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Commons Collections example source code file (ReferenceIdentityMap.java)

This example Commons Collections source code file (ReferenceIdentityMap.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

abstractreferencemap, classnotfoundexception, default_capacity, default_capacity, default_load_factor, default_load_factor, hard, io, ioexception, object, object, referenceidentitymap, referenceidentitymap, serializable, soft

The Commons Collections ReferenceIdentityMap.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;
import java.lang.ref.Reference;

/**
 * A <code>Map implementation that allows mappings to be
 * removed by the garbage collector and matches keys and values based
 * on <code>== not equals().
 * <p>
 * <p>
 * When you construct a <code>ReferenceIdentityMap, 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>==.
 * <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 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>ReferenceIdentityMap.
 * 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 ReferenceIdentityMap 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.
 *
 * @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 Stephen Colebourne
 */
public class ReferenceIdentityMap extends AbstractReferenceMap implements Serializable {

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

    /**
     * Constructs a new <code>ReferenceIdentityMap 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 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 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 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 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>==.
     * 
     * @param key1  the first key to compare passed in from outside
     * @param key2  the second key extracted from the entry via <code>entry.key
     * @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>==.
     * 
     * @param value1  the first value to compare passed in from outside
     * @param value2  the second value extracted from the entry via <code>getValue()
     * @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);
    }

}

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