals(Object)
,
get(Object)
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:
-
remove
in interface
Map
<
K
,
V
>
-
Specified by:
-
remove
in 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 null if the key did not have a mapping
-
Throws:
-
NullPointerException
- if the key is null
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:
-
putAll
in interface
Map
<
K
,
V
>
-
-
Parameters:
-
t - mappings to be stored in this map
-
Throws:
-
NullPointerException
- if the specified map is null
-
Since:
-
1.2
clear
public void clear()
-
Clears this hashtable so that it contains no keys.
-
-
Specified by:
-
clear
in interface
Map
<
K
,
V
>
-
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.
-
-
Overrides:
-
clone
in class
Object
-
-
Returns:
-
a clone of the hashtable
-
See Also:
-
Cloneable
toString
public String toString()
-
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.
-
-
Overrides:
-
toString
in class
Object
-
-
Returns:
-
a string representation of this hashtable
keySet
public Set<K> keySet()
-
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.
-
-
Specified by:
-
keySet
in interface
Map
<
K
,
V
>
-
-
Returns:
-
a set view of the keys contained in this map
-
Since:
-
1.2
entrySet
public Set<Map.Entry<K,V>> entrySet()
-
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.
-
-
Specified by:
-
entrySet
in interface
Map
<
K
,
V
>
-
-
Returns:
-
a set view of the mappings contained in this map
-
Since:
-
1.2
values
public Collection<V> values()
-
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.
-
-
Specified by:
-
values
in interface
Map
<
K
,
V
>
-
-
Returns:
-
a collection view of the values contained in this map
-
Since:
-
1.2
equals
public boolean equals(Object o)
-
Compares the specified Object with this Map for equality, as per the definition in the Map interface.
-
-
Specified by:
-
equals
in interface
Map
<
K
,
V
>
-
Overrides:
-
equals
in class
Object
-
-
Parameters:
-
o - object to be compared for equality with this hashtable
-
Returns:
-
true if the specified Object is equal to this Map
-
Since:
-
1.2
-
See Also:
-
Map.equals(Object)
hashCode
public int hashCode()
-
Returns the hash code value for this Map as per the definition in the Map interface.
-
-
Specified by:
-
hashCode
in interface
Map
<
K
,
V
>
-
Overrides:
-
hashCode
in class
Object
-
-
Returns:
-
a hash code value for this object.
-
Since:
-
1.2
-
See Also:
-
Map.hashCode()