Commons Collections example source code file (ReferenceMap.java)

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Java - Commons Collections tags/keywords

abstractreferencemap, classnotfoundexception, default_capacity, default_capacity, default_load_factor, default_load_factor, io, ioexception, ioexception, referencemap, referencemap, serializable, soft

The Commons Collections ReferenceMap.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.map;

import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

/**
 * A <code>Map implementation that allows mappings to be
 * removed by the garbage collector.
 * <p>
 * When you construct a <code>ReferenceMap, you can specify what kind
 * of references are used to store the map's keys and values.
 * If non-hard references are used, then the garbage collector can remove
 * mappings if a key or value becomes unreachable, or if the JVM's memory is
 * running low. For information on how the different reference types behave,
 * see {@link java.lang.ref.Reference Reference}.
 * <p>
 * Different types of references can be specified for keys and values.
 * The keys can be configured to be weak but the values hard,
 * in which case this class will behave like a
 * <a href="http://java.sun.com/j2se/1.4/docs/api/java/util/WeakHashMap.html">
 * <code>WeakHashMap. However, you can also specify hard keys and
 * weak values, or any other combination. The default constructor uses
 * hard keys and soft values, providing a memory-sensitive cache.
 * <p>
 * This map is similar to
 * {@link org.apache.commons.collections.map.ReferenceIdentityMap ReferenceIdentityMap}.
 * It differs in that keys and values in this class are compared using <code>equals().
 * <p>
 * This {@link java.util.Map Map} implementation does <i>not allow null elements.
 * Attempting to add a null key or value to the map will raise a <code>NullPointerException.
 * <p>
 * This implementation is not synchronized.
 * You can use {@link java.util.Collections#synchronizedMap} to 
 * provide synchronized access to a <code>ReferenceMap.
 * Remember that synchronization will not stop the garbage collecter removing entries.
 * <p>
 * All the available iterators can be reset back to the start by casting to
 * <code>ResettableIterator and calling reset().
 * <p>
 * <strong>Note that ReferenceMap 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 {@link java.util.Collections#synchronizedMap}. This class may throw 
 * exceptions when accessed by concurrent threads without synchronization.
 * <p>
 * NOTE: As from Commons Collections 3.1 this map extends <code>AbstractReferenceMap
 * (previously it extended AbstractMap). As a result, the implementation is now
 * extensible and provides a <code>MapIterator.
 *
 * @see java.lang.ref.Reference
 * 
 * @since Commons Collections 3.0 (previously in main package v2.1)
 * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
 * 
 * @author Paul Jack
 * @author Stephen Colebourne
 */
public class ReferenceMap extends AbstractReferenceMap implements Serializable {

    /** Serialization version */
    private static final long serialVersionUID = 1555089888138299607L;

    /**
     * Constructs a new <code>ReferenceMap that will
     * use hard references to keys and soft references to values.
     */
    public ReferenceMap() {
        super(HARD, SOFT, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false);
    }

    /**
     * Constructs a new <code>ReferenceMap that will
     * use the specified types of references.
     *
     * @param keyType  the type of reference to use for keys;
     *   must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
     * @param valueType  the type of reference to use for values;
     *   must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
     */
    public ReferenceMap(int keyType, int valueType) {
        super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false);
    }

    /**
     * Constructs a new <code>ReferenceMap that will
     * use the specified types of references.
     *
     * @param keyType  the type of reference to use for keys;
     *   must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
     * @param valueType  the type of reference to use for values;
     *   must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
     * @param purgeValues should the value be automatically purged when the 
     *   key is garbage collected 
     */
    public ReferenceMap(int keyType, int valueType, boolean purgeValues) {
        super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, purgeValues);
    }

    /**
     * Constructs a new <code>ReferenceMap with the
     * specified reference types, load factor and initial
     * capacity.
     *
     * @param keyType  the type of reference to use for keys;
     *   must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
     * @param valueType  the type of reference to use for values;
     *   must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
     * @param capacity  the initial capacity for the map
     * @param loadFactor  the load factor for the map
     */
    public ReferenceMap(int keyType, int valueType, int capacity, float loadFactor) {
        super(keyType, valueType, capacity, loadFactor, false);
    }

    /**
     * Constructs a new <code>ReferenceMap with the
     * specified reference types, load factor and initial
     * capacity.
     *
     * @param keyType  the type of reference to use for keys;
     *   must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
     * @param valueType  the type of reference to use for values;
     *   must be {@link #HARD}, {@link #SOFT}, {@link #WEAK}
     * @param capacity  the initial capacity for the map
     * @param loadFactor  the load factor for the map
     * @param purgeValues  should the value be automatically purged when the 
     *   key is garbage collected 
     */
    public ReferenceMap(int keyType, int valueType, int capacity,
                        float loadFactor, boolean purgeValues) {
        super(keyType, valueType, capacity, loadFactor, purgeValues);
    }

    //-----------------------------------------------------------------------
    /**
     * Write the map out using a custom routine.
     */
    private void writeObject(ObjectOutputStream out) throws IOException {
        out.defaultWriteObject();
        doWriteObject(out);
    }

    /**
     * Read the map in using a custom routine.
     */
    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
        in.defaultReadObject();
        doReadObject(in);
    }

}