Class java.util.Hashtable
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Class java.util.Hashtable

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

public class Hashtable
extends Dictionary
implements Cloneable
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(2));
	numbers.put("three", new Integer(3));
To retrieve a number use:
	Integer n = (Integer)numbers.get("two");
	if (n != null) {
	    System.out.println("two = " + n);
	}
See Also:
hashCode, equals

Constructor Index

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

Method Index

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

Constructors

 o 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 number of buckets
loadFactor - a number between 0.0 and 1.0, it defines the threshold for rehashing the hashtable into a bigger one.
Throws: IllegalArgumentException
If the initial capacity is less than or equal to zero.
Throws: IllegalArgumentException
If the load factor is less than or equal to zero.
 o Hashtable
  public Hashtable(int initialCapacity)
Constructs a new, empty hashtable with the specified initial capacity.
Parameters:
initialCapacity - the initial number of buckets
 o 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.

Methods

 o size
  public int size()
Returns the number of elements contained in the hashtable.
Overrides:
size in class Dictionary
 o isEmpty
  public boolean isEmpty()
Returns true if the hashtable contains no elements.
Overrides:
isEmpty in class Dictionary
 o keys
  public synchronized Enumeration keys()
Returns an enumeration of the hashtable's keys.
Overrides:
keys in class Dictionary
See Also:
elements, Enumeration
 o elements
  public synchronized Enumeration elements()
Returns an enumeration of the elements. Use the Enumeration methods on the returned object to fetch the elements sequentially.
Overrides:
elements in class Dictionary
See Also:
keys, Enumeration
 o 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 the containsKey() method.
Parameters:
value - the value that we are looking for
Throws: NullPointerException
If the value being searched for is equal to null.
See Also:
containsKey
 o 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
 o get
  public synchronized Object get(Object key)
Gets the object associated with the specified key in the hashtable.
Parameters:
key - the specified key
Returns:
s the element for the key or null if the key is not defined in the hash table.
Overrides:
get in class Dictionary
See Also:
put
 o rehash
  protected void rehash()
Rehashes the content of the table into a bigger table. This method is called automatically when the hashtable's size exceeds the threshold.
 o 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 and the element cannot be null.
Parameters:
key - the specified key in the hashtable
value - the specified element
Returns:
the old value of the key, or null if it did not have one.
Throws: NullPointerException
If the value of the element is equal to null.
Overrides:
put in class Dictionary
See Also:
get
 o remove
  public synchronized Object remove(Object key)
Removes the element corresponding to the key. Does nothing if the key is not present.
Parameters:
key - the key that needs to be removed
Returns:
the value of key, or null if the key was not found.
Overrides:
remove in class Dictionary
 o clear
  public synchronized void clear()
Clears the hash table so that it has no more elements in it.
 o 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
 o toString
  public synchronized String toString()
Converts to a rather lengthy String.
Overrides:
toString in class Object

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