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Class java.util.Hashtable

java.lang.Object
    |
    +----java.util.Dictionary
            |
            +----java.util.Hashtable

public class Hashtable

extends Dictionary

implements Map, Cloneable, Serializable

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);
     }
 

As of JDK1.2, this class has been retrofitted to implement Map, so that it becomes a part of Java's collection framework. Unlike the new collection implementations, Vector is synchronized.

The Iterators returned by the iterator and listIterator methods of the Collections returned by all of Hashtable's "collection view methods" are fail-fast: if the Hashtable is structurally modified at any time after the Iterator is created, in any way except through the Iterator's own remove or add methods, the Iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the Iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future. The Enumerations returned by Hashtable's keys and values methods are not fail-fast.

Since:

JDK1.0

See Also:

equals, hashCode, rehash, Collection, Map, HashMap, ArrayMap, TreeMap

Hashtable()

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

Hashtable(int)

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

Hashtable(int, float)

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

Hashtable(Map)

Constructs a new hashtable with the same mappings as the given Map.

clear()

Clears this hashtable so that it contains no keys.

clone()

Creates a shallow copy of this hashtable.

contains(Object)

Tests if some key maps into the specified value in this hashtable.

containsKey(Object)

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

containsValue(Object)

Returns true if this Hashtable maps one or more keys to this value.

elements()

Returns an enumeration of the values in this hashtable.

entries()

Returns a Collection view of the entries contained in this Hashtable.

equals(Object)

Compares the specified Object with this Map for equality, as per the definition in the Map interface.

get(Object)

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

hashCode()

Returns the hash code value for this Map as per the definition in the Map interface.

isEmpty()

Tests if this hashtable maps no keys to values.

keys()

Returns an enumeration of the keys in this hashtable.

keySet()

Returns a Set view of the keys contained in this Hashtable.

put(Object, Object)

Maps the specified key to the specified value in this hashtable.

putAll(Map)

Copies all of the mappings from the specified Map to this Hashtable These mappings will replace any mappings that this Hashtable had for any of the keys currently in the specified Map.

rehash()

Rehashes the contents of the hashtable into a hashtable with a larger capacity.

remove(Object)

Removes the key (and its corresponding value) from this hashtable.

size()

Returns the number of keys in this hashtable.

toString()

Returns a rather long string representation of this hashtable.

values()

Returns a Collection view of the values contained in this Hashtable.

public Hashtable(int initialCapacity,
                 float loadFactor)

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

Parameters:

initialCapacity - the initial capacity of the hashtable.

loadFactor - a number between 0.0 and 1.0.

Throws: IllegalArgumentException

if the initial capacity is less than or equal to zero, or if the load factor is less than or equal to zero.

public Hashtable(int initialCapacity)

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

Parameters:

initialCapacity - the initial capacity of the hashtable.

public Hashtable()

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

public Hashtable(Map t)

Constructs a new hashtable with the same mappings as the given Map. The hashtable is created with a capacity of twice the number of entries in the given Map, and a default load factor.

public int size()

Returns the number of keys in this hashtable.

Returns:

the number of keys in this hashtable.

Overrides:

size in class Dictionary

public boolean isEmpty()

Tests if this hashtable maps no keys to values.

Returns:

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

Overrides:

isEmpty in class Dictionary

public Enumeration keys()

Returns an enumeration of the keys in this hashtable.

Returns:

an enumeration of the keys in this hashtable.

Overrides:

keys in class Dictionary

See Also:

Enumeration, elements, keySet, Map

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.

Overrides:

elements in class Dictionary

See Also:

Enumeration, keys, values, Map

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.

Note that this method is identical in functionality to containsValue, (which is part of the Map interface in the collections framework).

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.

See Also:

containsKey, containsValue, Map

public boolean containsValue(Object value)

Returns true if this Hashtable maps one or more keys to this value.

Note that this method is identical in functionality to contains (which predates the Map interface).

Parameters:

value - value whose presence in this Hashtable is to be tested.

See Also:

Map

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.

See Also:

contains

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.

Overrides:

get in class Dictionary

See Also:

put

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.

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.

Overrides:

put in class Dictionary

See Also:

equals, get

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.

Overrides:

remove in class Dictionary

public void putAll(Map t)

Copies all of the mappings from the specified Map to this Hashtable These mappings will replace any mappings that this Hashtable had for any of the keys currently in the specified Map.

public void clear()

Clears this hashtable so that it contains no keys.

public Object clone()

Creates a shallow copy of this hashtable. The keys and values themselves are not cloned. This is a relatively expensive operation.

Returns:

a clone of the hashtable.

Overrides:

clone in class Object

public String toString()

Returns a rather long string representation of this hashtable.

Returns:

a string representation of this hashtable.

Overrides:

toString in class Object

public Set keySet()

Returns a Set view of the keys contained in this Hashtable. The Set is backed by the Hashtable, so changes to the Hashtable are reflected in the Set, and vice-versa. The Set supports element removal (which removes the corresponding entry from the Hashtable), but not element addition.

public Collection entries()

Returns a Collection view of the entries contained in this Hashtable. Each element in this collection is a Map.Entry. The Collection is backed by the Hashtable, so changes to the Hashtable are reflected in the Collection, and vice-versa. The Collection supports element removal (which removes the corresponding entry from the Hashtable) via its Iterator, but not element addition or "direct" element removal.

See Also:

Map.Entry

public Collection values()

Returns a Collection view of the values contained in this Hashtable. The Collection is backed by the Hashtable, so changes to the Hashtable are reflected in the Collection, and vice-versa. The Collection supports element removal (which removes the corresponding entry from the Hashtable) via its Iterator, but not element addition or "direct" element removal.

public boolean equals(Object o)

Compares the specified Object with this Map for equality, as per the definition in the Map interface.

Returns:

true if the specified Object is equal to this Map.

Overrides:

equals in class Object

See Also:

equals

public int hashCode()

Returns the hash code value for this Map as per the definition in the Map interface.

Overrides:

hashCode in class Object

See Also:

hashCode

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