public class Hashtable<K, V>

  1. Object
  2. Dictionary<K, V>
  3. Hashtable

ImplementsMap<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

Constructors

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

Methods

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

Inherited methods

From Object

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

From Map

getOrDefault, putIfAbsent, remove, replace, replace, forEach, replaceAll, computeIfAbsent, computeIfPresent, compute, merge

Constructor details

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 int
the initial capacity.

Hashtable

public Hashtable(int capacity, float loadFactor)
Constructs a new Hashtable using the specified capacity and load factor.

Parameters

capacity int
the initial capacity.
loadFactor float
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 Map<? extends K, ? extends V>
the mappings to add.

Method details

clear

public synchronized void clear()
Removes all key/value pairs from this Hashtable, leaving the size zero and the capacity unchanged.

Throws

UnsupportedOperationException
if removing elements from this Map is not supported.

See also

contains

public synchronized 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 Object
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

public synchronized boolean containsKey(Object key)
Returns true if this Hashtable contains the specified object as a key of one of the key/value pairs.

Parameters

key Object
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

containsValue

public boolean containsValue(Object value)
Searches this Hashtable for the specified value.

Parameters

value Object
the object to search for.

Returns

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

elements

public synchronized 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.

Returns

an enumeration of the values of this Hashtable.

See also

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.

Returns

a set of the mappings.

equals

public synchronized 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.

Parameters

object Object
the object to compare with this object.

Returns

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

See also

get

public synchronized V get(Object key)
Returns the value associated with the specified key in this Hashtable.

Parameters

key Object
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

hashCode

public synchronized int hashCode()
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.)

isEmpty

public synchronized boolean isEmpty()
Returns true if this Hashtable has no key/value pairs.

Returns

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

See also

keys

public synchronized 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.

Returns

an enumeration of the keys of this Hashtable.

See also

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.

Returns

a set of the keys.

put

public synchronized 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.

Parameters

key K
the key to add.
value V
the value to add.

Returns

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

See also

putAll

public synchronized void putAll(Map<? extends K, ? extends V> map)
Copies every mapping to this Hashtable from the specified map.

Parameters

map Map<? extends K, ? extends V>
the map to copy mappings from.

Throws

UnsupportedOperationException
if adding to this Map is not supported.
ClassCastException
if the class of a key or a value of the specified Map is inappropriate for this Map.
IllegalArgumentException
if a key or value cannot be added to this Map.
NullPointerException
if a key or value is null and this Map does not support null keys or values.

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 synchronized V remove(Object key)
Removes the key/value pair with the specified key from this Hashtable.

Parameters

key Object
the key to remove.

Returns

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

See also

size

public synchronized int size()
Returns the number of key/value pairs in this Hashtable.

Returns

the number of key/value pairs in this Hashtable.

See also

toString

public synchronized String toString()
Returns the string representation of this Hashtable.

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.

Returns

a collection of the values.