public class Hashtable extends Dictionary implements Map, Cloneable, Serializable
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.
When the number of entries in the hashtable exceeds the product of the load
factor and the current capacity, the capacity is increased by calling the
rehash method.
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.
Object.equals(java.lang.Object),
Object.hashCode(),
rehash(),
Collection,
Map,
HashMap,
TreeMap,
Serialized Form| Constructor and Description |
|---|
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 t)
Constructs a new hashtable with the same mappings as the given
Map.
|
| Modifier and Type | Method and Description |
|---|---|
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 |
elements()
Returns an enumeration of the values in this hashtable.
|
Set |
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.
|
Object |
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 |
keys()
Returns an enumeration of the keys in this hashtable.
|
Set |
keySet()
Returns a Set view of the keys contained in this Hashtable.
|
Object |
put(Object key,
Object value)
Maps the specified
key to the specified
value in this hashtable. |
void |
putAll(Map 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.
|
Object |
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 |
values()
Returns a Collection view of the values contained in this Hashtable.
|
public Hashtable(int initialCapacity,
float loadFactor)
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.public Hashtable(int initialCapacity)
initialCapacity - the initial capacity of the hashtable.IllegalArgumentException - if the initial capacity is less
than zero.public Hashtable()
public Hashtable(Map t)
t - the map whose mappings are to be placed in this map.NullPointerException - if the specified map is null.public int size()
size in interface Mapsize in class Dictionarypublic boolean isEmpty()
isEmpty in interface MapisEmpty in class Dictionarytrue if this hashtable maps no keys to values;
false otherwise.public Enumeration keys()
keys in class DictionaryEnumeration,
elements(),
keySet(),
Mappublic Enumeration elements()
elements in class DictionaryEnumeration,
keys(),
values(),
Mappublic boolean contains(Object value)
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.containsKey(Object),
containsValue(Object),
Mappublic boolean containsValue(Object value)
Note that this method is identical in functionality to contains (which predates the Map interface).
containsValue in interface Mapvalue - value whose presence in this Hashtable is to be tested.NullPointerException - if the value is null.Mappublic boolean containsKey(Object key)
containsKey in interface Mapkey - 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.contains(Object)public Object get(Object key)
get in interface Mapget in class Dictionarykey - a key in the hashtable.null if the key is not mapped to any value in
this hashtable.NullPointerException - if the key is null.put(Object, Object)protected void rehash()
public Object put(Object key, Object value)
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.
put in interface Mapput in class Dictionarykey - the hashtable key.value - the value.null if it did not have one.NullPointerException - if the key or value is
null.Object.equals(Object),
get(Object)public Object remove(Object key)
remove in interface Mapremove in class Dictionarykey - the key that needs to be removed.null if the key did not have a mapping.NullPointerException - if the key is null.public void putAll(Map t)
putAll in interface Mapt - Mappings to be stored in this map.NullPointerException - if the specified map is null.public void clear()
public Object clone()
public String toString()
Overrides to toString method of Object.
public Set keySet()
public Set entrySet()
public Collection values()
public boolean equals(Object o)
equals in interface Mapequals in class Objecto - object to be compared for equality with this HashtableMap.equals(Object)public int hashCode()
hashCode in interface MaphashCode in class ObjectMap.hashCode()Copyright © 2012. All Rights Reserved.