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.list; 018 019 import java.util.List; 020 021 import org.apache.commons.collections.Factory; 022 023 /** 024 * Decorates another <code>List</code> to create objects in the list on demand. 025 * <p> 026 * When the {@link #get(int)} method is called with an index greater than 027 * the size of the list, the list will automatically grow in size and return 028 * a new object from the specified factory. The gaps will be filled by null. 029 * If a get method call encounters a null, it will be replaced with a new 030 * object from the factory. Thus this list is unsuitable for storing null 031 * objects. 032 * <p> 033 * For instance: 034 * 035 * <pre> 036 * Factory factory = new Factory() { 037 * public Object create() { 038 * return new Date(); 039 * } 040 * } 041 * List lazy = LazyList.decorate(new ArrayList(), factory); 042 * Object obj = lazy.get(3); 043 * </pre> 044 * 045 * After the above code is executed, <code>obj</code> will contain 046 * a new <code>Date</code> instance. Furthermore, that <code>Date</code> 047 * instance is the fourth element in the list. The first, second, 048 * and third element are all set to <code>null</code>. 049 * <p> 050 * This class differs from {@link GrowthList} because here growth occurs on 051 * get, where <code>GrowthList</code> grows on set and add. However, they 052 * could easily be used together by decorating twice. 053 * <p> 054 * This class is Serializable from Commons Collections 3.1. 055 * 056 * @see GrowthList 057 * @since Commons Collections 3.0 058 * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $ 059 * 060 * @author Stephen Colebourne 061 * @author Arron Bates 062 * @author Paul Jack 063 */ 064 public class LazyList extends AbstractSerializableListDecorator { 065 066 /** Serialization version */ 067 private static final long serialVersionUID = -1708388017160694542L; 068 069 /** The factory to use to lazily instantiate the objects */ 070 protected final Factory factory; 071 072 /** 073 * Factory method to create a lazily instantiating list. 074 * 075 * @param list the list to decorate, must not be null 076 * @param factory the factory to use for creation, must not be null 077 * @throws IllegalArgumentException if list or factory is null 078 */ 079 public static List decorate(List list, Factory factory) { 080 return new LazyList(list, factory); 081 } 082 083 //----------------------------------------------------------------------- 084 /** 085 * Constructor that wraps (not copies). 086 * 087 * @param list the list to decorate, must not be null 088 * @param factory the factory to use for creation, must not be null 089 * @throws IllegalArgumentException if list or factory is null 090 */ 091 protected LazyList(List list, Factory factory) { 092 super(list); 093 if (factory == null) { 094 throw new IllegalArgumentException("Factory must not be null"); 095 } 096 this.factory = factory; 097 } 098 099 //----------------------------------------------------------------------- 100 /** 101 * Decorate the get method to perform the lazy behaviour. 102 * <p> 103 * If the requested index is greater than the current size, the list will 104 * grow to the new size and a new object will be returned from the factory. 105 * Indexes in-between the old size and the requested size are left with a 106 * placeholder that is replaced with a factory object when requested. 107 * 108 * @param index the index to retrieve 109 */ 110 public Object get(int index) { 111 int size = getList().size(); 112 if (index < size) { 113 // within bounds, get the object 114 Object object = getList().get(index); 115 if (object == null) { 116 // item is a place holder, create new one, set and return 117 object = factory.create(); 118 getList().set(index, object); 119 return object; 120 } else { 121 // good and ready to go 122 return object; 123 } 124 } else { 125 // we have to grow the list 126 for (int i = size; i < index; i++) { 127 getList().add(null); 128 } 129 // create our last object, set and return 130 Object object = factory.create(); 131 getList().add(object); 132 return object; 133 } 134 } 135 136 137 public List subList(int fromIndex, int toIndex) { 138 List sub = getList().subList(fromIndex, toIndex); 139 return new LazyList(sub, factory); 140 } 141 142 }