Java tutorial
/* * 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.collections4.map; import java.io.IOException; import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.io.Serializable; /** * A <code>Map</code> implementation that allows mappings to be * removed by the garbage collector. * <p> * When you construct a <code>ReferenceMap</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 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</code></a>. 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.collections4.map.ReferenceIdentityMap ReferenceIdentityMap}. * It differs in that keys and values in this class are compared using <code>equals()</code>. * <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>ReferenceMap</code>. * Remember that synchronization will not stop the garbage collector 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 ReferenceMap 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. * <p> * NOTE: As from Commons Collections 3.1 this map extends <code>AbstractReferenceMap</code> * (previously it extended AbstractMap). As a result, the implementation is now * extensible and provides a <code>MapIterator</code>. * * @see java.lang.ref.Reference * * @since 3.0 (previously in main package v2.1) * @version $Id: ReferenceMap.java 1477799 2013-04-30 19:56:11Z tn $ */ public class ReferenceMap<K, V> extends AbstractReferenceMap<K, V> implements Serializable { /** Serialization version */ private static final long serialVersionUID = 1555089888138299607L; /** * Constructs a new <code>ReferenceMap</code> that will * use hard references to keys and soft references to values. */ public ReferenceMap() { super(ReferenceStrength.HARD, ReferenceStrength.SOFT, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false); } /** * Constructs a new <code>ReferenceMap</code> that will * use the specified types of references. * * @param keyType the type of reference to use for keys; * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD}, * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT}, * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK} * @param valueType the type of reference to use for values; * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD}, * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT}, * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK} */ public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType) { super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, false); } /** * Constructs a new <code>ReferenceMap</code> that will * use the specified types of references. * * @param keyType the type of reference to use for keys; * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD}, * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT}, * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK} * @param valueType the type of reference to use for values; * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD}, * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT}, * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK} * @param purgeValues should the value be automatically purged when the * key is garbage collected */ public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType, final boolean purgeValues) { super(keyType, valueType, DEFAULT_CAPACITY, DEFAULT_LOAD_FACTOR, purgeValues); } /** * Constructs a new <code>ReferenceMap</code> with the * specified reference types, load factor and initial * capacity. * * @param keyType the type of reference to use for keys; * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD}, * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT}, * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK} * @param valueType the type of reference to use for values; * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD}, * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT}, * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK} * @param capacity the initial capacity for the map * @param loadFactor the load factor for the map */ public ReferenceMap(final ReferenceStrength keyType, final ReferenceStrength valueType, final int capacity, final float loadFactor) { super(keyType, valueType, capacity, loadFactor, false); } /** * Constructs a new <code>ReferenceMap</code> with the * specified reference types, load factor and initial * capacity. * * @param keyType the type of reference to use for keys; * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD}, * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT}, * {@link AbstractReferenceMap.ReferenceStrength#WEAK WEAK} * @param valueType the type of reference to use for values; * must be {@link AbstractReferenceMap.ReferenceStrength#HARD HARD}, * {@link AbstractReferenceMap.ReferenceStrength#SOFT SOFT}, * {@link AbstractReferenceMap.ReferenceStrength#WEAK 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(final ReferenceStrength keyType, final ReferenceStrength valueType, final int capacity, final float loadFactor, final boolean purgeValues) { super(keyType, valueType, capacity, loadFactor, purgeValues); } //----------------------------------------------------------------------- /** * Write the map out using a custom routine. */ private void writeObject(final ObjectOutputStream out) throws IOException { out.defaultWriteObject(); doWriteObject(out); } /** * Read the map in using a custom routine. */ private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException { in.defaultReadObject(); doReadObject(in); } }