java.util
Class LinkedHashMap<K,V>
- Cloneable, Map<K,V>, Serializable
This class provides a hashtable-backed implementation of the
Map interface, with predictable traversal order.
It uses a hash-bucket approach; that is, hash collisions are handled
by linking the new node off of the pre-existing node (or list of
nodes). In this manner, techniques such as linear probing (which
can cause primary clustering) and rehashing (which does not fit very
well with Java's method of precomputing hash codes) are avoided. In
addition, this maintains a doubly-linked list which tracks either
insertion or access order.
In insertion order, calling
put adds the key to the end of
traversal, unless the key was already in the map; changing traversal order
requires removing and reinserting a key. On the other hand, in access
order, all calls to
put and
get cause the
accessed key to move to the end of the traversal list. Note that any
accesses to the map's contents via its collection views and iterators do
not affect the map's traversal order, since the collection views do not
call
put or
get.
One of the nice features of tracking insertion order is that you can
copy a hashtable, and regardless of the implementation of the original,
produce the same results when iterating over the copy. This is possible
without needing the overhead of
TreeMap.
When using this
constructor,
you can build an access-order mapping. This can be used to implement LRU
caches, for example. By overriding
removeEldestEntry(Map.Entry),
you can also control the removal of the oldest entry, and thereby do
things like keep the map at a fixed size.
Under ideal circumstances (no collisions), LinkedHashMap offers O(1)
performance on most operations (
containsValue() is,
of course, O(n)). In the worst case (all keys map to the same
hash code -- very unlikely), most operations are O(n). Traversal is
faster than in HashMap (proportional to the map size, and not the space
allocated for the map), but other operations may be slower because of the
overhead of the maintaining the traversal order list.
LinkedHashMap accepts the null key and null values. It is not
synchronized, so if you need multi-threaded access, consider using:
Map m = Collections.synchronizedMap(new LinkedHashMap(...));
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.
LinkedHashMap()- Construct a new insertion-ordered LinkedHashMap with the default
capacity (11) and the default load factor (0.75).
|
LinkedHashMap(extends K, V> m)- Construct a new insertion-ordered LinkedHashMap from the given Map,
with initial capacity the greater of the size of
m or
the default of 11.
|
LinkedHashMap(int initialCapacity)- Construct a new insertion-ordered LinkedHashMap with a specific
inital capacity and default load factor of 0.75.
|
LinkedHashMap(int initialCapacity, float loadFactor)- Construct a new insertion-orderd LinkedHashMap with a specific
inital capacity and load factor.
|
LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder)- Construct a new LinkedHashMap with a specific inital capacity, load
factor, and ordering mode.
|
void | clear()- Clears the Map so it has no keys.
|
boolean | containsValue(Object value)- Returns
true if this HashMap contains a value
o, such that o.equals(value).
|
V | get(Object key)- Return the value in this Map associated with the supplied key,
or
null if the key maps to nothing.
|
protected boolean | removeEldestEntry(java.util.Map.Entry eldest)- Returns
true if this map should remove the eldest entry.
|
V>> entrySet, clear, clone, containsKey, containsValue, get, isEmpty, keySet, put, putAll, remove, size, values |
V>> entrySet, clear, clone, containsKey, containsValue, equals, get, hashCode, isEmpty, keySet, put, putAll, remove, size, toString, values |
clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait |
LinkedHashMap
public LinkedHashMap()
Construct a new insertion-ordered LinkedHashMap with the default
capacity (11) and the default load factor (0.75).
LinkedHashMap
public LinkedHashMap(extends K,
V> m) Construct a new insertion-ordered LinkedHashMap 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 HashMap, in the
order of m's iterator.
m - a Map whose key / value pairs will be put into
the new HashMap. NOTE: key / value pairs
are not cloned in this constructor.
LinkedHashMap
public LinkedHashMap(int initialCapacity)
Construct a new insertion-ordered LinkedHashMap with a specific
inital capacity and default load factor of 0.75.
initialCapacity - the initial capacity of this HashMap (>= 0)
LinkedHashMap
public LinkedHashMap(int initialCapacity,
float loadFactor) Construct a new insertion-orderd LinkedHashMap with a specific
inital capacity and load factor.
initialCapacity - the initial capacity (>= 0)loadFactor - the load factor (> 0, not NaN)
LinkedHashMap
public LinkedHashMap(int initialCapacity,
float loadFactor,
boolean accessOrder) Construct a new LinkedHashMap with a specific inital capacity, load
factor, and ordering mode.
initialCapacity - the initial capacity (>=0)loadFactor - the load factor (>0, not NaN)accessOrder - true for access-order, false for insertion-order
get
public V get(Object key)
Return the value in this Map associated with the supplied key,
or null if the key maps to nothing. If this is an
access-ordered Map and the key is found, this performs structural
modification, moving the key to the newest end of the list. NOTE:
Since the value could also be null, you must use containsKey to
see if this key actually maps to something.
- get in interface Map<K,V>
- get in interface HashMap<K,V>
key - the key for which to fetch an associated value
- what the key maps to, if present
removeEldestEntry
protected boolean removeEldestEntry(java.util.Map.Entry eldest)
Returns
true if this map should remove the eldest entry.
This method is invoked by all calls to
put and
putAll which place a new entry in the map, providing
the implementer an opportunity to remove the eldest entry any time
a new one is added. This can be used to save memory usage of the
hashtable, as well as emulating a cache, by deleting stale entries.
For example, to keep the Map limited to 100 entries, override as follows:
private static final int MAX_ENTRIES = 100;
protected boolean removeEldestEntry(Map.Entry eldest)
{
return size() > MAX_ENTRIES;
}
Typically, this method does not modify the map, but just uses the
return value as an indication to
put whether to proceed.
However, if you override it to modify the map, you must return false
(indicating that
put should leave the modified map alone),
or you face unspecified behavior. Remember that in access-order mode,
even calling
get is a structural modification, but using
the collections views (such as
keySet) is not.
This method is called after the eldest entry has been inserted, so
if
put was called on a previously empty map, the eldest
entry is the one you just put in! The default implementation just
returns
false, so that this map always behaves like
a normal one with unbounded growth.
eldest - the eldest element which would be removed if this
returns true. For an access-order map, this is the least
recently accessed; for an insertion-order map, this is the
earliest element inserted.
- true if
eldest should be removed
LinkedHashMap.java -- a class providing hashtable data structure,
mapping Object --> Object, with linked list traversal
Copyright (C) 2001, 2002, 2005 Free Software Foundation, Inc.
This file is part of GNU Classpath.
GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2, or (at your option)
any later version.
GNU Classpath is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License for more details.
You should have received a copy of the GNU General Public License
along with GNU Classpath; see the file COPYING. If not, write to the
Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
02110-1301 USA.
Linking this library statically or dynamically with other modules is
making a combined work based on this library. Thus, the terms and
conditions of the GNU General Public License cover the whole
combination.
As a special exception, the copyright holders of this library give you
permission to link this library with independent modules to produce an
executable, regardless of the license terms of these independent
modules, and to copy and distribute the resulting executable under
terms of your choice, provided that you also meet, for each linked
independent module, the terms and conditions of the license of that
module. An independent module is a module which is not derived from
or based on this library. If you modify this library, you may extend
this exception to your version of the library, but you are not
obligated to do so. If you do not wish to do so, delete this
exception statement from your version.