001/* 002 * Licensed to the Apache Software Foundation (ASF) under one or more 003 * contributor license agreements. See the NOTICE file distributed with 004 * this work for additional information regarding copyright ownership. 005 * The ASF licenses this file to You under the Apache License, Version 2.0 006 * (the "License"); you may not use this file except in compliance with 007 * the License. You may obtain a copy of the License at 008 * 009 * http://www.apache.org/licenses/LICENSE-2.0 010 * 011 * Unless required by applicable law or agreed to in writing, software 012 * distributed under the License is distributed on an "AS IS" BASIS, 013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 014 * See the License for the specific language governing permissions and 015 * limitations under the License. 016 */ 017package org.apache.commons.collections4.map; 018 019import java.io.IOException; 020import java.io.ObjectInputStream; 021import java.io.ObjectOutputStream; 022import java.io.Serializable; 023import java.util.Map; 024 025import org.apache.commons.collections4.Factory; 026import org.apache.commons.collections4.Transformer; 027import org.apache.commons.collections4.functors.FactoryTransformer; 028 029/** 030 * Decorates another <code>Map</code> to create objects in the map on demand. 031 * <p> 032 * When the {@link #get(Object)} method is called with a key that does not 033 * exist in the map, the factory is used to create the object. The created 034 * object will be added to the map using the requested key. 035 * </p> 036 * <p> 037 * For instance: 038 * </p> 039 * <pre> 040 * Factory<Date> factory = new Factory<Date>() { 041 * public Date create() { 042 * return new Date(); 043 * } 044 * } 045 * Map<String, Date> lazy = LazyMap.lazyMap(new HashMap<String, Date>(), factory); 046 * Date date = lazy.get("NOW"); 047 * </pre> 048 * 049 * <p> 050 * After the above code is executed, <code>date</code> will refer to 051 * a new <code>Date</code> instance. Furthermore, that <code>Date</code> 052 * instance is mapped to the "NOW" key in the map. 053 * </p> 054 * <p> 055 * <strong>Note that LazyMap is not synchronized and is not thread-safe.</strong> 056 * If you wish to use this map from multiple threads concurrently, you must use 057 * appropriate synchronization. The simplest approach is to wrap this map 058 * using {@link java.util.Collections#synchronizedMap(Map)}. This class may throw 059 * exceptions when accessed by concurrent threads without synchronization. 060 * </p> 061 * <p> 062 * This class is Serializable from Commons Collections 3.1. 063 * </p> 064 * 065 * @param <K> the type of the keys in this map 066 * @param <V> the type of the values in this map 067 * @since 3.0 068 */ 069public class LazyMap<K, V> extends AbstractMapDecorator<K, V> implements Serializable { 070 071 /** Serialization version */ 072 private static final long serialVersionUID = 7990956402564206740L; 073 074 /** The factory to use to construct elements */ 075 protected final Transformer<? super K, ? extends V> factory; 076 077 /** 078 * Factory method to create a lazily instantiated map. 079 * 080 * @param <K> the key type 081 * @param <V> the value type 082 * @param map the map to decorate, must not be null 083 * @param factory the factory to use, must not be null 084 * @return a new lazy map 085 * @throws NullPointerException if map or factory is null 086 * @since 4.0 087 */ 088 public static <K, V> LazyMap<K, V> lazyMap(final Map<K, V> map, final Factory< ? extends V> factory) { 089 return new LazyMap<>(map, factory); 090 } 091 092 /** 093 * Factory method to create a lazily instantiated map. 094 * 095 * @param <K> the key type 096 * @param <V> the value type 097 * @param map the map to decorate, must not be null 098 * @param factory the factory to use, must not be null 099 * @return a new lazy map 100 * @throws NullPointerException if map or factory is null 101 * @since 4.0 102 */ 103 public static <V, K> LazyMap<K, V> lazyMap(final Map<K, V> map, final Transformer<? super K, ? extends V> factory) { 104 return new LazyMap<>(map, factory); 105 } 106 107 //----------------------------------------------------------------------- 108 /** 109 * Constructor that wraps (not copies). 110 * 111 * @param map the map to decorate, must not be null 112 * @param factory the factory to use, must not be null 113 * @throws NullPointerException if map or factory is null 114 */ 115 protected LazyMap(final Map<K,V> map, final Factory<? extends V> factory) { 116 super(map); 117 if (factory == null) { 118 throw new NullPointerException("Factory must not be null"); 119 } 120 this.factory = FactoryTransformer.factoryTransformer(factory); 121 } 122 123 /** 124 * Constructor that wraps (not copies). 125 * 126 * @param map the map to decorate, must not be null 127 * @param factory the factory to use, must not be null 128 * @throws NullPointerException if map or factory is null 129 */ 130 protected LazyMap(final Map<K,V> map, final Transformer<? super K, ? extends V> factory) { 131 super(map); 132 if (factory == null) { 133 throw new NullPointerException("Factory must not be null"); 134 } 135 this.factory = factory; 136 } 137 138 //----------------------------------------------------------------------- 139 /** 140 * Write the map out using a custom routine. 141 * 142 * @param out the output stream 143 * @throws IOException if an error occurs while writing to the stream 144 * @since 3.1 145 */ 146 private void writeObject(final ObjectOutputStream out) throws IOException { 147 out.defaultWriteObject(); 148 out.writeObject(map); 149 } 150 151 /** 152 * Read the map in using a custom routine. 153 * 154 * @param in the input stream 155 * @throws IOException if an error occurs while reading from the stream 156 * @throws ClassNotFoundException if an object read from the stream can not be loaded 157 * @since 3.1 158 */ 159 @SuppressWarnings("unchecked") 160 private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException { 161 in.defaultReadObject(); 162 map = (Map<K, V>) in.readObject(); 163 } 164 165 //----------------------------------------------------------------------- 166 @Override 167 public V get(final Object key) { 168 // create value for key if key is not currently in the map 169 if (map.containsKey(key) == false) { 170 @SuppressWarnings("unchecked") 171 final K castKey = (K) key; 172 final V value = factory.transform(castKey); 173 map.put(castKey, value); 174 return value; 175 } 176 return map.get(key); 177 } 178 179 // no need to wrap keySet, entrySet or values as they are views of 180 // existing map entries - you can't do a map-style get on them. 181}