public class Hashtable<K,V> extends Dictionary<K,V> implements Map<K,V>, Cloneable
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.
HashMap,
TreeMap,
IdentityHashMap,
LinkedHashMap| Constructor and Description |
|---|
Hashtable()
Construct a new Hashtable with the default capacity (11) and the default
load factor (0.75).
|
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.
|
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. |
| Modifier and Type | Method and Description |
|---|---|
void |
clear()
Clears the hashtable so it has no keys.
|
Object |
clone()
Returns a shallow clone of this Hashtable.
|
boolean |
contains(Object value)
Returns true if this Hashtable contains a value
o,
such that o.equals(value). |
boolean |
containsKey(Object key)
Returns true if the supplied object
equals() a key
in this Hashtable. |
boolean |
containsValue(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<Map.Entry<K,V>> |
entrySet()
Returns a "set view" of this Hashtable's entries.
|
boolean |
equals(Object o)
Returns true if this Hashtable equals the supplied Object
o. |
V |
get(Object key)
Return the value in this Hashtable associated with the supplied key,
or
null if the key maps to nothing. |
int |
hashCode()
Returns the hashCode for this Hashtable.
|
boolean |
isEmpty()
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.
|
V |
put(K key,
V value)
Puts the supplied value into the Map, mapped by the supplied key.
|
void |
putAll(Map<? extends K,? extends V> m)
Copies all elements of the given map into this hashtable.
|
V |
remove(Object key)
Removes from the table and returns the value which is mapped by the
supplied key.
|
int |
size()
Returns the number of key-value mappings currently in this hashtable.
|
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.
|
Collection<V> |
values()
Returns a "collection view" (or "bag view") of this Hashtable's values.
|
public Hashtable()
public Hashtable(int initialCapacity)
initialCapacity - the initial capacity of this Hashtable (>= 0)IllegalArgumentException - if (initialCapacity < 0)public Hashtable(int initialCapacity,
float loadFactor)
initialCapacity - the initial capacity (>= 0)loadFactor - the load factor (> 0, not NaN)IllegalArgumentException - if (initialCapacity < 0) ||
! (loadFactor > 0.0)public Hashtable(Map<? extends K,? extends V> m)
m or the default of 11.
Every element in Map m will be put into this new Hashtable.
m - a Map whose key / value pairs will be put into
the new Hashtable. NOTE: key / value pairs
are not cloned in this constructor.NullPointerException - if m is null, or if m contains a mapping
to or from `null'.public void clear()
public Object clone()
public boolean contains(Object value)
o,
such that o.equals(value). This is the same as
containsValue(), and is O(n).
value - the value to search for in this HashtableNullPointerException - if value is nullcontainsValue(Object),
containsKey(Object)public boolean containsKey(Object key)
equals() a key
in this Hashtable.containsKey in interface Map<K,V>key - the key to search for in this HashtableNullPointerException - if key is nullcontainsValue(Object)public boolean containsValue(Object value)
o, such that
o.equals(value). This is the new API for the old
contains().containsValue in interface Map<K,V>value - the value to search for in this HashtableNullPointerException - if value is nullcontains(Object),
containsKey(Object)public Enumeration<V> elements()
public Set<Map.Entry<K,V>> entrySet()
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.
public boolean equals(Object o)
o.
As specified by Map, this is:
(o instanceof Map) && entrySet().equals(((Map) o).entrySet());
public V get(Object key)
null if the key maps to nothing.get in interface Map<K,V>get in class Dictionary<K,V>key - the key for which to fetch an associated valueNullPointerException - if key is nullput(Object, Object),
containsKey(Object)public int hashCode()
public boolean isEmpty()
public Enumeration<K> keys()
keys in class Dictionary<K,V>elements(),
keySet()public Set<K> keySet()
NullPointerException.public V put(K key, V value)
equals() this key.put in interface Map<K,V>put in class Dictionary<K,V>key - the key used to locate the valuevalue - the value to be stored in the tableNullPointerException - if key or value is nullget(Object),
Object.equals(Object)public void putAll(Map<? extends K,? extends V> m)
putAll in interface Map<K,V>m - the map to be hashed into thisNullPointerException - if m is null, or contains null keys or valuesMap.put(Object, Object)public V remove(Object key)
null is returned.public int size()
public String toString()
"{a=1, b=2}".
NOTE: if the toString() method of any key or value
throws an exception, this will fail for the same reason.
public Collection<V> values()
NullPointerException.