java.util

Class Hashtable<K,V>

java.lang.Object

java.util.Dictionary<K,V>

java.util.Hashtable<K,V>

All Implemented Interfaces:

Map<K,V>

public class Hashtable<K,V>
extends Dictionary<K,V>
implements Map<K,V>

Hashtable associates keys with values. Both keys and values cannot be null. The size of the Hashtable is the number of key/value pairs it contains. The capacity is the number of key/value pairs the Hashtable can hold. The load factor is a float value which determines how full the Hashtable gets before expanding the capacity. If the load factor of the Hashtable is exceeded, the capacity is doubled.

See Also:

Enumeration, Serializable, Object.equals(java.lang.Object), Object.hashCode()

Constructor Summary

Constructors

Constructor and Description
Hashtable() Constructs a new Hashtable using the default capacity and load factor.
Hashtable(int capacity) Constructs a new Hashtable using the specified capacity and the default load factor.
Hashtable(int capacity, float loadFactor) Constructs a new Hashtable using the specified capacity and load factor.
Hashtable(Map<? extends K,? extends V> map) Constructs a new instance of Hashtable containing the mappings from the specified map.

Method Summary

Modifier and Type	Method and Description
void	clear() Removes all key/value pairs from this Hashtable, leaving the size zero and the capacity unchanged.
boolean	contains(Object value) Returns true if this Hashtable contains the specified object as the value of at least one of the key/value pairs.
boolean	containsKey(Object key) Returns true if this Hashtable contains the specified object as a key of one of the key/value pairs.
boolean	containsValue(Object value) Searches this Hashtable for the specified value.
Enumeration<V>	elements() Returns an enumeration on the values of this Hashtable.
Set<Map.Entry<K,V>>	entrySet() Returns a set of the mappings contained in this Hashtable.
boolean	equals(Object object) Compares this Hashtable with the specified object and indicates if they are equal.
V	get(Object key) Returns the value associated with the specified key in this Hashtable.
int	hashCode() Returns a hash code value for the object.
boolean	isEmpty() Returns true if this Hashtable has no key/value pairs.
Enumeration<K>	keys() Returns an enumeration on the keys of this Hashtable instance.
Set<K>	keySet() Returns a set of the keys contained in this Hashtable.
V	put(K key, V value) Associate the specified value with the specified key in this Hashtable.
void	putAll(Map<? extends K,? extends V> map) Copies every mapping to this Hashtable from the specified map.
protected void	rehash() Increases the capacity of this Hashtable.
V	remove(Object key) Removes the key/value pair with the specified key from this Hashtable.
int	size() Returns the number of key/value pairs in this Hashtable.
String	toString() Returns the string representation of this Hashtable.
Collection<V>	values() Returns a collection of the values contained in this Hashtable.

Methods inherited from class java.lang.Object

clone, getClass, notify, notifyAll, wait, wait, wait

Constructor Detail

Hashtable

public Hashtable()

Constructs a new Hashtable using the default capacity and load factor.

Hashtable

public Hashtable(int capacity)

Constructs a new Hashtable using the specified capacity and the default load factor.

Parameters:

capacity - the initial capacity.

Hashtable

public Hashtable(int capacity,
                 float loadFactor)

Constructs a new Hashtable using the specified capacity and load factor.

Parameters:

capacity - the initial capacity.

loadFactor - the initial load factor.

Hashtable

public Hashtable(Map<? extends K,? extends V> map)

Constructs a new instance of Hashtable containing the mappings from the specified map.

Parameters:

map - the mappings to add.

Method Detail

clear

public void clear()

Removes all key/value pairs from this Hashtable, leaving the size zero and the capacity unchanged.

Specified by:

clear in interface Map<K,V>

See Also:

isEmpty(), size()

contains

public boolean contains(Object value)

Returns true if this Hashtable contains the specified object as the value of at least one of the key/value pairs.

Parameters:

value - the object to look for as a value in this Hashtable.

Returns:

true if object is a value in this Hashtable, false otherwise.

See Also:

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

containsKey

public boolean containsKey(Object key)

Returns true if this Hashtable contains the specified object as a key of one of the key/value pairs.

Specified by:

containsKey in interface Map<K,V>

Parameters:

key - the object to look for as a key in this Hashtable.

Returns:

true if object is a key in this Hashtable, false otherwise.

See Also:

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

containsValue

public boolean containsValue(Object value)

Searches this Hashtable for the specified value.

Specified by:

containsValue in interface Map<K,V>

Parameters:

value - the object to search for.

Returns:

true if value is a value of this Hashtable, false otherwise.

elements

public Enumeration<V> elements()

Returns an enumeration on the values of this Hashtable. The results of the Enumeration may be affected if the contents of this Hashtable are modified.

Specified by:

elements in class Dictionary<K,V>

Returns:

an enumeration of the values of this Hashtable.

See Also:

keys(), size(), Enumeration

entrySet

public Set<Map.Entry<K,V>> entrySet()

Returns a set of the mappings contained in this Hashtable. Each element in the set is a Map.Entry. The set is backed by this Hashtable so changes to one are reflected by the other. The set does not support adding.

Specified by:

entrySet in interface Map<K,V>

Returns:

a set of the mappings.

equals

public boolean equals(Object object)

Compares this Hashtable with the specified object and indicates if they are equal. In order to be equal, object must be an instance of Map and contain the same key/value pairs.

Specified by:

equals in interface Map<K,V>

Overrides:

equals in class Object

Parameters:

object - the object to compare with this object.

Returns:

true if the specified object is equal to this Map, false otherwise.

See Also:

hashCode()

get

public V get(Object key)

Returns the value associated with the specified key in this Hashtable.

Specified by:

get in interface Map<K,V>

Specified by:

get in class Dictionary<K,V>

Parameters:

key - the key of the value returned.

Returns:

the value associated with the specified key, or null if the specified key does not exist.

See Also:

put(K, V)

hashCode

public int hashCode()

Description copied from class: Object

Returns a hash code value for the object. This method is supported for the benefit of hashtables such as those provided by java.util.Hashtable. The general contract of hashCode is: Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application. If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result. It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables. As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the JavaTM programming language.)

Specified by:

hashCode in interface Map<K,V>

Overrides:

hashCode in class Object

Returns:

the receiver's hash.

See Also:

Map.equals(Object)

isEmpty

public boolean isEmpty()

Returns true if this Hashtable has no key/value pairs.

Specified by:

isEmpty in interface Map<K,V>

Specified by:

isEmpty in class Dictionary<K,V>

Returns:

true if this Hashtable has no key/value pairs, false otherwise.

See Also:

size()

keys

public Enumeration<K> keys()

Returns an enumeration on the keys of this Hashtable instance. The results of the enumeration may be affected if the contents of this Hashtable are modified.

Specified by:

keys in class Dictionary<K,V>

Returns:

an enumeration of the keys of this Hashtable.

See Also:

elements(), size(), Enumeration

keySet

public Set<K> keySet()

Returns a set of the keys contained in this Hashtable. The set is backed by this Hashtable so changes to one are reflected by the other. The set does not support adding.

Specified by:

keySet in interface Map<K,V>

Returns:

a set of the keys.

put

public V put(K key,
             V value)

Associate the specified value with the specified key in this Hashtable. If the key already exists, the old value is replaced. The key and value cannot be null.

Specified by:

put in interface Map<K,V>

Specified by:

put in class Dictionary<K,V>

Parameters:

key - the key to add.

value - the value to add.

Returns:

the old value associated with the specified key, or null if the key did not exist.

See Also:

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

putAll

public void putAll(Map<? extends K,? extends V> map)

Copies every mapping to this Hashtable from the specified map.

Specified by:

putAll in interface Map<K,V>

Parameters:

map - the map to copy mappings from.

rehash

protected void rehash()

Increases the capacity of this Hashtable. This method is called when the size of this Hashtable exceeds the load factor.

remove

public V remove(Object key)

Removes the key/value pair with the specified key from this Hashtable.

Specified by:

remove in interface Map<K,V>

Specified by:

remove in class Dictionary<K,V>

Parameters:

key - the key to remove.

Returns:

the value associated with the specified key, or null if the specified key did not exist.

See Also:

get(java.lang.Object), put(K, V)

size

public int size()

Returns the number of key/value pairs in this Hashtable.

Specified by:

size in interface Map<K,V>

Specified by:

size in class Dictionary<K,V>

Returns:

the number of key/value pairs in this Hashtable.

See Also:

elements(), keys()

toString

public String toString()

Returns the string representation of this Hashtable.

Overrides:

toString in class Object

Returns:

the string representation of this Hashtable.

values

public Collection<V> values()

Returns a collection of the values contained in this Hashtable. The collection is backed by this Hashtable so changes to one are reflected by the other. The collection does not support adding.

Specified by:

values in interface Map<K,V>

Returns:

a collection of the values.