dependencies/plugins/org.apache.commons.collections/source-bundle/org/apache/commons/collections/map/ReferenceIdentityMap.java

/*
 *  Copyright 2004 The Apache Software Foundation
 *
 *  Licensed 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;
import java.lang.ref.Reference;

/**
 * A <code>Map</code> implementation that allows mappings to be
 * removed by the garbage collector and matches keys and values based
 * on <code>==</code> not <code>equals()</code>.
 * <p>
 * <p>
 * When you construct a <code>ReferenceIdentityMap</code>, 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 Reference}.
 * <p>
 * Different types of references can be specified for keys and values.
 * 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.ReferenceMap ReferenceMap}.
 * It differs in that keys and values in this class are compared using <code>==</code>.
 * <p>
 * This map will violate the detail of various Map and map view contracts.
 * As a general rule, don't compare this map to other maps.
 * <p>
 * This {@link java.util.Map Map} implementation does <i>not</i> allow null elements.
 * Attempting to add a null key or value to the map will raise a <code>NullPointerException</code>.
 * <p>
 * This implementation is not synchronized.
 * You can use {@link java.util.Collections#synchronizedMap} to 
 * provide synchronized access to a <code>ReferenceIdentityMap</code>.
 * 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</code> and calling <code>reset()</code>.
 * <p>
 * <strong>Note that ReferenceIdentityMap is not synchronized and is not thread-safe.</strong>
 * 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.
 *
 * @see java.lang.ref.Reference
 * 
 * @since Commons Collections 3.0 (previously in main package v2.1)
 * @version $Revision: 1.1 $ $Date: 2009/05/27 22:16:30 $
 * 
 * @author Stephen Colebourne
 */
public class ReferenceIdentityMap extends AbstractReferenceMap implements Serializable {

    /** Serialization version */
    private static final long serialVersionUID = -1266190134568365852L;

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

    /**
     * Constructs a new <code>ReferenceIdentityMap</code> 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 ReferenceIdentityMap(int keyType, int valueType) {
        super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false);
    }

    /**
     * Constructs a new <code>ReferenceIdentityMap</code> 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 ReferenceIdentityMap(int keyType, int valueType, boolean purgeValues) {
        super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, purgeValues);
    }

    /**
     * Constructs a new <code>ReferenceIdentityMap</code> 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 ReferenceIdentityMap(int keyType, int valueType, int capacity, float loadFactor) {
        super(keyType, valueType, capacity, loadFactor, false);
    }

    /**
     * Constructs a new <code>ReferenceIdentityMap</code> 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 ReferenceIdentityMap(int keyType, int valueType, int capacity,
                        float loadFactor, boolean purgeValues) {
        super(keyType, valueType, capacity, loadFactor, purgeValues);
    }

    //-----------------------------------------------------------------------
    /**
     * Gets the hash code for the key specified.
     * <p>
     * This implementation uses the identity hash code.
     * 
     * @param key  the key to get a hash code for
     * @return the hash code
     */
    protected int hash(Object key) {
        return System.identityHashCode(key);
    }

    /**
     * Gets the hash code for a MapEntry.
     * <p>
     * This implementation uses the identity hash code.
     * 
     * @param key  the key to get a hash code for, may be null
     * @param value  the value to get a hash code for, may be null
     * @return the hash code, as per the MapEntry specification
     */
    protected int hashEntry(Object key, Object value) {
        return System.identityHashCode(key) ^
               System.identityHashCode(value);
    }

    /**
     * Compares two keys for equals.
     * <p>
     * This implementation converts the key from the entry to a real reference
     * before comparison and uses <code>==</code>.
     * 
     * @param key1  the first key to compare passed in from outside
     * @param key2  the second key extracted from the entry via <code>entry.key</code>
     * @return true if equal by identity
     */
    protected boolean isEqualKey(Object key1, Object key2) {
        key2 = (keyType > HARD ? ((Reference) key2).get() : key2);
        return (key1 == key2);
    }

    /**
     * Compares two values for equals.
     * <p>
     * This implementation uses <code>==</code>.
     * 
     * @param value1  the first value to compare passed in from outside
     * @param value2  the second value extracted from the entry via <code>getValue()</code>
     * @return true if equal by identity
     */
    protected boolean isEqualValue(Object value1, Object value2) {
        return (value1 == value2);
    }

    //-----------------------------------------------------------------------
    /**
     * 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);
    }

}