public class LinkedHashMap extends HashMap
Hash table and linked list implementation of the Map interface, with predictable iteration order. This implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (insertion-order). Note that insertion order is not affected if a key is re-inserted into the map. (A key k is reinserted into a map m if m.put(k, v) is invoked when m.containsKey(k) would return true immediately prior to the invocation.)
This implementation spares its clients from the unspecified, generally
chaotic ordering provided by HashMap (and Hashtable),
without incurring the increased cost associated with TreeMap. It
can be used to produce a copy of a map that has the same order as the
original, regardless of the original map's implementation:
void foo(Map m) {
Map copy = new LinkedHashMap(m);
...
}
This technique is particularly useful if a module takes a map on input,
copies it, and later returns results whose order is determined by that of
the copy. (Clients generally appreciate having things returned in the same
order they were presented.)
A special constructor is
provided to create a linked hash map whose order of iteration is the order
in which its entries were last accessed, from least-recently accessed to
most-recently (access-order). This kind of map is well-suited to
building LRU caches. Invoking the put or get method
results in an access to the corresponding entry (assuming it exists after
the invocation completes). The putAll method generates one entry
access for each mapping in the specified map, in the order that key-value
mappings are provided by the specified map's entry set iterator. No
other methods generate entry accesses. In particular, operations on
collection-views do not affect the order of iteration of the backing
map.
The removeEldestEntry(Map.Entry) method may be overridden to
impose a policy for removing stale mappings automatically when new mappings
are added to the map.
This class provides all of the optional Map operations, and permits null elements. Like HashMap, it provides constant-time performance for the basic operations (add, contains and remove), assuming the the hash function disperses elements properly among the buckets. Performance is likely to be just slightly below that of HashMap, due to the added expense of maintaining the linked list, with one exception: Iteration over the collection-views of a LinkedHashMap requires time proportional to the size of the map, regardless of its capacity. Iteration over a HashMap is likely to be more expensive, requiring time proportional to its capacity.
A linked hash map has two parameters that affect its performance: initial capacity and load factor. They are defined precisely as for HashMap. Note, however, that the penalty for choosing an excessively high value for initial capacity is less severe for this class than for HashMap, as iteration times for this class are unaffected by capacity.
Note that this implementation is not synchronized. If multiple threads access a linked hash map concurrently, and at least one of the threads modifies the map structurally, it must be synchronized externally. This is typically accomplished by synchronizing on some object that naturally encapsulates the map. If no such object exists, the map should be "wrapped" using the Collections.synchronizedMapmethod. This is best done at creation time, to prevent accidental unsynchronized access:
Map m = Collections.synchronizedMap(new LinkedHashMap(...));
A structural modification is any operation that adds or deletes one or more
mappings or, in the case of access-ordered linked hash maps, affects
iteration order. In insertion-ordered linked hash maps, merely changing
the value associated with a key that is already contained in the map is not
a structural modification. In access-ordered linked hash maps,
merely querying the map with get is a structural
modification.)
The iterators returned by the iterator methods of the collections returned by all of this class's collection view methods are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the Iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.
Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.
This class is a member of the Java Collections Framework.
Object.hashCode(),
Collection,
Map,
HashMap,
TreeMap,
Hashtable,
Serialized Form| Constructor and Description |
|---|
LinkedHashMap()
Constructs an empty insertion-ordered LinkedHashMap instance
with a default capacity (16) and load factor (0.75).
|
LinkedHashMap(int initialCapacity)
Constructs an empty insertion-ordered LinkedHashMap instance
with the specified initial capacity and a default load factor (0.75).
|
LinkedHashMap(int initialCapacity,
float loadFactor)
Constructs an empty insertion-ordered LinkedHashMap instance
with the specified initial capacity and load factor.
|
LinkedHashMap(int initialCapacity,
float loadFactor,
boolean accessOrder)
Constructs an empty LinkedHashMap instance with the
specified initial capacity, load factor and ordering mode.
|
LinkedHashMap(Map m)
Constructs an insertion-ordered LinkedHashMap instance with
the same mappings as the specified map.
|
| Modifier and Type | Method and Description |
|---|---|
void |
clear()
Removes all mappings from this map.
|
boolean |
containsValue(Object value)
Returns true if this map maps one or more keys to the
specified value.
|
Object |
get(Object key)
Returns the value to which this map maps the specified key.
|
protected boolean |
removeEldestEntry(Map.Entry eldest)
Returns true if this map should remove its eldest entry.
|
clone, containsKey, entrySet, isEmpty, keySet, put, putAll, remove, size, valuesequals, hashCode, toStringpublic LinkedHashMap(int initialCapacity,
float loadFactor)
initialCapacity - the initial capacity.loadFactor - the load factor.IllegalArgumentException - if the initial capacity is negative
or the load factor is nonpositive.public LinkedHashMap(int initialCapacity)
initialCapacity - the initial capacity.IllegalArgumentException - if the initial capacity is negative.public LinkedHashMap()
public LinkedHashMap(Map m)
m - the map whose mappings are to be placed in this map.NullPointerException - if the specified map is null.public LinkedHashMap(int initialCapacity,
float loadFactor,
boolean accessOrder)
initialCapacity - the initial capacity.loadFactor - the load factor.accessOrder - the ordering mode - true for
access-order, false for insertion-order.IllegalArgumentException - if the initial capacity is negative
or the load factor is nonpositive.public boolean containsValue(Object value)
containsValue in interface MapcontainsValue in class HashMapvalue - value whose presence in this map is to be tested.public Object get(Object key)
get in interface Mapget in class HashMapkey - key whose associated value is to be returned.HashMap.put(Object, Object)public void clear()
protected boolean removeEldestEntry(Map.Entry eldest)
Sample use: this override will allow the map to grow up to 100 entries and then delete the eldest entry each time a new entry is added, maintaining a steady state of 100 entries.
private static final int MAX_ENTRIES = 100;
protected boolean removeEldestEntry(Map.Entry eldest) {
return size() > MAX_ENTRIES;
}
This method typically does not modify the map in any way, instead allowing the map to modify itself as directed by its return value. It is permitted for this method to modify the map directly, but if it does so, it must return false (indicating that the map should not attempt any further modification). The effects of returning true after modifying the map from within this method are unspecified.
This implementation merely returns false (so that this map acts like a normal map - the eldest element is never removed).
eldest - The least recently inserted entry in the map, or if
this is an access-ordered map, the least recently accessed
entry. This is the entry that will be removed it this
method returns true. If the map was empty prior
to the put or putAll invocation resulting
in this invocation, this will be the entry that was just
inserted; in other words, if the map contains a single
entry, the eldest entry is also the newest.Copyright © 2012. All Rights Reserved.