java.util
class Hashtable

java.lang.Object java.util.Dictionary java.util.Hashtable

All Implemented Interfaces:

Serializable, Cloneable, Map

Direct Known Subclasses:

Properties, UIDefaults

Most common way to construct:

Hashtable numbers = new Hashtable();

Based on 213 examples

public class Hashtable
extends Dictionary
implements Map, Cloneable, Serializable

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 performance: initial capacity and load factor. The capacity is the number of buckets in the hash table, and the initial capacity is simply the capacity at the time the hash table is created. Note that the hash table is open: in the case of a "hash collision", a single bucket stores multiple entries, which must be searched sequentially. The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. The initial capacity and load factor parameters are merely hints to the implementation. The exact details as to when and whether the rehash method is invoked are implementation-dependent.

Generally, the default load factor (.75) offers a good tradeoff between time and space costs. Higher values decrease the space overhead but increase the time cost to look up an entry (which is reflected in most Hashtable operations, including get and put).

The initial capacity controls a tradeoff between wasted space and the need for rehash operations, which are time-consuming. No rehash operations will ever occur if the initial capacity is greater than the maximum number of entries the Hashtable will contain divided by its load factor. However, setting the initial capacity too high can waste space.

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:

   {@code
   Hashtable numbers
     = new Hashtable();
   numbers.put("one", 1);
   numbers.put("two", 2);
   numbers.put("three", 3);}

To retrieve a number, use the following code:

   {@code
   Integer n = numbers.get("two");
   if (n != null) {
     System.out.println("two = " + n);
   }}

The iterators returned by the iterator method of the collections returned by all of this class'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 method, the iterator will throw a {@link 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 elements methods are not fail-fast.

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

As of the Java 2 platform v1.2, this class was retrofitted to implement the {@link Map} interface, making it a member of the Java Collections Framework. Unlike the new collection implementations, {@code Hashtable} is synchronized.

Hashtable

public Hashtable()

Constructs a new, empty hashtable with a default initial capacity (11) and load factor (0.75).

Hashtable

public Hashtable(int initialCapacity)

Constructs a new, empty hashtable with the specified initial capacity and default load factor (0.75).

Parameters:

initialCapacity - the initial capacity of the hashtable.

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 - the load factor of the hashtable.

Hashtable

public Hashtable(Map t)

Constructs a new hashtable with the same mappings as the given Map. The hashtable is created with an initial capacity sufficient to hold the mappings in the given Map and a default load factor (0.75).

Parameters:

t - the map whose mappings are to be placed in this map.

clear

public synchronized void clear()

Clears this hashtable so that it contains no keys.

clone

public synchronized Object clone()

Creates a shallow copy of this hashtable. All the structure of the hashtable itself is copied, but the keys and values are not cloned. This is a relatively expensive operation.

Overrides:

clone in class Object

Returns:

a clone of the hashtable

contains

public synchronized boolean contains(Object value)

Tests if some key maps into the specified value in this hashtable. This operation is more expensive than the {@link #containsKey containsKey} method.

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

Parameters:

value - a value to search for

Returns:

true if and only if some key maps to the value argument in this hashtable as determined by the equals method; false otherwise.

containsKey

public synchronized boolean containsKey(Object key)

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

Parameters:

key - possible key

Returns:

true if and only if the specified object is a key in this hashtable, as determined by the equals method; false otherwise.

containsValue

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 {@link #contains contains} (which predates the {@link Map} interface).

Parameters:

value - value whose presence in this hashtable is to be tested

Returns:

true if this map maps one or more keys to the specified value

elements

public synchronized Enumeration elements()

Returns an enumeration of the values in this hashtable. Use the Enumeration methods on the returned object to fetch the elements sequentially.

Overrides:

elements in class Dictionary

Returns:

an enumeration of the values in this hashtable.

entrySet

public Set entrySet()

Returns a {@link Set} view of the mappings contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation, or through the setValue operation on a map entry returned by the iterator) the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations.

equals

public synchronized boolean equals(Object o)

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

Overrides:

equals in class Object

Parameters:

o - object to be compared for equality with this hashtable

Returns:

true if the specified Object is equal to this Map

get

public synchronized Object get(Object key)

Returns the value to which the specified key is mapped, or {@code null} if this map contains no mapping for the key.

More formally, if this map contains a mapping from a key {@code k} to a value {@code v} such that {@code (key.equals(k))}, then this method returns {@code v}; otherwise it returns {@code null}. (There can be at most one such mapping.)

Overrides:

get in class Dictionary

Parameters:

key - the key whose associated value is to be returned

Returns:

the value to which the specified key is mapped, or {@code null} if this map contains no mapping for the key

hashCode

public synchronized int hashCode()

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

Overrides:

hashCode in class Object

isEmpty

public synchronized boolean isEmpty()

Tests if this hashtable maps no keys to values.

Overrides:

isEmpty in class Dictionary

Returns:

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

keys

public synchronized Enumeration keys()

Returns an enumeration of the keys in this hashtable.

Overrides:

keys in class Dictionary

Returns:

an enumeration of the keys in this hashtable.

keySet

public Set keySet()

Returns a {@link Set} view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

put

public synchronized 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.

Overrides:

put in class Dictionary

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

putAll

public synchronized 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.

Parameters:

t - mappings to be stored in this map

rehash

protected void rehash()

Increases the capacity of and internally reorganizes this hashtable, in order to accommodate and access its entries more efficiently. This method is called automatically when the number of keys in the hashtable exceeds this hashtable's capacity and load factor.

remove

public synchronized 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.

Overrides:

remove in class Dictionary

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

size

public synchronized int size()

Returns the number of keys in this hashtable.

Overrides:

size in class Dictionary

Returns:

the number of keys in this hashtable.

toString

public synchronized String toString()

Returns a string representation of this Hashtable object in the form of a set of entries, enclosed in braces and separated by the ASCII characters ", " (comma and space). Each entry is rendered as the key, an equals sign =, and the associated element, where the toString method is used to convert the key and element to strings.

Overrides:

toString in class Object

Returns:

a string representation of this hashtable

values

public Collection values()

Returns a {@link Collection} view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. If the map is modified while an iteration over the collection is in progress (except through the iterator's own remove operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Collection.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations.