Package java.util

Class Hashtable<K,​V>

java.lang.Object
java.util.Dictionary<K,​V>
java.util.Hashtable<K,​V>
Type Parameters:
K - the type of keys maintained by this map
V - the type of mapped values
All Implemented Interfaces:
Serializable, Cloneable, Map<K,​V>
Direct Known Subclasses:
CountersTable, Properties, WeakHashtable
public class Hashtable<K,​V>
extends Dictionary<K,​V>
implements Map<K,​V>, Cloneable, Serializable
Hashtable is a synchronized implementation of Map. All optional operations are supported.

Neither keys nor values can be null. (Use HashMap or LinkedHashMap if you need null keys or values.)

See Also:
HashMap, Serialized Form

  • Nested Class Summary

    Nested classes/interfaces inherited from interface java.util.Map

    Map.Entry<K,​V>

  • Constructor Summary

    Constructors
    Constructor 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 Description
    void clear()
    Removes all key/value pairs from this Hashtable, leaving the size zero and the capacity unchanged.
    Object clone()
    Returns a new Hashtable with the same key/value pairs, capacity and load factor.
    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 an integer hash code for this 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

    finalize, getClass, notify, notifyAll, wait, wait, wait

  • 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 - 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 Details

    • clone

      public Object clone()
      Returns a new Hashtable with the same key/value pairs, capacity and load factor.
      Overrides:
      clone in class Object
      Returns:
      a shallow copy of this Hashtable.
      See Also:
      Cloneable

    • 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

    • 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()

    • 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)

    • 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.

    • 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)

    • 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)

    • 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

    • 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.

    • 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.

    • 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.

    • 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

    • 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

    • 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()

    • hashCode

      public int hashCode()
      Description copied from class: Object
      Returns an integer hash code for this object. By contract, any two objects for which Object.equals(java.lang.Object) returns true must return the same hash code value. This means that subclasses of Object usually override both methods or neither method.

      Note that hash values must not change over time unless information used in equals comparisons also changes.

      See Writing a correct hashCode method if you intend implementing your own hashCode method.

      Specified by:
      hashCode in interface Map<K,​V>
      Overrides:
      hashCode in class Object
      Returns:
      this object's hash code.
      See Also:
      Object.equals(java.lang.Object)

    • toString

      public String toString()
      Returns the string representation of this Hashtable.
      Overrides:
      toString in class Object
      Returns:
      the string representation of this Hashtable.