    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}