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 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.
Object.hashCode(),
Collection,
Map,
HashMap,
TreeMap,
HashtableAbstractMap.SimpleEntry<K,V>, AbstractMap.SimpleImmutableEntry<K,V>| Constructor and Description |
|---|
LinkedHashMap()
Construct a new insertion-ordered LinkedHashMap with the default
capacity (11) and the default load factor (0.75).
|
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.
|
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. |
| Modifier and Type | Method and Description |
|---|---|
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. |
clone, containsKey, entrySet, isEmpty, keySet, put, putAll, remove, size, valuesequals, hashCode, toStringpublic LinkedHashMap()
public LinkedHashMap(int initialCapacity)
initialCapacity - the initial capacity of this HashMap (>= 0)IllegalArgumentException - if (initialCapacity < 0)public LinkedHashMap(int initialCapacity,
float loadFactor)
initialCapacity - the initial capacity (>= 0)loadFactor - the load factor (> 0, not NaN)IllegalArgumentException - if (initialCapacity < 0) ||
! (loadFactor > 0.0)public LinkedHashMap(int initialCapacity,
float loadFactor,
boolean accessOrder)
initialCapacity - the initial capacity (>=0)loadFactor - the load factor (>0, not NaN)accessOrder - true for access-order, false for insertion-orderIllegalArgumentException - if (initialCapacity < 0) ||
! (loadFactor > 0.0)public LinkedHashMap(Map<? extends K,? extends V> m)
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.NullPointerException - if m is nullpublic void clear()
public boolean containsValue(Object value)
true if this HashMap contains a value
o, such that o.equals(value).containsValue in interface Map<K,V>containsValue in class HashMap<K,V>value - the value to search for in this HashMaptrue if at least one key maps to the valueHashMap.containsKey(Object)public V get(Object key)
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 class HashMap<K,V>key - the key for which to fetch an associated valueHashMap.put(Object, Object),
HashMap.containsKey(Object)