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aaron.util
Class LinkedHashMap  

java.lang.Object
  aaron.util.AbstractMap
      aaron.util.HashMap
          aaron.util.LinkedHashMap

All Implemented Interfaces:

java.lang.Cloneable, java.util.Map, java.io.Serializable

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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 java.util.Hashtable), without incurring the increased cost associated with java.util.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 55 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) 55 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.

Since:

JDK1.4

Version:

1.9, 12/03/01

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Nested Class Summary
private static class	LinkedHashMap.Entry           LinkedHashMap entry.
private  class	LinkedHashMap.EntryIterator
private  class	LinkedHashMap.KeyIterator
private  class	LinkedHashMap.LinkedHashIterator
private  class	LinkedHashMap.ValueIterator

 

Nested classes inherited from class aaron.util.HashMap

 

Nested classes inherited from class aaron.util.AbstractMap

AbstractMap.SimpleEntry

 

Field Summary
private  boolean	accessOrder           The iteration ordering method for this linked hash map: true for access-order, false for insertion-order.
private  LinkedHashMap.Entry	header           The head of the doubly linked list.

 

Fields inherited from class aaron.util.HashMap

DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, loadFactor, MAXIMUM_CAPACITY, modCount, NULL_KEY, size, table, threshold

 

Fields inherited from class aaron.util.AbstractMap

keySet, values

 

Constructor Summary

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(java.util.Map m)           Constructs an insertion-ordered LinkedHashMap instance with the same mappings as the specified map.

 

Method Summary
(package private)  void	addEntry(int hash, java.lang.Object key, java.lang.Object value, int bucketIndex)           This override alters behavior of superclass put method.
void	clear()           Removes all mappings from this map.
boolean	containsValue(java.lang.Object value)           Returns true if this map maps one or more keys to the specified value.
(package private)  void	createEntry(int hash, java.lang.Object key, java.lang.Object value, int bucketIndex)           This override differs from addEntry in that it doesn't resize the table or remove the eldest entry.
java.lang.Object	get(java.lang.Object key)           Returns the value to which this map maps the specified key.
(package private)  void	init()           Called by superclass constructors and pseudoconstructors (clone, readObject) before any entries are inserted into the map.
(package private)  java.util.Iterator	newEntryIterator()
(package private)  java.util.Iterator	newKeyIterator()
(package private)  java.util.Iterator	newValueIterator()
protected  boolean	removeEldestEntry(java.util.Map.Entry eldest)           Returns true if this map should remove its eldest entry.
(package private)  void	transfer(HashMap.Entry[] newTable)           Transfer all entries to new table array.

 

Methods inherited from class aaron.util.HashMap

capacity, clone, containsKey, entrySet, eq, getEntry, hash, indexFor, isEmpty, keySet, loadFactor, maskNull, put, putAll, putAllForCreate, remove, removeEntryForKey, removeMapping, resize, size, unmaskNull, values

 

Methods inherited from class aaron.util.AbstractMap

equals, hashCode, toString

 

Methods inherited from class java.lang.Object

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

 

Methods inherited from interface java.util.Map

equals, hashCode

 

Field Detail

header

private transient LinkedHashMap.Entry header

The head of the doubly linked list.

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accessOrder

private final boolean accessOrder

The iteration ordering method for this linked hash map: true for access-order, false for insertion-order.

Constructor Detail

LinkedHashMap

public LinkedHashMap(int initialCapacity,
                     float loadFactor)

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

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LinkedHashMap

public LinkedHashMap(int initialCapacity)

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

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LinkedHashMap

public LinkedHashMap()

Constructs an empty insertion-ordered LinkedHashMap instance with a default capacity (16) and load factor (0.75).

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LinkedHashMap

public LinkedHashMap(java.util.Map m)

Constructs an insertion-ordered LinkedHashMap instance with the same mappings as the specified map. The LinkedHashMap instance is created with a a default load factor (0.75) and an initial capacity sufficient to hold the mappings in the specified map.

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LinkedHashMap

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

Constructs an empty LinkedHashMap instance with the specified initial capacity, load factor and ordering mode.

Method Detail

init

void init()

Called by superclass constructors and pseudoconstructors (clone, readObject) before any entries are inserted into the map. Initializes the chain.

Overrides:

init in class HashMap

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transfer

void transfer(HashMap.Entry[] newTable)

Transfer all entries to new table array. This method is called by superclass resize. It is overridden for performance, as it is faster to iterate using our linked list.

Overrides:

transfer in class HashMap

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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 java.util.Map

Overrides:

containsValue in class HashMap

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get

public java.lang.Object 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 java.util.Map

Overrides:

get in class HashMap

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clear

public void clear()

Removes all mappings from this map.

Specified by:

clear in interface java.util.Map

Overrides:

clear in class HashMap

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newKeyIterator

java.util.Iterator newKeyIterator()

Overrides:

newKeyIterator in class HashMap

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newValueIterator

java.util.Iterator newValueIterator()

Overrides:

newValueIterator in class HashMap

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newEntryIterator

java.util.Iterator newEntryIterator()

Overrides:

newEntryIterator in class HashMap

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addEntry

void addEntry(int hash,
              java.lang.Object key,
              java.lang.Object value,
              int bucketIndex)

This override alters behavior of superclass put method. It causes newly allocated entry to get inserted at the end of the linked list and removes the eldest entry if appropriate.

Overrides:

addEntry in class HashMap

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createEntry

void createEntry(int hash,
                 java.lang.Object key,
                 java.lang.Object value,
                 int bucketIndex)

This override differs from addEntry in that it doesn't resize the table or remove the eldest entry.

Overrides:

createEntry in class HashMap

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removeEldestEntry

protected boolean removeEldestEntry(java.util.Map.Entry eldest)

Returns true if this map should remove its eldest entry. This method is invoked by put and putAll after inserting a new entry into the map. It provides the implementer with the opportunity to remove the eldest entry each time a new one is added. This is useful if the map represents a cache: it allows the map to reduce memory consumption by deleting stale entries.

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).

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Overview  Package   Class  Use  Deprecated  Index
Home >> All >> aaron >> [ util overview ]	PREV CLASS  NEXT CLASS
SUMMARY:  JAVADOC |  SOURCE |  DOWNLOAD | NESTED | FIELD | CONSTR | METHOD	DETAIL: FIELD | CONSTR | METHOD

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