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 */ 017 package org.apache.commons.collections.map; 018 019 import java.io.IOException; 020 import java.io.ObjectInputStream; 021 import java.io.ObjectOutputStream; 022 import java.io.Serializable; 023 import java.util.Map; 024 025 import org.apache.commons.collections.Factory; 026 import org.apache.commons.collections.Transformer; 027 import org.apache.commons.collections.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 * For instance: 037 * <pre> 038 * Factory factory = new Factory() { 039 * public Object create() { 040 * return new Date(); 041 * } 042 * } 043 * Map lazy = Lazy.map(new HashMap(), factory); 044 * Object obj = lazy.get("NOW"); 045 * </pre> 046 * 047 * After the above code is executed, <code>obj</code> will contain 048 * a new <code>Date</code> instance. Furthermore, that <code>Date</code> 049 * instance is mapped to the "NOW" key in the map. 050 * <p> 051 * <strong>Note that LazyMap is not synchronized and is not thread-safe.</strong> 052 * If you wish to use this map from multiple threads concurrently, you must use 053 * appropriate synchronization. The simplest approach is to wrap this map 054 * using {@link java.util.Collections#synchronizedMap(Map)}. This class may throw 055 * exceptions when accessed by concurrent threads without synchronization. 056 * <p> 057 * This class is Serializable from Commons Collections 3.1. 058 * 059 * @since Commons Collections 3.0 060 * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $ 061 * 062 * @author Stephen Colebourne 063 * @author Paul Jack 064 */ 065 public class LazyMap 066 extends AbstractMapDecorator 067 implements Map, Serializable { 068 069 /** Serialization version */ 070 private static final long serialVersionUID = 7990956402564206740L; 071 072 /** The factory to use to construct elements */ 073 protected final Transformer factory; 074 075 /** 076 * Factory method to create a lazily instantiated map. 077 * 078 * @param map the map to decorate, must not be null 079 * @param factory the factory to use, must not be null 080 * @throws IllegalArgumentException if map or factory is null 081 */ 082 public static Map decorate(Map map, Factory factory) { 083 return new LazyMap(map, factory); 084 } 085 086 /** 087 * Factory method to create a lazily instantiated map. 088 * 089 * @param map the map to decorate, must not be null 090 * @param factory the factory to use, must not be null 091 * @throws IllegalArgumentException if map or factory is null 092 */ 093 public static Map decorate(Map map, Transformer factory) { 094 return new LazyMap(map, factory); 095 } 096 097 //----------------------------------------------------------------------- 098 /** 099 * Constructor that wraps (not copies). 100 * 101 * @param map the map to decorate, must not be null 102 * @param factory the factory to use, must not be null 103 * @throws IllegalArgumentException if map or factory is null 104 */ 105 protected LazyMap(Map map, Factory factory) { 106 super(map); 107 if (factory == null) { 108 throw new IllegalArgumentException("Factory must not be null"); 109 } 110 this.factory = FactoryTransformer.getInstance(factory); 111 } 112 113 /** 114 * Constructor that wraps (not copies). 115 * 116 * @param map the map to decorate, must not be null 117 * @param factory the factory to use, must not be null 118 * @throws IllegalArgumentException if map or factory is null 119 */ 120 protected LazyMap(Map map, Transformer factory) { 121 super(map); 122 if (factory == null) { 123 throw new IllegalArgumentException("Factory must not be null"); 124 } 125 this.factory = factory; 126 } 127 128 //----------------------------------------------------------------------- 129 /** 130 * Write the map out using a custom routine. 131 * 132 * @param out the output stream 133 * @throws IOException 134 * @since Commons Collections 3.1 135 */ 136 private void writeObject(ObjectOutputStream out) throws IOException { 137 out.defaultWriteObject(); 138 out.writeObject(map); 139 } 140 141 /** 142 * Read the map in using a custom routine. 143 * 144 * @param in the input stream 145 * @throws IOException 146 * @throws ClassNotFoundException 147 * @since Commons Collections 3.1 148 */ 149 private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException { 150 in.defaultReadObject(); 151 map = (Map) in.readObject(); 152 } 153 154 //----------------------------------------------------------------------- 155 public Object get(Object key) { 156 // create value for key if key is not currently in the map 157 if (map.containsKey(key) == false) { 158 Object value = factory.transform(key); 159 map.put(key, value); 160 return value; 161 } 162 return map.get(key); 163 } 164 165 // no need to wrap keySet, entrySet or values as they are views of 166 // existing map entries - you can't do a map-style get on them. 167 }