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.list;
018
019import java.util.List;
020
021import org.apache.commons.collections4.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&lt;Date&gt; factory = new Factory&lt;Date&gt;() {
037 *     public Date create() {
038 *         return new Date();
039 *     }
040 * }
041 * List&lt;Date&gt; lazy = LazyList.decorate(new ArrayList&lt;Date&gt;(), factory);
042 * Date date = lazy.get(3);
043 * </pre>
044 *
045 * After the above code is executed, <code>date</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 3.0
058 */
059public class LazyList<E> extends AbstractSerializableListDecorator<E> {
060
061    /** Serialization version */
062    private static final long serialVersionUID = -1708388017160694542L;
063
064    /** The factory to use to lazily instantiate the objects */
065    private final Factory<? extends E> factory;
066
067    /**
068     * Factory method to create a lazily instantiating list.
069     *
070     * @param <E> the type of the elements in the list
071     * @param list  the list to decorate, must not be null
072     * @param factory  the factory to use for creation, must not be null
073     * @return a new lazy list
074     * @throws NullPointerException if list or factory is null
075     * @since 4.0
076     */
077    public static <E> LazyList<E> lazyList(final List<E> list, final Factory<? extends E> factory) {
078        return new LazyList<>(list, factory);
079    }
080
081    //-----------------------------------------------------------------------
082    /**
083     * Constructor that wraps (not copies).
084     *
085     * @param list  the list to decorate, must not be null
086     * @param factory  the factory to use for creation, must not be null
087     * @throws NullPointerException if list or factory is null
088     */
089    protected LazyList(final List<E> list, final Factory<? extends E> factory) {
090        super(list);
091        if (factory == null) {
092            throw new IllegalArgumentException("Factory must not be null");
093        }
094        this.factory = factory;
095    }
096
097    //-----------------------------------------------------------------------
098    /**
099     * Decorate the get method to perform the lazy behaviour.
100     * <p>
101     * If the requested index is greater than the current size, the list will
102     * grow to the new size and a new object will be returned from the factory.
103     * Indexes in-between the old size and the requested size are left with a
104     * placeholder that is replaced with a factory object when requested.
105     *
106     * @param index  the index to retrieve
107     * @return the element at the given index
108     */
109    @Override
110    public E get(final int index) {
111        final int size = decorated().size();
112        if (index < size) {
113            // within bounds, get the object
114            E object = decorated().get(index);
115            if (object == null) {
116                // item is a place holder, create new one, set and return
117                object = factory.create();
118                decorated().set(index, object);
119                return object;
120            }
121            // good and ready to go
122            return object;
123        }
124        // we have to grow the list
125        for (int i = size; i < index; i++) {
126            decorated().add(null);
127        }
128        // create our last object, set and return
129        final E object = factory.create();
130        decorated().add(object);
131        return object;
132    }
133
134    @Override
135    public List<E> subList(final int fromIndex, final int toIndex) {
136        final List<E> sub = decorated().subList(fromIndex, toIndex);
137        return new LazyList<>(sub, factory);
138    }
139
140}