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 should be between 0.0 and 1.0. 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. Larger load factors use memory more efficiently, at the expense of larger expected time per lookup.

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);
     }
 

Note: To conserve space, the CLDC implementation is based on JDK 1.1.8, not JDK 1.3.

Since:

JDK1.0, CLDC 1.0

See Also:

Object.equals(java.lang.Object), Object.hashCode(), rehash()

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(int initialCapacity)

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

Parameters:

initialCapacity - the initial capacity of the hashtable.

Throws:

IllegalArgumentException - if the initial capacity is less than zero

Since:

JDK1.0

Hashtable

public Hashtable()

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

Since:

JDK1.0

Method Detail

size

public int size()

Returns the number of keys in this hashtable.

Returns:

the number of keys in this hashtable.

Since:

JDK1.0

isEmpty

public boolean isEmpty()

Tests if this hashtable maps no keys to values.

Returns:

true if this hashtable maps no keys to values; false otherwise.

Since:

JDK1.0

keys

public Enumeration keys()

Returns an enumeration of the keys in this hashtable.

Returns:

an enumeration of the keys in this hashtable.

Since:

JDK1.0

See Also:

Enumeration, elements()

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.

Returns:

an enumeration of the values in this hashtable.

Since:

JDK1.0

See Also:

Enumeration, 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.

Parameters:

value - a value to search for.

Returns:

true if some key maps to the value argument in this hashtable; false otherwise.

Throws:

NullPointerException - if the value is null.

Since:

JDK1.0

See Also:

containsKey(java.lang.Object)

containsKey

public boolean containsKey(Object key)

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

Parameters:

key - possible key.

Returns:

true if the specified object is a key in this hashtable; false otherwise.

Since:

JDK1.0

See Also:

contains(java.lang.Object)

get

public Object get(Object key)

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

Parameters:

key - a key in the hashtable.

Returns:

the value to which the key is mapped in this hashtable; null if the key is not mapped to any value in this hashtable.

Since:

JDK1.0

See Also:

put(java.lang.Object, java.lang.Object)

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.

Since:

JDK1.0

put

public Object put(Object key,
                  Object 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.

Parameters:

key - the hashtable key.

value - the value.

Returns:

the previous value of the specified key in this hashtable, or null if it did not have one.

Throws:

NullPointerException - if the key or value is null.

Since:

JDK1.0

See Also:

Object.equals(java.lang.Object), get(java.lang.Object)

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.

Parameters:

key - the key that needs to be removed.

Returns:

the value to which the key had been mapped in this hashtable, or null if the key did not have a mapping.

Since:

JDK1.0

clear

public void clear()

Clears this hashtable so that it contains no keys.

Since:

JDK1.0

toString

public String toString()

Returns a rather long string representation of this hashtable.

Overrides:

toString in class Object

Returns:

a string representation of this hashtable.

Since:

JDK1.0