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The Apache Commons Collections package contains types that extend and augment the Java Collections Framework.
There is a newer version: 4.5.0-M1
/*
 * 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.map;

import java.util.Comparator;
import java.util.SortedMap;

import org.apache.commons.collections4.Factory;
import org.apache.commons.collections4.Transformer;

/**
 * Decorates another SortedMap to create objects in the map on demand.
 * 
 * When the {@link #get(Object)} method is called with a key that does not
 * exist in the map, the factory is used to create the object. The created
 * object will be added to the map using the requested key.
 * 

 * For instance:
 * 
 * Factory<Date> factory = new Factory<Date>() {
 *     public Date create() {
 *         return new Date();
 *     }
 * }
 * SortedMap<String, Date> lazy =
 *     LazySortedMap.lazySortedMap(new HashMap<String, Date>(), factory);
 * Date date = lazy.get("NOW");
 * 
 *
 * After the above code is executed, date will refer to
 * a new Date instance. Furthermore, that Date
 * instance is mapped to the "NOW" key in the map.
 * 
 * Note that LazySortedMap is not synchronized and is not thread-safe.
 * If you wish to use this map from multiple threads concurrently, you must use
 * appropriate synchronization. The simplest approach is to wrap this map
 * using {@link java.util.Collections#synchronizedSortedMap}. This class may throw
 * exceptions when accessed by concurrent threads without synchronization.
 * 
 * This class is Serializable from Commons Collections 3.1.
 *
 * @param  the type of the keys in this map
 * @param  the type of the values in this map
 * @since 3.0
 */
public class LazySortedMap extends LazyMap implements SortedMap {

    /** Serialization version */
    private static final long serialVersionUID = 2715322183617658933L;

    /**
     * Factory method to create a lazily instantiated sorted map.
     *
     * @param   the key type
     * @param   the value type
     * @param map  the map to decorate, must not be null
     * @param factory  the factory to use, must not be null
     * @return a new lazy sorted map
     * @throws NullPointerException if map or factory is null
     * @since 4.0
     */
    public static  LazySortedMap lazySortedMap(final SortedMap map,
                                                           final Factory factory) {
        return new LazySortedMap<>(map, factory);
    }

    /**
     * Factory method to create a lazily instantiated sorted map.
     *
     * @param   the key type
     * @param   the value type
     * @param map  the map to decorate, must not be null
     * @param factory  the factory to use, must not be null
     * @return a new lazy sorted map
     * @throws NullPointerException if map or factory is null
     * @since 4.0
     */
    public static  LazySortedMap lazySortedMap(final SortedMap map,
                                                           final Transformer factory) {
        return new LazySortedMap<>(map, factory);
    }

    //-----------------------------------------------------------------------
    /**
     * Constructor that wraps (not copies).
     *
     * @param map  the map to decorate, must not be null
     * @param factory  the factory to use, must not be null
     * @throws NullPointerException if map or factory is null
     */
    protected LazySortedMap(final SortedMap map, final Factory factory) {
        super(map, factory);
    }

    /**
     * Constructor that wraps (not copies).
     *
     * @param map  the map to decorate, must not be null
     * @param factory  the factory to use, must not be null
     * @throws NullPointerException if map or factory is null
     */
    protected LazySortedMap(final SortedMap map, final Transformer factory) {
        super(map, factory);
    }

    //-----------------------------------------------------------------------
    /**
     * Gets the map being decorated.
     *
     * @return the decorated map
     */
    protected SortedMap getSortedMap() {
        return (SortedMap) map;
    }

    //-----------------------------------------------------------------------
    @Override
    public K firstKey() {
        return getSortedMap().firstKey();
    }

    @Override
    public K lastKey() {
        return getSortedMap().lastKey();
    }

    @Override
    public Comparator comparator() {
        return getSortedMap().comparator();
    }

    @Override
    public SortedMap subMap(final K fromKey, final K toKey) {
        final SortedMap map = getSortedMap().subMap(fromKey, toKey);
        return new LazySortedMap<>(map, factory);
    }

    @Override
    public SortedMap headMap(final K toKey) {
        final SortedMap map = getSortedMap().headMap(toKey);
        return new LazySortedMap<>(map, factory);
    }

    @Override
    public SortedMap tailMap(final K fromKey) {
        final SortedMap map = getSortedMap().tailMap(fromKey);
        return new LazySortedMap<>(map, factory);
    }

}