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Class java.util.Hashtable

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

public class Hashtable

extends Dictionary

implements Cloneable

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));

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

See Also:

hashCode, equals

Hashtable(int, float)

Constructs 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.

clear()

Clears the hash table so that it has no more elements in it.

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 the specified 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 number of elements contained in the hashtable.

toString()

Converts to a rather lengthy String.

  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.

  public Hashtable(int initialCapacity)

Constructs a new, empty hashtable with the specified initial capacity.

Parameters:

initialCapacity - the initial number of buckets

  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.

  public int size()

Returns the number of elements contained in the hashtable.

Overrides:

size in class Dictionary

  public boolean isEmpty()

Returns true if the hashtable contains no elements.

Overrides:

isEmpty in class Dictionary

  public synchronized Enumeration keys()

Returns an enumeration of the hashtable's keys.

Overrides:

keys in class Dictionary

See Also:

elements, Enumeration

  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

  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

  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

  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

  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.

  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

  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

  public synchronized void clear()

Clears the hash table so that it has no more elements in it.

  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

  public synchronized String toString()

Converts to a rather lengthy String.

Overrides:

toString in class Object

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