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Java > Open Source Codes > org > apache > commons > collections > list > LazyList


1 /*
2  * Copyright 2003-2004 The Apache Software Foundation
3  *
4  * Licensed under the Apache License, Version 2.0 (the "License");
5  * you may not use this file except in compliance with the License.
6  * You may obtain a copy of the License at
7  *
8  * http://www.apache.org/licenses/LICENSE-2.0
9  *
10  * Unless required by applicable law or agreed to in writing, software
11  * distributed under the License is distributed on an "AS IS" BASIS,
12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13  * See the License for the specific language governing permissions and
14  * limitations under the License.
15  */
16 package org.apache.commons.collections.list;
17
18 import java.util.List JavaDoc;
19
20 import org.apache.commons.collections.Factory;
21
22 /**
23  * Decorates another <code>List</code> to create objects in the list on demand.
24  * <p>
25  * When the {@link #get(int)} method is called with an index greater than
26  * the size of the list, the list will automatically grow in size and return
27  * a new object from the specified factory. The gaps will be filled by null.
28  * If a get method call encounters a null, it will be replaced with a new
29  * object from the factory. Thus this list is unsuitable for storing null
30  * objects.
31  * <p>
32  * For instance:
33  *
34  * <pre>
35  * Factory factory = new Factory() {
36  * public Object create() {
37  * return new Date();
38  * }
39  * }
40  * List lazy = LazyList.decorate(new ArrayList(), factory);
41  * Object obj = lazy.get(3);
42  * </pre>
43  *
44  * After the above code is executed, <code>obj</code> will contain
45  * a new <code>Date</code> instance. Furthermore, that <code>Date</code>
46  * instance is the fourth element in the list. The first, second,
47  * and third element are all set to <code>null</code>.
48  * <p>
49  * This class is Serializable from Commons Collections 3.1.
50  *
51  * @since Commons Collections 3.0
52  * @version $Revision: 1.6 $ $Date: 2004/06/03 22:02:13 $
53  *
54  * @author Stephen Colebourne
55  * @author Arron Bates
56  * @author Paul Jack
57  */
58 public class LazyList extends AbstractSerializableListDecorator {
59
60     /** Serialization version */
61     private static final long serialVersionUID = -1708388017160694542L;
62
63     /** The factory to use to lazily instantiate the objects */
64     protected final Factory factory;
65
66     /**
67      * Factory method to create a lazily instantiating list.
68      *
69      * @param list the list to decorate, must not be null
70      * @param factory the factory to use for creation, must not be null
71      * @throws IllegalArgumentException if list or factory is null
72      */
73     public static List JavaDoc decorate(List JavaDoc list, Factory factory) {
74         return new LazyList(list, factory);
75     }
76     
77     //-----------------------------------------------------------------------
78 /**
79      * Constructor that wraps (not copies).
80      *
81      * @param list the list to decorate, must not be null
82      * @param factory the factory to use for creation, must not be null
83      * @throws IllegalArgumentException if list or factory is null
84      */
85     protected LazyList(List JavaDoc list, Factory factory) {
86         super(list);
87         if (factory == null) {
88             throw new IllegalArgumentException JavaDoc("Factory must not be null");
89         }
90         this.factory = factory;
91     }
92
93     //-----------------------------------------------------------------------
94 /**
95      * Decorate the get method to perform the lazy behaviour.
96      * <p>
97      * If the requested index is greater than the current size, the list will
98      * grow to the new size and a new object will be returned from the factory.
99      * Indexes in-between the old size and the requested size are left with a
100      * placeholder that is replaced with a factory object when requested.
101      *
102      * @param index the index to retrieve
103      */
104     public Object JavaDoc get(int index) {
105         int size = getList().size();
106         if (index < size) {
107             // within bounds, get the object
108 Object JavaDoc object = getList().get(index);
109             if (object == null) {
110                 // item is a place holder, create new one, set and return
111 object = factory.create();
112                 getList().set(index, object);
113                 return object;
114             } else {
115                 // good and ready to go
116 return object;
117             }
118         } else {
119             // we have to grow the list
120 for (int i = size; i < index; i++) {
121                 getList().add(null);
122             }
123             // create our last object, set and return
124 Object JavaDoc object = factory.create();
125             getList().add(object);
126             return object;
127         }
128     }
129
130
131     public List JavaDoc subList(int fromIndex, int toIndex) {
132         List JavaDoc sub = getList().subList(fromIndex, toIndex);
133         return new LazyList(sub, factory);
134     }
135
136 }
137
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