java.util
public class LinkedHashMap<K,V> extends HashMap<K,V>
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 {@link #LinkedHashMap(int, float, boolean) constructor}, you can build an access-order mapping. This can be used to implement LRU caches, for example. By overriding {@link #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
{@link ConcurrentModificationException} rather than exhibit
non-deterministic behavior.
Since: 1.4
See Also: hashCode Collection Map HashMap TreeMap Hashtable
UNKNOWN: updated to 1.4
| Constructor Summary | |
|---|---|
| LinkedHashMap()
Construct a new insertion-ordered LinkedHashMap with the default
capacity (11) and the default load factor (0.75). | |
| LinkedHashMap(Map<? extends K,? extends 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.
| |
| Method Summary | |
|---|---|
| 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(Entry<K,V> eldest)
Returns true if this map should remove the eldest entry.
|
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.
Parameters: m a Map whose key / value pairs will be put into the new HashMap. NOTE: key / value pairs are not cloned in this constructor.
Throws: NullPointerException if m is null
Parameters: initialCapacity the initial capacity of this HashMap (>= 0)
Throws: IllegalArgumentException if (initialCapacity < 0)
Parameters: initialCapacity the initial capacity (>= 0) loadFactor the load factor (> 0, not NaN)
Throws: IllegalArgumentException if (initialCapacity < 0) || ! (loadFactor > 0.0)
Parameters: initialCapacity the initial capacity (>=0) loadFactor the load factor (>0, not NaN) accessOrder true for access-order, false for insertion-order
Throws: IllegalArgumentException if (initialCapacity < 0) || ! (loadFactor > 0.0)
true if this HashMap contains a value
o, such that o.equals(value).
Parameters: value the value to search for in this HashMap
Returns: true if at least one key maps to the value
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.
Parameters: key the key for which to fetch an associated value
Returns: what the key maps to, if present
See Also: LinkedHashMap LinkedHashMap
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.
Parameters: 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.
Returns: true if eldest should be removed