- /*
- * Copyright 2003-2004 The Apache Software Foundation
- *
- * Licensed 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 <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 is Serializable from Commons Collections 3.1.
- *
- * @since Commons Collections 3.0
- * @version $Revision: 1.6 $ $Date: 2004/06/03 22:02:13 $
- *
- * @author Stephen Colebourne
- * @author Arron Bates
- * @author Paul Jack
- */
- public class LazyList extends AbstractSerializableListDecorator {
-
- /** Serialization version */
- private static final long serialVersionUID = -1708388017160694542L;
-
- /** The factory to use to lazily instantiate the objects */
- protected final Factory factory;
-
- /**
- * 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).
- *
- * @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.
- * <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);
- }
-
- }