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java.util
public class: Hashtable [javadoc | source]
java.lang.Object
   java.util.Dictionary<K, V>
      java.util.Hashtable

All Implemented Interfaces:
    Cloneable, Map, java$io$Serializable

Direct Known Subclasses:
    NoneProvider, UIDefaults, BasicDefaults, Properties, MultiUIDefaults, Provider, MultiUIDefaults, AuthProvider

This class implements a hash table, 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:

   {@code
  Hashtable numbers
    = new Hashtable();
  numbers.put("one", 1);
  numbers.put("two", 2);
  numbers.put("three", 3);}

To retrieve a number, use the following code:

   {@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, {@code Hashtable} is synchronized. If a thread-safe implementation is not needed, it is recommended to use HashMap in place of {@code Hashtable}. If a thread-safe highly-concurrent implementation is desired, then it is recommended to use java.util.concurrent.ConcurrentHashMap in place of {@code Hashtable}.

Constructor:
 public Hashtable() 
 public Hashtable(int initialCapacity) 
 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 -
 public Hashtable(int initialCapacity,
    float loadFactor) 
Method from java.util.Hashtable Summary:
clear,   clone,   contains,   containsKey,   containsValue,   elements,   entrySet,   equals,   get,   hashCode,   isEmpty,   keySet,   keys,   put,   putAll,   rehash,   remove,   size,   toString,   values
Methods from java.util.Dictionary:
elements,   get,   isEmpty,   keys,   put,   remove,   size
Methods from java.lang.Object:
clone,   equals,   finalize,   getClass,   hashCode,   notify,   notifyAll,   toString,   wait,   wait,   wait
Method from java.util.Hashtable Detail:
 public synchronized  void clear() 
    Clears this hashtable so that it contains no keys.
 public synchronized 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.
 public synchronized 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).

 public synchronized boolean containsKey(Object key) 
    Tests if the specified object is a key in this hashtable.
 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).

 public synchronized 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.
 public Set<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.
 public synchronized boolean equals(Object o) 
    Compares the specified Object with this Map for equality, as per the definition in the Map interface.
 public synchronized V get(Object key) 
    Returns the value to which the specified key is mapped, or {@code null} if this map contains no mapping for the key.

    More formally, if this map contains a mapping from a key {@code k} to a value {@code v} such that {@code (key.equals(k))}, then this method returns {@code v}; otherwise it returns {@code null}. (There can be at most one such mapping.)

 public synchronized int hashCode() 
    Returns the hash code value for this Map as per the definition in the Map interface.
 public synchronized boolean isEmpty() 
    Tests if this hashtable maps no keys to values.
 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.
 public synchronized Enumeration<K> keys() 
    Returns an enumeration of the keys in this hashtable.
 public synchronized 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.

 public synchronized  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. This method is called automatically when the number of keys in the hashtable exceeds this hashtable's capacity and load factor.
 public synchronized 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.
 public synchronized int size() 
    Returns the number of keys in this hashtable.
 public synchronized 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.
 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.