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java.util

Class Hashtable<K,V>

public class Hashtable<K,V> extends Dictionary<K,V> implements Map<K,V>, Cloneable, Serializable

A class which implements a hashtable data structure.

This implementation of Hashtable uses a hash-bucket approach. That is: linear probing and rehashing is avoided; instead, each hashed value maps to a simple linked-list which, in the best case, only has one node. Assuming a large enough table, low enough load factor, and / or well implemented hashCode() methods, Hashtable should provide O(1) insertion, deletion, and searching of keys. Hashtable is O(n) in the worst case for all of these (if all keys hash to the same bucket).

This is a JDK-1.2 compliant implementation of Hashtable. As such, it belongs, partially, to the Collections framework (in that it implements Map). For backwards compatibility, it inherits from the obsolete and utterly useless Dictionary class.

Being a hybrid of old and new, Hashtable has methods which provide redundant capability, but with subtle and even crucial differences. For example, one can iterate over various aspects of a Hashtable with either an Iterator (which is the JDK-1.2 way of doing things) or with an Enumeration. The latter can end up in an undefined state if the Hashtable changes while the Enumeration is open.

Unlike HashMap, Hashtable does not accept `null' as a key value. Also, all accesses are synchronized: in a single thread environment, this is expensive, but in a multi-thread environment, this saves you the effort of extra synchronization. However, the old-style enumerators are not synchronized, because they can lead to unspecified behavior even if they were synchronized. You have been warned.

The iterators are fail-fast, meaning that any structural modification, except for remove() called on the iterator itself, cause the iterator to throw a ConcurrentModificationException rather than exhibit non-deterministic behavior.

Since: 1.0

See Also: HashMap TreeMap IdentityHashMap LinkedHashMap

UNKNOWN: updated to 1.4

Constructor Summary
Hashtable()
Construct a new Hashtable with the default capacity (11) and the default load factor (0.75).
Hashtable(Map<? extends K,? extends V> m)
Construct a new Hashtable from the given Map, with initial capacity the greater of the size of m or the default of 11.
Hashtable(int initialCapacity)
Construct a new Hashtable with a specific inital capacity and default load factor of 0.75.
Hashtable(int initialCapacity, float loadFactor)
Construct a new Hashtable with a specific initial capacity and load factor.
Method Summary
voidclear()
Clears the hashtable so it has no keys.
Objectclone()
Returns a shallow clone of this Hashtable.
booleancontains(Object value)
Returns true if this Hashtable contains a value o, such that o.equals(value).
booleancontainsKey(Object key)
Returns true if the supplied object equals() a key in this Hashtable.
booleancontainsValue(Object value)
Returns true if this Hashtable contains a value o, such that o.equals(value).
Enumeration<V>elements()
Return an enumeration of the values of this table.
Set<Entry<K,V>>entrySet()
Returns a "set view" of this Hashtable's entries.
booleanequals(Object o)
Returns true if this Hashtable equals the supplied Object o.
Vget(Object key)
Return the value in this Hashtable associated with the supplied key, or null if the key maps to nothing.
inthashCode()
Returns the hashCode for this Hashtable.
booleanisEmpty()
Returns true if there are no key-value mappings currently in this table.
Enumeration<K>keys()
Return an enumeration of the keys of this table.
Set<K>keySet()
Returns a "set view" of this Hashtable's keys.
Vput(K key, V value)
Puts the supplied value into the Map, mapped by the supplied key.
voidputAll(Map<? extends K,? extends V> m)
Copies all elements of the given map into this hashtable.
protected voidrehash()
Increases the size of the Hashtable and rehashes all keys to new array indices; this is called when the addition of a new value would cause size() > threshold.
Vremove(Object key)
Removes from the table and returns the value which is mapped by the supplied key.
intsize()
Returns the number of key-value mappings currently in this hashtable.
StringtoString()
Converts this Hashtable to a String, surrounded by braces, and with key/value pairs listed with an equals sign between, separated by a comma and space.
Collection<V>values()
Returns a "collection view" (or "bag view") of this Hashtable's values.

Constructor Detail

Hashtable

public Hashtable()
Construct a new Hashtable with the default capacity (11) and the default load factor (0.75).

Hashtable

public Hashtable(Map<? extends K,? extends V> m)
Construct a new Hashtable from the given Map, with initial capacity the greater of the size of m or the default of 11.

Every element in Map m will be put into this new Hashtable.

Parameters: m a Map whose key / value pairs will be put into the new Hashtable. NOTE: key / value pairs are not cloned in this constructor.

Throws: NullPointerException if m is null, or if m contains a mapping to or from `null'.

Since: 1.2

Hashtable

public Hashtable(int initialCapacity)
Construct a new Hashtable with a specific inital capacity and default load factor of 0.75.

Parameters: initialCapacity the initial capacity of this Hashtable (>= 0)

Throws: IllegalArgumentException if (initialCapacity < 0)

Hashtable

public Hashtable(int initialCapacity, float loadFactor)
Construct a new Hashtable with a specific initial capacity and load factor.

Parameters: initialCapacity the initial capacity (>= 0) loadFactor the load factor (> 0, not NaN)

Throws: IllegalArgumentException if (initialCapacity < 0) || ! (loadFactor > 0.0)

Method Detail

clear

public void clear()
Clears the hashtable so it has no keys. This is O(1).

clone

public Object clone()
Returns a shallow clone of this Hashtable. The Map itself is cloned, but its contents are not. This is O(n).

Returns: the clone

contains

public boolean contains(Object value)
Returns true if this Hashtable contains a value o, such that o.equals(value). This is the same as containsValue(), and is O(n).

Parameters: value the value to search for in this Hashtable

Returns: true if at least one key maps to the value

Throws: NullPointerException if value is null

See Also: containsValue containsKey

containsKey

public boolean containsKey(Object key)
Returns true if the supplied object equals() a key in this Hashtable.

Parameters: key the key to search for in this Hashtable

Returns: true if the key is in the table

Throws: NullPointerException if key is null

See Also: containsValue

containsValue

public boolean containsValue(Object value)
Returns true if this Hashtable contains a value o, such that o.equals(value). This is the new API for the old contains().

Parameters: value the value to search for in this Hashtable

Returns: true if at least one key maps to the value

Throws: NullPointerException if value is null

Since: 1.2

See Also: contains containsKey

elements

public Enumeration<V> elements()
Return an enumeration of the values of this table. There's no point in synchronizing this, as you have already been warned that the enumeration is not specified to be thread-safe.

Returns: the values

See Also: keys values

entrySet

public Set<Entry<K,V>> entrySet()
Returns a "set view" of this Hashtable's entries. The set is backed by the hashtable, so changes in one show up in the other. The set supports element removal, but not element addition. The set is properly synchronized on the original hashtable. Sun has not documented the proper interaction of null with this set, but has inconsistent behavior in the JDK. Therefore, in this implementation, contains, remove, containsAll, retainAll, removeAll, and equals just ignore a null entry, or an entry with a null key or value, rather than throwing a {@link NullPointerException}. However, calling entry.setValue(null) will fail.

Note that the iterators for all three views, from keySet(), entrySet(), and values(), traverse the hashtable in the same sequence.

Returns: a set view of the entries

Since: 1.2

See Also: keySet values Entry

equals

public boolean equals(Object o)
Returns true if this Hashtable equals the supplied Object o. As specified by Map, this is: (o instanceof Map) && entrySet().equals(((Map) o).entrySet());

Parameters: o the object to compare to

Returns: true if o is an equal map

Since: 1.2

get

public V get(Object key)
Return the value in this Hashtable associated with the supplied key, or null if the key maps to nothing.

Parameters: key the key for which to fetch an associated value

Returns: what the key maps to, if present

Throws: NullPointerException if key is null

See Also: Hashtable containsKey

hashCode

public int hashCode()
Returns the hashCode for this Hashtable. As specified by Map, this is the sum of the hashCodes of all of its Map.Entry objects

Returns: the sum of the hashcodes of the entries

Since: 1.2

isEmpty

public boolean isEmpty()
Returns true if there are no key-value mappings currently in this table.

Returns: size() == 0

keys

public Enumeration<K> keys()
Return an enumeration of the keys of this table. There's no point in synchronizing this, as you have already been warned that the enumeration is not specified to be thread-safe.

Returns: the keys

See Also: elements keySet

keySet

public Set<K> keySet()
Returns a "set view" of this Hashtable's keys. The set is backed by the hashtable, so changes in one show up in the other. The set supports element removal, but not element addition. The set is properly synchronized on the original hashtable. Sun has not documented the proper interaction of null with this set, but has inconsistent behavior in the JDK. Therefore, in this implementation, contains, remove, containsAll, retainAll, removeAll, and equals just ignore a null key rather than throwing a {@link NullPointerException}.

Returns: a set view of the keys

Since: 1.2

See Also: values entrySet

put

public V put(K key, V value)
Puts the supplied value into the Map, mapped by the supplied key. Neither parameter may be null. The value may be retrieved by any object which equals() this key.

Parameters: key the key used to locate the value value the value to be stored in the table

Returns: the prior mapping of the key, or null if there was none

Throws: NullPointerException if key or value is null

See Also: get equals

putAll

public void putAll(Map<? extends K,? extends V> m)
Copies all elements of the given map into this hashtable. However, no mapping can contain null as key or value. If this table already has a mapping for a key, the new mapping replaces the current one.

Parameters: m the map to be hashed into this

Throws: NullPointerException if m is null, or contains null keys or values

rehash

protected void rehash()
Increases the size of the Hashtable and rehashes all keys to new array indices; this is called when the addition of a new value would cause size() > threshold. Note that the existing Entry objects are reused in the new hash table.

This is not specified, but the new size is twice the current size plus one; this number is not always prime, unfortunately. This implementation is not synchronized, as it is only invoked from synchronized methods.

remove

public V remove(Object key)
Removes from the table and returns the value which is mapped by the supplied key. If the key maps to nothing, then the table remains unchanged, and null is returned.

Parameters: key the key used to locate the value to remove

Returns: whatever the key mapped to, if present

size

public int size()
Returns the number of key-value mappings currently in this hashtable.

Returns: the size

toString

public String toString()
Converts this Hashtable to a String, surrounded by braces, and with key/value pairs listed with an equals sign between, separated by a comma and space. For example, "{a=1, b=2}".

NOTE: if the toString() method of any key or value throws an exception, this will fail for the same reason.

Returns: the string representation

values

public Collection<V> values()
Returns a "collection view" (or "bag view") of this Hashtable's values. The collection is backed by the hashtable, so changes in one show up in the other. The collection supports element removal, but not element addition. The collection is properly synchronized on the original hashtable. Sun has not documented the proper interaction of null with this set, but has inconsistent behavior in the JDK. Therefore, in this implementation, contains, remove, containsAll, retainAll, removeAll, and equals just ignore a null value rather than throwing a {@link NullPointerException}.

Returns: a bag view of the values

Since: 1.2

See Also: keySet entrySet

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Summary: Nested Field Constr Method Detail: Field Constr Method