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 *  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.collections.list;

import java.util.List;

import org.apache.commons.collections.Factory;

Decorates another List to create objects in the list on demand.
 When the 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. 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. 

For instance:
Factory factory = new Factory() {
    public Object create() {
        return new Date();
    }
}
List lazy = LazyList.decorate(new ArrayList(), factory);
Object obj = lazy.get(3);

After the above code is executed, obj will contain
a new Date instance.  Furthermore, that Date
instance is the fourth element in the list.  The first, second, 
and third element are all set to null.
 This class differs from GrowthList because here growth occurs on get, where GrowthList grows on set and add. However, they
could easily be used together by decorating twice.

This class is Serializable from Commons Collections 3.1.
Author:
Stephen Colebourne, Arron Bates, Paul Jack
See Also:
GrowthList
Since:
Commons Collections 3.0
Version:
$Revision: 646777 $ $Date: 2008-04-10 14:33:15 +0200 (Thu, 10 Apr 2008) $/**
 * Decorates another <code>List</code> 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. 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>
 * For instance:
 *
 * <pre>
 * Factory factory = new Factory() {
 *     public Object create() {
 *         return new Date();
 *     }
 * }
 * List lazy = LazyList.decorate(new ArrayList(), factory);
 * Object obj = lazy.get(3);
 * </pre>
 *
 * After the above code is executed, <code>obj</code> will contain
 * a new <code>Date</code> instance.  Furthermore, that <code>Date</code>
 * instance is the fourth element in the list.  The first, second, 
 * and third element are all set to <code>null</code>.
 * <p>
 * This class differs from {@link GrowthList} because here growth occurs on
 * get, where <code>GrowthList</code> grows on set and add. However, they
 * could easily be used together by decorating twice.
 * <p>
 * This class is Serializable from Commons Collections 3.1.
 *
 * @see GrowthList
 * @since Commons Collections 3.0
 * @version $Revision: 646777 $ $Date: 2008-04-10 14:33:15 +0200 (Thu, 10 Apr 2008) $
 * 
 * @author Stephen Colebourne
 * @author Arron Bates
 * @author Paul Jack
 */
public class LazyList extends AbstractSerializableListDecorator {

    
Serialization version /** Serialization version */
    private static final long serialVersionUID = -1708388017160694542L;

    
The factory to use to lazily instantiate the objects /** The factory to use to lazily instantiate the objects */
    protected final Factory factory;

    
Factory method to create a lazily instantiating list.
Params:
list –  the list to decorate, must not be null
factory –  the factory to use for creation, must not be null
Throws:
IllegalArgumentException – if list or factory is null/**
     * Factory method to create a lazily instantiating list.
     * 
     * @param list  the list to decorate, must not be null
     * @param factory  the factory to use for creation, must not be null
     * @throws IllegalArgumentException if list or factory is null
     */
    public static List decorate(List list, Factory factory) {
        return new LazyList(list, factory);
    }
    
    //-----------------------------------------------------------------------
    
Constructor that wraps (not copies).
Params:
list –  the list to decorate, must not be null
factory –  the factory to use for creation, must not be null
Throws:
IllegalArgumentException – if list or factory is null/**
     * 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 IllegalArgumentException if list or factory is null
     */
    protected LazyList(List list, Factory factory) {
        super(list);
        if (factory == null) {
            throw new IllegalArgumentException("Factory must not be null");
        }
        this.factory = factory;
    }

    //-----------------------------------------------------------------------
    
Decorate the get method to perform the lazy behaviour.

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.
Indexes in-between the old size and the requested size are left with a 
placeholder that is replaced with a factory object when requested.
Params:
index –  the index to retrieve/**
     * Decorate the get method to perform the lazy behaviour.
     * <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.
     * Indexes in-between the old size and the requested size are left with a 
     * placeholder that is replaced with a factory object when requested.
     * 
     * @param index  the index to retrieve
     */
    public Object get(int index) {
        int size = getList().size();
        if (index < size) {
            // within bounds, get the object
            Object object = getList().get(index);
            if (object == null) {
                // item is a place holder, create new one, set and return
                object = factory.create();
                getList().set(index, object);
                return object;
            } else {
                // good and ready to go
                return object;
            }
        } else {
            // we have to grow the list
            for (int i = size; i < index; i++) {
                getList().add(null);
            }
            // create our last object, set and return
            Object object = factory.create();
            getList().add(object);
            return object;
        }
    }


    public List subList(int fromIndex, int toIndex) {
        List sub = getList().subList(fromIndex, toIndex);
        return new LazyList(sub, factory);
    }

}