java.util
Class LinkedHashMap<K,V>

java.lang.Object
  |
  +--java.util.AbstractMap<A,B>
        |
        +--java.util.HashMap<K,V>
              |
              +--java.util.LinkedHashMap<K,V>

All Implemented Interfaces:

java.lang.Cloneable, Map<K,V>, java.io.Serializable

public class LinkedHashMap<K,V>

extends HashMap<K,V>

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.

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.

Since:

JDK1.4

See Also:

Object.hashCode(), Collection, Map, HashMap, TreeMap, Hashtable, Serialized Form

Nested classes inherited from class java.util.Map<K,V>

Map.Entry<K,V>

Constructor Summary

LinkedHashMap()
Constructs a new, empty insertion-ordered LinkedHashMap instance with a default capacity and load factor (0.75).

LinkedHashMap(int initialCapacity)
Constructs a new, empty insertion-ordered LinkedHashMap instance with the specified initial capacity and a default load factor (0.75).

LinkedHashMap(int initialCapacity, float loadFactor)
Constructs a new, empty insertion-ordered LinkedHashMap instance with the specified initial capacity and load factor.

LinkedHashMap(int initialCapacity, float loadFactor, boolean accessOrder)
Constructs a new, empty linked hash map with the specified initial capacity, load factor and ordering mode.

<K2 extends K, V2 extends V> LinkedHashMap(java.util.Map<K2,V2> t)
Constructs a new insertion-ordered LinkedHashMap instance with the same mappings as the specified map.

Method Summary

void	clear() Removes all mappings from this map.
java.lang.Object	clone() Returns a shallow copy of this LinkedHashMap instance: the keys and values themselves are not cloned.
boolean	containsValue(java.lang.Object value) Returns true if this map maps one or more keys to the specified value.
V	get(java.lang.Object key) Returns the value to which this map maps the specified key.
V	remove(java.lang.Object key) Removes the mapping for this key from this map if present.

Methods inherited from class java.util.HashMap<K,V>

containsKey, entrySet, isEmpty, keySet, put, size, values

Methods inherited from class java.util.AbstractMap<A,B>

equals, hashCode, put, putAll, toString

Methods inherited from class java.lang.Object

finalize, getClass, notify, notifyAll, wait, wait, wait

Methods inherited from interface java.util.Map<K,V>

equals, hashCode, putAll

Constructor Detail

LinkedHashMap

public LinkedHashMap(int initialCapacity,
                     float loadFactor)

Constructs a new, empty insertion-ordered LinkedHashMap instance with the specified initial capacity and load factor.

Parameters:

initialCapacity - the initial capacity of the LinkedHashMap.

loadFactor - a number between 0.0 and 1.0.

Throws:

java.lang.IllegalArgumentException - if the initial capacity is less than or equal to zero, or if the load factor is less than or equal to zero.

LinkedHashMap

public LinkedHashMap(int initialCapacity)

Constructs a new, empty insertion-ordered LinkedHashMap instance with the specified initial capacity and a default load factor (0.75).

Parameters:

initialCapacity - the initial capacity of the LinkedHashMap.

LinkedHashMap

public LinkedHashMap()

Constructs a new, empty insertion-ordered LinkedHashMap instance with a default capacity and load factor (0.75).

LinkedHashMap

public <K2 extends K, V2 extends V> LinkedHashMap(java.util.Map<K2,V2> t)

Constructs a new insertion-ordered LinkedHashMap instance with the same mappings as the specified map. The LinkedHashMap instance is created with a capacity of thrice the number of mappings in the specified map or 11 (whichever is greater), and a default load factor (0.75).

LinkedHashMap

public LinkedHashMap(int initialCapacity,
                     float loadFactor,
                     boolean accessOrder)

Constructs a new, empty linked hash map with the specified initial capacity, load factor and ordering mode.

Parameters:

initialCapacity - the initial capacity of the HashMap.

loadFactor - the load factor of the HashMap

accessOrder - the ordering mode - true for access-order, false for insertion-order.

Throws:

java.lang.IllegalArgumentException - if the initial capacity is less than zero, or if the load factor is nonpositive.

Method Detail

containsValue

public boolean containsValue(java.lang.Object value)

Returns true if this map maps one or more keys to the specified value.

Specified by:

containsValue in interface Map<K,V>

Overrides:

containsValue in class HashMap<K,V>

Parameters:

value - value whose presence in this map is to be tested.

Returns:

true if this map maps one or more keys to the specified value.

get

public V get(java.lang.Object key)

Returns the value to which this map maps the specified key. Returns null if the map contains no mapping for this key. A return value of null does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to null. The containsKey operation may be used to distinguish these two cases.

Specified by:

get in interface Map<K,V>

Overrides:

get in class HashMap<K,V>

Parameters:

key - key whose associated value is to be returned.

Returns:

the value to which this map maps the specified key.

remove

public V remove(java.lang.Object key)

Removes the mapping for this key from this map if present.

Specified by:

remove in interface Map<K,V>

Overrides:

remove in class HashMap<K,V>

Parameters:

key - key whose mapping is to be removed from the map.

Returns:

previous value associated with specified key, or null if there was no mapping for key. A null return can also indicate that the map previously associated null with the specified key.

clear

public void clear()

Removes all mappings from this map.

Specified by:

clear in interface Map<K,V>

Overrides:

clear in class HashMap<K,V>

Following copied from interface: java.util.Map<K,V>

Throws:

java.lang.UnsupportedOperationException - clear is not supported by this map.

clone

public java.lang.Object clone()

Returns a shallow copy of this LinkedHashMap instance: the keys and values themselves are not cloned.

Overrides:

clone in class HashMap<K,V>

Returns:

a shallow copy of this map.