org.apache.commons.collections
Class LRUMap

java.lang.Object
  |
  +--org.apache.commons.collections.SequencedHashMap
        |
        +--org.apache.commons.collections.LRUMap

All Implemented Interfaces:

Cloneable, Externalizable, Map, Serializable

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().

Since:

1.0

Author:

James Strachan, Morgan Delagrange

See Also:

Serialized Form

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, getFirst, getFirstKey, getFirstValue, getLast, getLastKey, getLastValue, getValue, hashCode, indexOf, isEmpty, iterator, keySet, lastIndexOf, putAll, remove, remove, sequence, size, toString, values

Methods inherited from class java.lang.Object

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

Constructor Detail

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 Detail

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.

Specified by:

get in interface Map

Overrides:

get in class 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.)

Specified by:

put in interface Map

Overrides:

put in class SequencedHashMap

Parameters:

key - Key of the Object to add.

value - Object to add

Returns:

Former value of the key

See Also:

removeLRU()

readExternal

public void readExternal(ObjectInput in)
                  throws IOException,
                         ClassNotFoundException

Description copied from class: SequencedHashMap

Deserializes this map from the given stream.

Specified by:

readExternal in interface Externalizable

Overrides:

readExternal in class SequencedHashMap

Parameters:

in - the stream to deserialize from

Throws:

ClassNotFoundException - if the stream raises it

IOException - if the stream raises it

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

Description copied from class: SequencedHashMap

Serializes this map to the given stream.

Specified by:

writeExternal in interface Externalizable

Overrides:

writeExternal in class SequencedHashMap

Parameters:

out - the stream to serialize to

Throws:

IOException - if the stream raises it