org.apache.commons.collections.map

Class LRUMap

public class LRUMap extends AbstractLinkedMap implements BoundedMap, Serializable, Cloneable

A Map implementation with a fixed maximum size which removes the least recently used entry if an entry is added when full.

The least recently used algorithm works on the get and put operations only. Iteration of any kind, including setting the value by iteration, does not change the order. Queries such as containsKey and containsValue or access via views also do not change the order.

The map implements OrderedMap and entries may be queried using the bidirectional OrderedMapIterator. The order returned is least recently used to most recently used. Iterators from map views can also be cast to OrderedIterator if required.

All the available iterators can be reset back to the start by casting to ResettableIterator and calling reset().

Note that LRUMap is not synchronized and is not thread-safe. If you wish to use this map from multiple threads concurrently, you must use appropriate synchronization. The simplest approach is to wrap this map using java.util.Collections#synchronizedMap(Map). This class may throw NullPointerException's when accessed by concurrent threads.

Since: Commons Collections 3.0 (previously in main package v1.0)

Version: $Revision: 348299 $ $Date: 2005-11-22 23:51:45 +0000 (Tue, 22 Nov 2005) $

Author: James Strachan Morgan Delagrange Stephen Colebourne Mike Pettypiece Mario Ivankovits

Field Summary
protected static intDEFAULT_MAX_SIZE
Default maximum size
Constructor Summary
LRUMap()
Constructs a new empty map with a maximum size of 100.
LRUMap(int maxSize)
Constructs a new, empty map with the specified maximum size.
LRUMap(int maxSize, boolean scanUntilRemovable)
Constructs a new, empty map with the specified maximum size.
LRUMap(int maxSize, float loadFactor)
Constructs a new, empty map with the specified initial capacity and load factor.
LRUMap(int maxSize, float loadFactor, boolean scanUntilRemovable)
Constructs a new, empty map with the specified initial capacity and load factor.
LRUMap(Map map)
Constructor copying elements from another map.
LRUMap(Map map, boolean scanUntilRemovable)
Constructor copying elements from another map.
Method Summary
protected voidaddMapping(int hashIndex, int hashCode, Object key, Object value)
Adds a new key-value mapping into this map.
Objectclone()
Clones the map without cloning the keys or values.
protected voiddoReadObject(ObjectInputStream in)
Reads the data necessary for put() to work in the superclass.
protected voiddoWriteObject(ObjectOutputStream out)
Writes the data necessary for put() to work in deserialization.
Objectget(Object key)
Gets the value mapped to the key specified.
booleanisFull()
Returns true if this map is full and no new mappings can be added.
booleanisScanUntilRemovable()
Whether this LRUMap will scan until a removable entry is found when the map is full.
intmaxSize()
Gets the maximum size of the map (the bound).
protected voidmoveToMRU(LinkEntry entry)
Moves an entry to the MRU position at the end of the list.
protected booleanremoveLRU(LinkEntry entry)
Subclass method to control removal of the least recently used entry from the map.
protected voidreuseMapping(LinkEntry entry, int hashIndex, int hashCode, Object key, Object value)
Reuses an entry by removing it and moving it to a new place in the map.
protected voidupdateEntry(HashEntry entry, Object newValue)
Updates an existing key-value mapping.

Field Detail

DEFAULT_MAX_SIZE

protected static final int DEFAULT_MAX_SIZE
Default maximum size

Constructor Detail

LRUMap

public LRUMap()
Constructs a new empty map with a maximum size of 100.

LRUMap

public LRUMap(int maxSize)
Constructs a new, empty map with the specified maximum size.

Parameters: maxSize the maximum size of the map

Throws: IllegalArgumentException if the maximum size is less than one

LRUMap

public LRUMap(int maxSize, boolean scanUntilRemovable)
Constructs a new, empty map with the specified maximum size.

Parameters: maxSize the maximum size of the map scanUntilRemovable scan until a removeable entry is found, default false

Throws: IllegalArgumentException if the maximum size is less than one

Since: Commons Collections 3.1

LRUMap

public LRUMap(int maxSize, float loadFactor)
Constructs a new, empty map with the specified initial capacity and load factor.

Parameters: maxSize the maximum size of the map, -1 for no limit, loadFactor the load factor

Throws: IllegalArgumentException if the maximum size is less than one IllegalArgumentException if the load factor is less than zero

LRUMap

public LRUMap(int maxSize, float loadFactor, boolean scanUntilRemovable)
Constructs a new, empty map with the specified initial capacity and load factor.

Parameters: maxSize the maximum size of the map, -1 for no limit, loadFactor the load factor scanUntilRemovable scan until a removeable entry is found, default false

Throws: IllegalArgumentException if the maximum size is less than one IllegalArgumentException if the load factor is less than zero

Since: Commons Collections 3.1

LRUMap

public LRUMap(Map map)
Constructor copying elements from another map.

The maximum size is set from the map's size.

Parameters: map the map to copy

Throws: NullPointerException if the map is null IllegalArgumentException if the map is empty

LRUMap

public LRUMap(Map map, boolean scanUntilRemovable)
Constructor copying elements from another map.

The maximum size is set from the map's size.

Parameters: map the map to copy scanUntilRemovable scan until a removeable entry is found, default false

Throws: NullPointerException if the map is null IllegalArgumentException if the map is empty

Since: Commons Collections 3.1

Method Detail

addMapping

protected void addMapping(int hashIndex, int hashCode, Object key, Object value)
Adds a new key-value mapping into this map.

This implementation checks the LRU size and determines whether to discard an entry or not using LRUMap.

From Commons Collections 3.1 this method uses isFull rather than accessing size and maxSize directly. It also handles the scanUntilRemovable functionality.

Parameters: hashIndex the index into the data array to store at hashCode the hash code of the key to add key the key to add value the value to add

clone

public Object clone()
Clones the map without cloning the keys or values.

Returns: a shallow clone

doReadObject

protected void doReadObject(ObjectInputStream in)
Reads the data necessary for put() to work in the superclass.

doWriteObject

protected void doWriteObject(ObjectOutputStream out)
Writes the data necessary for put() to work in deserialization.

get

public Object get(Object key)
Gets the value mapped to the key specified.

This operation changes the position of the key in the map to the most recently used position (first).

Parameters: key the key

Returns: the mapped value, null if no match

isFull

public boolean isFull()
Returns true if this map is full and no new mappings can be added.

Returns: true if the map is full

isScanUntilRemovable

public boolean isScanUntilRemovable()
Whether this LRUMap will scan until a removable entry is found when the map is full.

Returns: true if this map scans

Since: Commons Collections 3.1

maxSize

public int maxSize()
Gets the maximum size of the map (the bound).

Returns: the maximum number of elements the map can hold

moveToMRU

protected void moveToMRU(LinkEntry entry)
Moves an entry to the MRU position at the end of the list.

This implementation moves the updated entry to the end of the list.

Parameters: entry the entry to update

removeLRU

protected boolean removeLRU(LinkEntry entry)
Subclass method to control removal of the least recently used entry from the map.

This method exists for subclasses to override. A subclass may wish to provide cleanup of resources when an entry is removed. For example:

 protected boolean removeLRU(LinkEntry entry) {
   releaseResources(entry.getValue());  // release resources held by entry
   return true;  // actually delete entry
 }
 

Alternatively, a subclass may choose to not remove the entry or selectively keep certain LRU entries. For example:

 protected boolean removeLRU(LinkEntry entry) {
   if (entry.getKey().toString().startsWith("System.")) {
     return false;  // entry not removed from LRUMap
   } else {
     return true;  // actually delete entry
   }
 }
 
The effect of returning false is dependent on the scanUntilRemovable flag. If the flag is true, the next LRU entry will be passed to this method and so on until one returns false and is removed, or every entry in the map has been passed. If the scanUntilRemovable flag is false, the map will exceed the maximum size.

NOTE: Commons Collections 3.0 passed the wrong entry to this method. This is fixed in version 3.1 onwards.

Parameters: entry the entry to be removed

reuseMapping

protected void reuseMapping(LinkEntry entry, int hashIndex, int hashCode, Object key, Object value)
Reuses an entry by removing it and moving it to a new place in the map.

This method uses LRUMap, LRUMap and LRUMap.

Parameters: entry the entry to reuse hashIndex the index into the data array to store at hashCode the hash code of the key to add key the key to add value the value to add

updateEntry

protected void updateEntry(HashEntry entry, Object newValue)
Updates an existing key-value mapping.

This implementation moves the updated entry to the top of the list using LRUMap.

Parameters: entry the entry to update newValue the new value to store

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