org.apache.commons.collections

Class LRUMap

Implemented Interfaces:
Cloneable, Externalizable, Map

public class LRUMap
extends SequencedHashMap
implements Externalizable

An implementation of a Map which has a maximum size and uses a Least Recently Used algorithm to remove items from the Map when the maximum size is reached and new items are added.

A synchronized version can be obtained with: Collections.synchronizedMap( theMapToSynchronize ) If it will be accessed by multiple threads, you _must_ synchronize access to this Map. Even concurrent get(Object) operations produce indeterminate behaviour.

Unlike the Collections 1.0 version, this version of LRUMap does use a true LRU algorithm. The keys for all gets and puts are moved to the front of the list. LRUMap is now a subclass of SequencedHashMap, and the "LRU" key is now equivalent to LRUMap.getFirst().

Version:
$Revision: 1.23 $ $Date: 2004/02/18 01:15:42 $
Authors:
James Strachan
Morgan Delagrange
Since:
Commons Collections 1.0

Constructor Summary

LRUMap()
Default constructor, primarily for the purpose of de-externalization.
LRUMap(int i)
Create a new LRUMap with a maximum capacity of i.

Method Summary

Object
get(Object key)
Get the value for a key from the Map.
int
getMaximumSize()
Getter for property maximumSize.
protected void
processRemovedLRU(Object key, Object value)
Subclasses of LRUMap may hook into this method to provide specialized actions whenever an Object is automatically removed from the cache.
Object
put(Object key, Object value)
Removes the key and its Object from the Map.
void
readExternal(ObjectInput in)
Deserializes this map from the given stream.
protected void
removeLRU()
This method is used internally by the class for finding and removing the LRU Object.
void
setMaximumSize(int maximumSize)
Setter for property maximumSize.
void
writeExternal(ObjectOutput out)
Serializes this map to the given stream.

Methods inherited from class org.apache.commons.collections.SequencedHashMap

clear, clone, containsKey, containsValue, entrySet, equals, get, get, getFirst, getFirstKey, getFirstValue, getLast, getLastKey, getLastValue, getValue, hashCode, indexOf, isEmpty, iterator, keySet, lastIndexOf, put, putAll, readExternal, remove, remove, sequence, size, toString, values, writeExternal

Constructor Details

LRUMap

public LRUMap()
Default constructor, primarily for the purpose of de-externalization. This constructors sets a default LRU limit of 100 keys, but this value may be overridden internally as a result of de-externalization.

LRUMap

public LRUMap(int i)
Create a new LRUMap with a maximum capacity of i. Once i capacity is achieved, subsequent gets and puts will push keys out of the map. See .
Parameters:
i - Maximum capacity of the LRUMap

Method Details

get

public Object get(Object key)
Get the value for a key from the Map. The key will be promoted to the Most Recently Used position. Note that get(Object) operations will modify the underlying Collection. Calling get(Object) inside of an iteration over keys, values, etc. is currently unsupported.
Overrides:
get in interface SequencedHashMap
Parameters:
key - Key to retrieve
Returns:
Returns the value. Returns null if the key has a null value or if the key has no value.

getMaximumSize

public int getMaximumSize()
Getter for property maximumSize.
Returns:
Value of property maximumSize.

processRemovedLRU

protected void processRemovedLRU(Object key,
                                 Object value)
Subclasses of LRUMap may hook into this method to provide specialized actions whenever an Object is automatically removed from the cache. By default, this method does nothing.
Parameters:
key - key that was removed
value - value of that key (can be null)

put

public Object put(Object key,
                  Object value)
Removes the key and its Object from the Map.

(Note: this may result in the "Least Recently Used" object being removed from the Map. In that case, the removeLRU() method is called. See javadoc for removeLRU() for more details.)

Overrides:
put in interface SequencedHashMap
Parameters:
key - Key of the Object to add.
value - Object to add
Returns:
Former value of the key

readExternal

public void readExternal(ObjectInput in)
            throws IOException,
                   ClassNotFoundException
Deserializes this map from the given stream.
Overrides:
readExternal in interface SequencedHashMap
Parameters:
in - the stream to deserialize from

removeLRU

protected void removeLRU()
This method is used internally by the class for finding and removing the LRU Object.

setMaximumSize

public void setMaximumSize(int maximumSize)
Setter for property maximumSize.
Parameters:
maximumSize - New value of property maximumSize.

writeExternal

public void writeExternal(ObjectOutput out)
            throws IOException
Serializes this map to the given stream.
Overrides:
writeExternal in interface SequencedHashMap
Parameters:
out - the stream to serialize to

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