| /* |
| * Licensed to the Apache Software Foundation (ASF) under one or more |
| * contributor license agreements. See the NOTICE file distributed with |
| * this work for additional information regarding copyright ownership. |
| * The ASF licenses this file to You under the Apache License, Version 2.0 |
| * (the "License"); you may not use this file except in compliance with |
| * the License. You may obtain a copy of the License at |
| * |
| * http://www.apache.org/licenses/LICENSE-2.0 |
| * |
| * Unless required by applicable law or agreed to in writing, software |
| * distributed under the License is distributed on an "AS IS" BASIS, |
| * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| * See the License for the specific language governing permissions and |
| * limitations under the License. |
| */ |
| package org.apache.commons.collections4.list; |
| |
| import java.util.List; |
| import java.util.Objects; |
| |
| import org.apache.commons.collections4.Factory; |
| import org.apache.commons.collections4.Transformer; |
| |
| /** |
| * Decorates another {@code List} to create objects in the list on demand. |
| * <p> |
| * When the {@link #get(int)} method is called with an index greater than |
| * the size of the list, the list will automatically grow in size and return |
| * a new object from the specified factory or transformer. The gaps will be |
| * filled by null. If a get method call encounters a null, it will be replaced |
| * with a new object from the factory. Thus this list is unsuitable for |
| * storing null objects. |
| * </p> |
| * <p> |
| * For instance: |
| * </p> |
| * |
| * <pre> |
| * Factory<Date> factory = new Factory<Date>() { |
| * public Date create() { |
| * return new Date(); |
| * } |
| * } |
| * List<Date> lazy = LazyList.decorate(new ArrayList<Date>(), factory); |
| * Date date = lazy.get(3); |
| * </pre> |
| * |
| * <p> |
| * After the above code is executed, {@code date} will contain |
| * a new {@code Date} instance. Furthermore, that {@code Date} |
| * instance is the fourth element in the list. The first, second, |
| * and third element are all set to {@code null}. |
| * </p> |
| * <p> |
| * This class differs from {@link GrowthList} because here growth occurs on |
| * get, where {@code GrowthList} grows on set and add. However, they |
| * could easily be used together by decorating twice. |
| * </p> |
| * <p> |
| * This class is Serializable from Commons Collections 3.1. |
| * </p> |
| * |
| * @see GrowthList |
| * @since 3.0 |
| */ |
| public class LazyList<E> extends AbstractSerializableListDecorator<E> { |
| |
| /** Serialization version */ |
| private static final long serialVersionUID = -3677737457567429713L; |
| |
| /** The factory to use to lazily instantiate the objects */ |
| private final Factory<? extends E> factory; |
| |
| /** The transformer to use to lazily instantiate the objects */ |
| private final Transformer<Integer, ? extends E> transformer; |
| |
| /** |
| * Factory method to create a lazily instantiating list. |
| * |
| * @param <E> the type of the elements in the list |
| * @param list the list to decorate, must not be null |
| * @param factory the factory to use for creation, must not be null |
| * @return a new lazy list |
| * @throws NullPointerException if list or factory is null |
| * @since 4.0 |
| */ |
| public static <E> LazyList<E> lazyList(final List<E> list, final Factory<? extends E> factory) { |
| return new LazyList<>(list, factory); |
| } |
| |
| /** |
| * Transformer method to create a lazily instantiating list. |
| * |
| * @param <E> the type of the elements in the list |
| * @param list the list to decorate, must not be null |
| * @param transformer the transformer to use for creation, must not be null |
| * @return a new lazy list |
| * @throws NullPointerException if list or transformer is null |
| * @since 4.4 |
| */ |
| public static <E> LazyList<E> lazyList(final List<E> list, final Transformer<Integer, ? extends E> transformer) { |
| return new LazyList<>(list, transformer); |
| } |
| |
| //----------------------------------------------------------------------- |
| /** |
| * Constructor that wraps (not copies). |
| * |
| * @param list the list to decorate, must not be null |
| * @param factory the factory to use for creation, must not be null |
| * @throws NullPointerException if list or factory is null |
| */ |
| protected LazyList(final List<E> list, final Factory<? extends E> factory) { |
| super(list); |
| this.factory = Objects.requireNonNull(factory); |
| this.transformer = null; |
| } |
| |
| /** |
| * Constructor that wraps (not copies). |
| * |
| * @param list the list to decorate, must not be null |
| * @param transformer the transformer to use for creation, must not be null |
| * @throws NullPointerException if list or transformer is null |
| */ |
| protected LazyList(final List<E> list, final Transformer<Integer, ? extends E> transformer) { |
| super(list); |
| this.factory = null; |
| this.transformer = Objects.requireNonNull(transformer); |
| } |
| |
| //----------------------------------------------------------------------- |
| /** |
| * Decorate the get method to perform the lazy behavior. |
| * <p> |
| * If the requested index is greater than the current size, the list will |
| * grow to the new size and a new object will be returned from the factory |
| * or transformer. Indexes in-between the old size and the requested size |
| * are left with a placeholder that is replaced with a factory or |
| * transformer object when requested. |
| * |
| * @param index the index to retrieve |
| * @return the element at the given index |
| */ |
| @Override |
| public E get(final int index) { |
| final int size = decorated().size(); |
| if (index < size) { |
| // within bounds, get the object |
| E object = decorated().get(index); |
| if (object == null) { |
| // item is a place holder, create new one, set and return |
| object = element(index); |
| decorated().set(index, object); |
| } |
| // good and ready to go |
| return object; |
| } |
| // we have to grow the list |
| for (int i = size; i < index; i++) { |
| decorated().add(null); |
| } |
| // create our last object, set and return |
| final E object = element(index); |
| decorated().add(object); |
| return object; |
| } |
| |
| @Override |
| public List<E> subList(final int fromIndex, final int toIndex) { |
| final List<E> sub = decorated().subList(fromIndex, toIndex); |
| if (factory != null) { |
| return new LazyList<>(sub, factory); |
| } |
| if (transformer != null) { |
| return new LazyList<>(sub, transformer); |
| } |
| throw new IllegalStateException("Factory and Transformer are both null!"); |
| } |
| |
| private E element(final int index) { |
| if (factory != null) { |
| return factory.create(); |
| } |
| if (transformer != null) { |
| return transformer.transform(index); |
| } |
| throw new IllegalStateException("Factory and Transformer are both null!"); |
| } |
| |
| } |