org.apache.commons.collections.map

Class LRUMap

Implemented Interfaces:
BoundedMap, Cloneable, Map, Serializable, IterableMap, OrderedMap

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().

Version:
$Revision: 1.14 $ $Date: 2004/06/07 22:13:53 $
Authors:
James Strachan
Morgan Delagrange
Stephen Colebourne
Mike Pettypiece
Mario Ivankovits
Since:
Commons Collections 3.0 (previously in main package v1.0)

Nested Class Summary

Nested classes/interfaces inherited from class org.apache.commons.collections.map.AbstractLinkedMap

AbstractLinkedMap.EntrySetIterator, AbstractLinkedMap.KeySetIterator, AbstractLinkedMap.LinkEntry, AbstractLinkedMap.LinkIterator, AbstractLinkedMap.LinkMapIterator, AbstractLinkedMap.ValuesIterator

Nested classes/interfaces inherited from class org.apache.commons.collections.map.AbstractHashedMap

AbstractHashedMap.EntrySet, AbstractHashedMap.EntrySetIterator, AbstractHashedMap.HashEntry, AbstractHashedMap.HashIterator, AbstractHashedMap.HashMapIterator, AbstractHashedMap.KeySet, AbstractHashedMap.KeySetIterator, AbstractHashedMap.Values, AbstractHashedMap.ValuesIterator

Field Summary

protected static int
DEFAULT_MAX_SIZE
Default maximum size

Fields inherited from class org.apache.commons.collections.map.AbstractLinkedMap

header

Fields inherited from class org.apache.commons.collections.map.AbstractHashedMap

DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_THRESHOLD, GETKEY_INVALID, GETVALUE_INVALID, MAXIMUM_CAPACITY, NO_NEXT_ENTRY, NO_PREVIOUS_ENTRY, NULL, REMOVE_INVALID, SETVALUE_INVALID, data, entrySet, keySet, loadFactor, modCount, size, threshold, values

Constructor Summary

LRUMap()
Constructs a new empty map with a maximum size of 100.
LRUMap(Map map)
Constructor copying elements from another map.
LRUMap(Map map, boolean scanUntilRemovable)
Constructor copying elements from another map.
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.

Method Summary

protected void
addMapping(int hashIndex, int hashCode, Object key, Object value)
Adds a new key-value mapping into this map.
Object
clone()
Clones the map without cloning the keys or values.
protected void
doReadObject(ObjectInputStream in)
Reads the data necessary for put() to work in the superclass.
protected void
doWriteObject(ObjectOutputStream out)
Writes the data necessary for put() to work in deserialization.
Object
get(Object key)
Gets the value mapped to the key specified.
boolean
isFull()
Returns true if this map is full and no new mappings can be added.
boolean
isScanUntilRemovable()
Whether this LRUMap will scan until a removable entry is found when the map is full.
int
maxSize()
Gets the maximum size of the map (the bound).
protected void
moveToMRU(AbstractLinkedMap.LinkEntry entry)
Moves an entry to the MRU position at the end of the list.
protected boolean
removeLRU(AbstractLinkedMap.LinkEntry entry)
Subclass method to control removal of the least recently used entry from the map.
protected void
reuseMapping(AbstractLinkedMap.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 void
updateEntry(AbstractHashedMap.HashEntry entry, Object newValue)
Updates an existing key-value mapping.

Methods inherited from class org.apache.commons.collections.map.AbstractLinkedMap

addEntry, clear, containsValue, createEntry, createEntrySetIterator, createKeySetIterator, createValuesIterator, entryAfter, entryBefore, firstKey, getEntry, init, lastKey, mapIterator, nextKey, orderedMapIterator, previousKey, removeEntry

Methods inherited from class org.apache.commons.collections.map.AbstractHashedMap

addEntry, addMapping, calculateNewCapacity, calculateThreshold, checkCapacity, clear, clone, containsKey, containsValue, convertKey, createEntry, createEntrySetIterator, createKeySetIterator, createValuesIterator, destroyEntry, doReadObject, doWriteObject, ensureCapacity, entryHashCode, entryKey, entryNext, entrySet, entryValue, equals, get, getEntry, hash, hashCode, hashIndex, init, isEmpty, isEqualKey, isEqualValue, keySet, mapIterator, put, putAll, remove, removeEntry, removeMapping, reuseEntry, size, toString, updateEntry, values

Field Details

DEFAULT_MAX_SIZE

protected static final int DEFAULT_MAX_SIZE
Default maximum size
Field Value:
100

Constructor Details

LRUMap

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

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

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
Since:
Commons Collections 3.1

LRUMap

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

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
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

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
Since:
Commons Collections 3.1

Method Details

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 removeLRU(AbstractLinkedMap.LinkEntry).

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

Overrides:
addMapping in interface AbstractHashedMap
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.
Overrides:
clone in interface AbstractHashedMap
Returns:
a shallow clone

doReadObject

protected void doReadObject(ObjectInputStream in)
            throws IOException,
                   ClassNotFoundException
Reads the data necessary for put() to work in the superclass.
Overrides:
doReadObject in interface AbstractHashedMap

doWriteObject

protected void doWriteObject(ObjectOutputStream out)
            throws IOException
Writes the data necessary for put() to work in deserialization.
Overrides:
doWriteObject in interface AbstractHashedMap

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).

Overrides:
get in interface AbstractHashedMap
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.
Specified by:
isFull in interface BoundedMap
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).
Specified by:
maxSize in interface BoundedMap
Returns:
the maximum number of elements the map can hold

moveToMRU

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

This implementation moves the updated entry to the top of the list using moveToMRU(AbstractLinkedMap.LinkEntry).

Overrides:
updateEntry in interface AbstractHashedMap
Parameters:
entry - the entry to update
newValue - the new value to store

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