java.lang.Object | +--java.util.AbstractMap | +--java.util.HashMap | +--java.util.LinkedHashMap
LinkedHashMap() Construct a new insertion-ordered LinkedHashMap with the default capacity (11) and the default load factor (0.75). |
LinkedHashMap(java.util.Map m) Construct a new insertion-ordered LinkedHashMap from the given Map,
with initial capacity the greater of the size of |
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(java.lang.Object value) Returns |
java.lang.Object | get(java.lang.Object key) Return the value in this Map associated with the supplied key,
or |
boolean | removeEldestEntry(java.util.Map.Entry eldest) Returns |
public 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(java.util.Map 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.NullPointerException
- if m is nullpublic void clear()
public boolean containsValue(java.lang.Object value)
true
if this HashMap contains a value
o
, such that o.equals(value)
.
value
- the value to search for in this HashMaptrue
if at least one key maps to the valuepublic Object get(java.lang.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.
key
- the key for which to fetch an associated valueprotected boolean removeEldestEntry(java.util.Map.Entry eldest)
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.eldest
should be removed
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 toput
andget
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 callput
orget
.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 #LinkedHashMap(int, float, boolean) 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.