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Commons Collections example source code file (LRUMap.java)

This example Commons Collections source code file (LRUMap.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Commons Collections tags/keywords

classnotfoundexception, externalizable, externalizable, io, ioexception, ioexception, iterator, lrumap, lrumap, object, object, objectinput, objectoutput, sequencedhashmap, util

The Commons Collections LRUMap.java source code

/*
 *  Licensed to the Apache Software Foundation (ASF) under one or more
 *  contributor license agreements.  See the NOTICE file distributed with
 *  this work for additional information regarding copyright ownership.
 *  The ASF licenses this file to You under the Apache License, Version 2.0
 *  (the "License"); you may not use this file except in compliance with
 *  the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */
package org.apache.commons.collections;

import java.io.Externalizable;
import java.io.IOException;
import java.io.ObjectInput;
import java.io.ObjectOutput;
import java.util.Iterator;

/**
 * <p>
 * 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.
 * </p>
 * 
 * <p>
 * A synchronized version can be obtained with:
 * <code>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.
 * </p>
 * 
 * <p>
 * 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().
 * </p>
 * 
 * @deprecated Moved to map subpackage. Due to be removed in v4.0.
 * @since Commons Collections 1.0
 * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
 * 
 * @author <a href="mailto:jstrachan@apache.org">James Strachan
 * @author <a href="mailto:morgand@apache.org">Morgan Delagrange
 */
public class LRUMap extends SequencedHashMap implements Externalizable {
        
    private int maximumSize = 0;

    /**
     * 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.
     */
    public LRUMap() {
        this( 100 );
    }

    /**
     * Create a new LRUMap with a maximum capacity of <i>i.
     * Once <i>i capacity is achieved, subsequent gets
     * and puts will push keys out of the map.  See .
     * 
     * @param i      Maximum capacity of the LRUMap
     */
    public LRUMap(int i) {
        super( i );
        maximumSize = i;
    }

    /**
     * <p>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.</p>
     * 
     * @param key    Key to retrieve
     * @return Returns the value.  Returns null if the key has a
     *         null value <i>or if the key has no value.
     */
    public Object get(Object key) {
        if(!containsKey(key)) return null;

        Object value = remove(key);
        super.put(key,value);
        return value;
    }

     /**
      * <p>Removes the key and its Object from the Map.

* * <p>(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.)</p> * * @param key Key of the Object to add. * @param value Object to add * @return Former value of the key */ public Object put( Object key, Object value ) { int mapSize = size(); Object retval = null; if ( mapSize >= maximumSize ) { // don't retire LRU if you are just // updating an existing key if (!containsKey(key)) { // lets retire the least recently used item in the cache removeLRU(); } } retval = super.put(key,value); return retval; } /** * This method is used internally by the class for * finding and removing the LRU Object. */ protected void removeLRU() { Object key = getFirstKey(); // be sure to call super.get(key), or you're likely to // get infinite promotion recursion Object value = super.get(key); remove(key); processRemovedLRU(key,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. * * @param key key that was removed * @param value value of that key (can be null) */ protected void processRemovedLRU(Object key, Object value) { } // Externalizable interface //------------------------------------------------------------------------- public void readExternal( ObjectInput in ) throws IOException, ClassNotFoundException { maximumSize = in.readInt(); int size = in.readInt(); for( int i = 0; i < size; i++ ) { Object key = in.readObject(); Object value = in.readObject(); put(key,value); } } public void writeExternal( ObjectOutput out ) throws IOException { out.writeInt( maximumSize ); out.writeInt( size() ); for( Iterator iterator = keySet().iterator(); iterator.hasNext(); ) { Object key = iterator.next(); out.writeObject( key ); // be sure to call super.get(key), or you're likely to // get infinite promotion recursion Object value = super.get( key ); out.writeObject( value ); } } // Properties //------------------------------------------------------------------------- /** Getter for property maximumSize. * @return Value of property maximumSize. */ public int getMaximumSize() { return maximumSize; } /** Setter for property maximumSize. * @param maximumSize New value of property maximumSize. */ public void setMaximumSize(int maximumSize) { this.maximumSize = maximumSize; while (size() > maximumSize) { removeLRU(); } } // add a serial version uid, so that if we change things in the future // without changing the format, we can still deserialize properly. private static final long serialVersionUID = 2197433140769957051L; }

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