Java Doc for LinkedHashMap.java in  » 6.0-JDK-Modules » j2me » java » util » Java Source Code / Java DocumentationJava Source Code and Java Documentation

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);
 ...
 }
 

A special LinkedHashMap.LinkedHashMap(int,float,boolean) 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 LinkedHashMap.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(...));
 

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.
author:
   Josh Bloch
version:
   1.14, 10/10/06
See Also:   Object.hashCode
See Also:   Collection
See Also:   Map
See Also:   HashMap
See Also:   TreeMap
See Also:   Hashtable
since:
   JDK1.4

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).
Parameters:
  eldest - The least recently inserted entry in the map, or if this is an access-ordered map, the least recently accessedentry. This is the entry that will be removed it thismethod returns true. If the map was empty priorto the put or putAll invocation resultingin this invocation, this will be the entry that was justinserted; in other words, if the map contains a singleentry, the eldest entry is also the newest. true if the eldest entry should be removedfrom the map; false if it should be retained.