java.lang.Object
java.util.Dictionary<K,V>
java.util.Hashtable<K,V>
public class Hashtable<K,V>
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.
Object.equals(java.lang.Object), Object.hashCode(), rehash(), Collection, Map, HashMap, TreeMap, Serialized Form
Hashtable()
Hashtable(int initialCapacity)
Hashtable(int initialCapacity, float loadFactor)
Hashtable(Map<? extends K,? extends V> t)
void clear()
Object clone()
boolean contains(Object value)
boolean containsKey(Object key)
boolean containsValue(Object value)
Enumeration<V> elements()
Set<Map.Entry<K,V>> entrySet()
boolean equals(Object o)
V get(Object key)
int hashCode()
boolean isEmpty()
Enumeration<K> keys()
Set<K> keySet()
V put(K key, V value)
key to the specified value in this hashtable. void putAll(Map<? extends K,? extends V> t)
protected void rehash()
V remove(Object key)
int size()
String toString()
Collection<V> values()
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<? extends K,? extends V> 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 Map<K,V>
size in class Dictionary<K,V>
public boolean isEmpty()
isEmpty in interface Map<K,V>
isEmpty in class Dictionary<K,V>
true if this hashtable maps no keys to values; false otherwise.
public Enumeration<K> keys()
keys in class Dictionary<K,V>
Enumeration, elements(), keySet(), Map
public Enumeration<V> elements()
elements in class Dictionary<K,V>
Enumeration, keys(), values(), Map
public 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), Map
public boolean containsValue(Object value)
Note that this method is identical in functionality to contains (which predates the Map interface).
containsValue in interface Map<K,V>
value - value whose presence in this Hashtable is to be tested.
NullPointerException - if the value is null.
Map
public boolean containsKey(Object key)
containsKey in interface Map<K,V>
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.
contains(Object)
public V get(Object key)
get in interface Map<K,V>
get in class Dictionary<K,V>
key - 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 V put(K key,
V 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 Map<K,V>
put in class Dictionary<K,V>
key - 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 V remove(Object key)
remove in interface Map<K,V>
remove in class Dictionary<K,V>
key - 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<? extends K,? extends V> t)
putAll in interface Map<K,V>
t - Mappings to be stored in this map.
NullPointerException - if the specified map is null.
public void clear()
clear in interface Map<K,V>
public Object clone()
clone in class Object
Cloneable
public String toString()
Overrides to toString method of Object.
toString in class Object
public Set<K> keySet()
keySet in interface Map<K,V>
public Set<Map.Entry<K,V>> entrySet()
entrySet in interface Map<K,V>
Map.Entry
public Collection<V> values()
values in interface Map<K,V>
public boolean equals(Object o)
equals in interface Map<K,V>
equals in class Object
o - object to be compared for equality with this Hashtable
Map.equals(Object)
public int hashCode()
hashCode in interface Map<K,V>
hashCode in class Object
Map.hashCode()
Copyright © 2004, 2010 Oracle and/or its affiliates. All rights reserved. Use is subject to license terms. Also see the documentation redistribution policy.
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