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 }