java.util
Class Hashtable

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

Direct Known Subclasses:

Properties

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 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. 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.

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:

     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, Hashtable 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:

Object.equals(java.lang.Object), Object.hashCode(), rehash(), Collection, Map, HashMap, TreeMap, Serialized Form

Inner Class Summary

private static class	Hashtable.Entry Hashtable collision list.
private class	Hashtable.EntrySet
private class	Hashtable.Enumerator A hashtable enumerator class.
private class	Hashtable.KeySet
private class	Hashtable.ValueCollection

Field Summary

private int	count The total number of entries in the hash table.
private static int	ENTRIES
private Set	entrySet
private static int	KEYS
private Set	keySet
private float	loadFactor The load factor for the hashtable.
private int	modCount The number of times this Hashtable has been structurally modified Structural modifications are those that change the number of entries in the Hashtable or otherwise modify its internal structure (e.g., rehash).
private static long	serialVersionUID use serialVersionUID from JDK 1.0.2 for interoperability
private Hashtable.Entry[]	table The hash table data.
private int	threshold The table is rehashed when its size exceeds this threshold.
private Collection	values
private static int	VALUES

Constructor Summary

Hashtable()
Constructs a new, empty hashtable with a default capacity and load factor, which is 0.75.

Hashtable(int initialCapacity)
Constructs a new, empty hashtable with the specified initial capacity and default load factor, which is 0.75.

Hashtable(int initialCapacity, float loadFactor)
Constructs a new, empty hashtable with the specified initial capacity and the specified load factor.

Hashtable(Map t)
Constructs a new hashtable with the same mappings as the given Map.

Method Summary

void	clear() Clears this hashtable so that it contains no keys.
Object	clone() Creates a shallow copy of this hashtable.
boolean	contains(Object value) Tests if some key maps into the specified value in this hashtable.
boolean	containsKey(Object key) Tests if the specified object is a key in this hashtable.
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).
Enumeration	elements() Returns an enumeration of the values in this hashtable.
Set	entrySet() Returns a Set view of the entries contained in this Hashtable.
boolean	equals(Object o) Compares the specified Object with this Map for equality, as per the definition in the Map interface.
Object	get(Object key) Returns the value to which the specified key is mapped in this hashtable.
int	hashCode() Returns the hash code value for this Map as per the definition in the Map interface.
boolean	isEmpty() Tests if this hashtable maps no keys to values.
Enumeration	keys() Returns an enumeration of the keys in this hashtable.
Set	keySet() Returns a Set view of the keys contained in this Hashtable.
Object	put(Object key, Object value) Maps the specified key to the specified value in this hashtable.
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.
private void	readObject(ObjectInputStream s) Reconstitute the Hashtable from a stream (i.e., deserialize it).
protected void	rehash() Increases the capacity of and internally reorganizes this hashtable, in order to accommodate and access its entries more efficiently.
Object	remove(Object key) Removes the key (and its corresponding value) from this hashtable.
int	size() Returns the number of keys in this hashtable.
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).
Collection	values() Returns a Collection view of the values contained in this Hashtable.
private void	writeObject(ObjectOutputStream s) Save the state of the Hashtable to a stream (i.e., serialize it).

Methods inherited from class java.lang.Object

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

Field Detail

table

private transient Hashtable.Entry[] table

The hash table data.

count

private transient int count

The total number of entries in the hash table.

threshold

private int threshold

The table is rehashed when its size exceeds this threshold. (The value of this field is (int)(capacity * loadFactor).)

loadFactor

private float loadFactor

The load factor for the hashtable.

modCount

private transient int modCount

The number of times this Hashtable has been structurally modified Structural modifications are those that change the number of entries in the Hashtable or otherwise modify its internal structure (e.g., rehash). This field is used to make iterators on Collection-views of the Hashtable fail-fast. (See ConcurrentModificationException).

serialVersionUID

private static final long serialVersionUID

use serialVersionUID from JDK 1.0.2 for interoperability

keySet

private transient Set keySet

entrySet

private transient Set entrySet

values

private transient Collection values

KEYS

private static final int KEYS

VALUES

private static final int VALUES

ENTRIES

private static final int ENTRIES

Constructor Detail

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.

Throws:

IllegalArgumentException - if the initial capacity is less than zero, or if the load factor is nonpositive.

Hashtable

public Hashtable(int initialCapacity)

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

Parameters:

initialCapacity - the initial capacity of the hashtable.

Throws:

IllegalArgumentException - if the initial capacity is less than zero.

Hashtable

public Hashtable()

Constructs a new, empty hashtable with a default capacity and load factor, which is 0.75.

Hashtable

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 or 11 (whichever is greater), and a default load factor, which is 0.75.

Since:

JDK1.2

Method Detail

size

public int size()

Returns the number of keys in this hashtable.

Specified by:

size in interface Map

Returns:

the number of keys in this hashtable.

Overrides:

size in class Dictionary

isEmpty

public boolean isEmpty()

Tests if this hashtable maps no keys to values.

Specified by:

isEmpty in interface Map

Returns:

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

Overrides:

isEmpty in class Dictionary

keys

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

elements

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

contains

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

Throws:

NullPointerException - if the value is null.

See Also:

containsKey(Object), containsValue(Object), Map

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 contains (which predates the Map interface).

Specified by:

containsValue in interface Map

Parameters:

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

Since:

JDK1.2

See Also:

Map

containsKey

public boolean containsKey(Object key)

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

Specified by:

containsKey in interface Map

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.

See Also:

contains(Object)

get

public Object get(Object key)

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

Specified by:

get in interface Map

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(Object, Object)

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.

put

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.

Specified by:

put in interface Map

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:

Object.equals(Object), get(Object)

remove

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.

Specified by:

remove in interface Map

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

putAll

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.

Specified by:

putAll in interface Map

Since:

JDK1.2

clear

public void clear()

Clears this hashtable so that it contains no keys.

Specified by:

clear in interface Map

clone

public 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.

Returns:

a clone of the hashtable.

Overrides:

clone in class Object

toString

public 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 to toString method of Object.

Returns:

a string representation of this hashtable.

Overrides:

toString in class Object

keySet

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.

Specified by:

keySet in interface Map

Since:

JDK1.2

entrySet

public Set entrySet()

Returns a Set view of the entries contained in this Hashtable. Each element in this collection is a Map.Entry. 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.

Specified by:

entrySet in interface Map

Since:

JDK1.2

See Also:

Map.Entry

values

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), but not element addition.

Specified by:

values in interface Map

Since:

JDK1.2

equals

public boolean equals(Object o)

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

Specified by:

equals in interface Map

Returns:

true if the specified Object is equal to this Map.

Overrides:

equals in class Object

Since:

JDK1.2

See Also:

Map.equals(Object)

hashCode

public int hashCode()

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

Specified by:

hashCode in interface Map

Overrides:

hashCode in class Object

Since:

JDK1.2

See Also:

Map.hashCode()

writeObject

private void writeObject(ObjectOutputStream s)
                  throws IOException

Save the state of the Hashtable to a stream (i.e., serialize it).

readObject

private void readObject(ObjectInputStream s)
                 throws IOException,
                        ClassNotFoundException

Reconstitute the Hashtable from a stream (i.e., deserialize it).