java.util
Class Hashtable

java.lang.Object
  java.util.Hashtable

public class Hashtable
extends Object

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 efficiency: its capacity and its load factor. The load factor in the CLDC implementation of the hashtable class is always 75 percent. When the number of entries in the hashtable exceeds the product of the load factor and the current capacity, the capacity is increased by calling the rehash method. 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); }

Constructor Summary

Hashtable()
Constructs a new, empty hashtable with a default capacity and load factor.

Hashtable(int initialCapacity)
Constructs a new, empty hashtable with the specified initial capacity.

Method Summary

void	clear() Clears this hashtable so that it contains no keys.
boolean	contains(Object value) Tests if some key maps into the specified value in this hashtable.
boolean	containsKey(Object key) Tests if the specified object is a key in this hashtable.
Enumeration	elements() Returns an enumeration of the values in this hashtable.
Object	get(Object key) Returns the value to which the specified key is mapped in this hashtable.
boolean	isEmpty() Tests if this hashtable maps no keys to values.
Enumeration	keys() Returns an enumeration of the keys in this hashtable.
Object	put(Object key, Object value) Maps the specified key to the specified value in this hashtable.
protected void	rehash() Rehashes the contents of the hashtable into a hashtable with a larger capacity.
Object	remove(Object key) Removes the key (and its corresponding value) from this hashtable.
int	size() Returns the number of keys in this hashtable.
String	toString() Returns a rather long string representation of this hashtable.

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, wait, wait, wait

Constructor Detail

Hashtable

public Hashtable()

Constructs a new, empty hashtable with a default capacity and load factor.

Hashtable

public Hashtable(int initialCapacity)

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

Method Detail

clear

public void clear()

Clears this hashtable so that it contains no keys.

contains

public 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.

containsKey

public boolean containsKey(Object key)

Tests if the specified object is a key in this hashtable.

elements

public Enumeration elements()

Returns an enumeration of the values in this hashtable. Use the Enumeration methods on the returned object to fetch the elements sequentially.

get

public Object get(Object key)

Returns the value to which the specified key is mapped in this hashtable. Use equals

isEmpty

public boolean isEmpty()

Tests if this hashtable maps no keys to values.

keys

public Enumeration keys()

Returns an enumeration of the keys in this hashtable.

put

public Object put(Object key,
                  Object value)
           throws NullPointerException

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.

Throws:

NullPointerException

rehash

protected void rehash()

Rehashes the contents of the hashtable into a hashtable with a larger capacity. This method is called automatically when the number of keys in the hashtable exceeds this hashtable's capacity and load factor.

remove

public Object 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.

size

public int size()

Returns the number of keys in this hashtable.

toString

public String toString()

Returns a rather long string representation of this hashtable.

Overrides:

toString in class Object