Java tutorial
/* * 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>. * * @see java.lang.ref.Reference * * @since Commons Collections 3.0 (previously in main package v2.1) * @version $Revision: 1.2 $ $Date: 2004/10/16 21:26:14 $ * * @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); } }