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# Class java.util.Hashtable

    java.lang.Object
       |
       +----java.util.Hashtable

public class Hashtable
extends Object

Hashtable class. Maps keys to values. Any object can be used as a key and/or value.

To sucessfully store and retrieve objects from a hash table the object used as the key must implement the hashCode() and equals() methods.

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(1));
    	numbers.put("three", new Integer(1));

To retrieve a number use:

    	Integer n = (Integer)numbers.get("two");
    	if (n != null) {
    	    System.out.println("two = " + n);
    	}

See Also:
  hashCode, equals
Version:
  1.20, 14 Mar 1995
Author:
  Arthur van Hoff

##

Hashtable(int, float)
  Construct a new, empty hashtable with the specified initial capacity and the specified load factor.
Hashtable(int)
  Constructs a new, empty hashtable with the specified initial capacity.
Hashtable()
  Constructs a new, empty hashtable.

##

clone()
  Creates a clone of the hashtable.
contains(Object)
  Returns true if the specified object is an element of the hashtable.
containsKey(Object)
  Returns true if the collection contains an element for the key.
elements()
  Returns an enumeration of the elements.
get(Object)
  Gets the object associated with a key in the hashtable.
isEmpty()
  Returns true if the hashtable contains no elements.
keys()
  Returns an enumeration of the hashtable's keys.
put(Object, Object)
  Puts the specified element into the hashtable, using the specified key.
rehash()
  Rehashes the content of the table into a bigger table.
remove(Object)
  Removes the element corresponding to the key.
size()
  Returns the hashtable's size (the number of elements it contains).
toString()
  Converts to a rather lengthy string.

##

Hashtable

      public Hashtable(int initialCapacity,
                       float loadFactor)

  Construct a new, empty hashtable with the specified initial capacity and the specified load factor.
  Parameters:
    initialCapacity - the initial number of buckets
    loadFactor - a number between 0.0 and 1.0, it defines the threshold for rehashing the hashtable into a bigger one.

Hashtable

      public Hashtable(int initialCapacity)

  Constructs a new, empty hashtable with the specified initial capacity.
  Parameters:
    initialCapacity - the initial number of buckets

Hashtable

      public Hashtable()

  Constructs a new, empty hashtable. A default capacity and load factor is used. Note that the hashtable will automatically grow when it gets full.

##

size

      public int size()

  Returns the hashtable's size (the number of elements it contains).

isEmpty

      public boolean isEmpty()

  Returns true if the hashtable contains no elements.

keys

      public synchronized Enumeration keys()

  Returns an enumeration of the hashtable's keys.
  See Also:
    elements, Enumeration

elements

      public synchronized Enumeration elements()

  Returns an enumeration of the elements. Use the Enumeration methods on the returned object to fetch the elements sequentially.
  See Also:
    keys, Enumeration

contains

      public synchronized boolean contains(Object value)

  Returns true if the specified object is an element of the hashtable. This operation is more expensive than containsKey()
  Parameters:
    value - the value that we are looking for
  See Also:
    containsKey

containsKey

      public synchronized boolean containsKey(Object key)

  Returns true if the collection contains an element for the key.
  Parameters:
    key - the key that we are looking for
  See Also:
    contains

get

      public synchronized Object get(Object key)

  Gets the object associated with a key in the hashtable.
  Returns:
    the element for the key or null if the key is not defined in the hash table.
  See Also:
    put

rehash

      protected void rehash()

  Rehashes the content of the table into a bigger table. This is method called automatically when the hashtable's size exeeds a threshold.

put

      public synchronized Object put(Object key,
                                     Object value)

  Puts the specified element into the hashtable, using the specified key. The element may be retrieved by doing a get() with the same key. The key can't be null and the element can't be null.
  See Also:
    get
  Returns:
    the old value of the key, or null if it didn't have one

remove

      public synchronized Object remove(Object key)

  Removes the element corresponding to the key. Does nothing if the key isn't present.
  Parameters:
    key - the key that needs to be removed
  Returns:
    the value of key, or null if the key was not found

clone

      public synchronized Object clone()

  Creates a clone of the hashtable. A shallow copy is made, the keys and elements themselves are NOT cloned. This is a relatively expensive operation.
  Overrides:
    clone in class Object

toString

      public synchronized String toString()

  Converts to a rather lengthy string.
  Overrides:
    toString in class Object

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