LazyList

/*
 * 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&lt;Date&gt; factory = new Factory&lt;Date&gt;() {
 *     public Date create() {
 *         return new Date();
 *     }
 * }
 * List&lt;Date&gt; lazy = LazyList.decorate(new ArrayList&lt;Date&gt;(), 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;

    /**
     * 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);
    }

    /** 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;

    /**
     * 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);
    }

    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!");
    }

    /**
     * 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 placeholder, 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!");
    }

}