java.lang.Object | +--java.util.Dictionary | +--java.util.HashtableAll Implemented Interfaces:
Hashtable() Construct a new Hashtable with the default capacity (11) and the default load factor (0.75). |
Hashtable(java.util.Map m) Construct a new Hashtable from the given Map, with initial capacity
the greater of the size of |
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. |
synchronized void | clear() Clears the hashtable so it has no keys. |
synchronized java.lang.Object | clone() Returns a shallow clone of this Hashtable. |
synchronized boolean | contains(java.lang.Object value) Returns true if this Hashtable contains a value |
synchronized boolean | containsKey(java.lang.Object key) Returns true if the supplied object |
boolean | containsValue(java.lang.Object value) Returns true if this Hashtable contains a value |
java.util.Enumeration | elements() Return an enumeration of the values of this table. |
java.util.Set | entrySet() Returns a "set view" of this Hashtable's entries. |
boolean | equals(java.lang.Object o) Returns true if this Hashtable equals the supplied Object |
synchronized java.lang.Object | get(java.lang.Object key) Return the value in this Hashtable associated with the supplied key,
or |
synchronized int | hashCode() Returns the hashCode for this Hashtable. |
synchronized boolean | isEmpty() Returns true if there are no key-value mappings currently in this table. |
java.util.Set | keySet() Returns a "set view" of this Hashtable's keys. |
java.util.Enumeration | keys() Return an enumeration of the keys of this table. |
synchronized java.lang.Object | put(java.lang.Object key, java.lang.Object value) Puts the supplied value into the Map, mapped by the supplied key. |
synchronized void | putAll(java.util.Map m) Copies all elements of the given map into this hashtable. |
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. |
synchronized java.lang.Object | remove(java.lang.Object key) Removes from the table and returns the value which is mapped by the supplied key. |
synchronized int | size() Returns the number of key-value mappings currently in this hashtable. |
synchronized java.lang.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. |
java.util.Collection | 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(java.util.Map 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.NullPointerException
- if m is null, or if m contains a mapping
to or from `null'.public synchronized void clear()
public synchronized Object clone()
public synchronized boolean contains(java.lang.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 nullpublic synchronized boolean containsKey(java.lang.Object key)
equals()
a key
in this Hashtable.
key
- the key to search for in this HashtableNullPointerException
- if key is nullpublic boolean containsValue(java.lang.Object value)
o
, such that
o.equals(value)
. This is the new API for the old
contains()
.
value
- the value to search for in this HashtableNullPointerException
- if value
is nullpublic Enumeration elements()
public Set entrySet()
Note that the iterators for all three views, from keySet(), entrySet(), and values(), traverse the hashtable in the same sequence.
public boolean equals(java.lang.Object o)
o
.
As specified by Map, this is:
(o instanceof Map) && entrySet().equals(((Map) o).entrySet());
o
- the object to compare topublic synchronized Object get(java.lang.Object key)
null
if the key maps to nothing.
key
- the key for which to fetch an associated valueNullPointerException
- if key is nullpublic synchronized int hashCode()
public synchronized boolean isEmpty()
size() == 0
public Set keySet()
public Enumeration keys()
public synchronized Object put(java.lang.Object key, java.lang.Object value)
equals()
this key.
key
- the key used to locate the valuevalue
- the value to be stored in the tableNullPointerException
- if key or value is nullpublic synchronized void putAll(java.util.Map m)
m
- the map to be hashed into thisNullPointerException
- if m is null, or contains null keys or valuesprotected void rehash()
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.
public synchronized Object remove(java.lang.Object key)
null
is returned.
key
- the key used to locate the value to removepublic synchronized int size()
public synchronized 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 values()
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 aConcurrentModificationException
rather than exhibit non-deterministic behavior.