JavaTM 2 Platform Std. Ed. v1.5.0
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 numbers = new Hashtable();
numbers.put("one", new Integer(1));
numbers.put("two", new Integer(2));
numbers.put("three", new Integer(3));
To retrieve a number, use the following code:
Integer n = (Integer)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 and listIterator methods
of the Collections returned by all of Hashtable'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 or add methods, 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 values 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.
This class is a member of the
Java Collections Framework.
- 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, which is 0.75. |
Hashtable(int initialCapacity)
Constructs a new, empty hashtable with the specified initial capacity
and default load factor, which is 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 entries contained in this Hashtable. |
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 in this hashtable. |
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 Hashtable. |
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
These mappings will replace any mappings that this Hashtable had for any
of the keys currently in the specified Map. |
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 Hashtable. |
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, which is 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, which is 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, which is
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 .- See Also:
containsKey(Object) ,
containsValue(Object) ,
Map
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
- See Also:
Map
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 in this hashtable.
- Specified by:
get in interface Map<K,V> - Specified by:
get in class Dictionary<K,V>
- Parameters:
key - a key in the hashtable.
- Returns:
- the value to which the key is mapped in this hashtable;
null if the key is not mapped to any value in
this hashtable.
- Throws:
NullPointerException - if the 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 to
toString method of Object.
- 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 Hashtable. The Set
is backed by the Hashtable, so changes to the Hashtable are reflected
in the Set, and vice-versa. The Set supports element removal
(which removes the corresponding entry from the Hashtable), but not
element addition.
- 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 entries contained in this Hashtable.
Each element in this collection is a Map.Entry. The Set is
backed by the Hashtable, so changes to the Hashtable are reflected in
the Set, and vice-versa. The Set supports element removal
(which removes the corresponding entry from the Hashtable),
but not element addition.
- Specified by:
entrySet in interface Map<K,V>
- Returns:
- a set view of the mappings contained in this map.
- Since:
- 1.2
- See Also:
Map.Entry
values
public Collection<V> values()
- Returns a Collection view of the values contained in this Hashtable.
The Collection is backed by the Hashtable, so changes to the Hashtable
are reflected in the Collection, and vice-versa. The Collection
supports element removal (which removes the corresponding entry from
the Hashtable), but not element addition.
- 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()
Copyright 2003 Sun Microsystems, Inc. All rights reserved
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