    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *      http://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017package org.apache.commons.collections4.map;
    018
    019import java.io.IOException;
    020import java.io.ObjectInputStream;
    021import java.io.ObjectOutputStream;
    022import java.io.Serializable;
    023import java.util.Map;
    024
    025import org.apache.commons.collections4.Factory;
    026import org.apache.commons.collections4.Transformer;
    027import org.apache.commons.collections4.functors.FactoryTransformer;
    028
    029/**
    030 * Decorates another <code>Map</code> to create objects in the map on demand.
    031 * <p>
    032 * When the {@link #get(Object)} method is called with a key that does not
    033 * exist in the map, the factory is used to create the object. The created
    034 * object will be added to the map using the requested key.
    035 * <p>
    036 * For instance:
    037 * <pre>
    038 * Factory&lt;Date&gt; factory = new Factory&lt;Date&gt;() {
    039 *     public Date create() {
    040 *         return new Date();
    041 *     }
    042 * }
    043 * Map&lt;String, Date&gt; lazy = LazyMap.lazyMap(new HashMap&lt;String, Date&gt;(), factory);
    044 * Date date = lazy.get("NOW");
    045 * </pre>
    046 *
    047 * After the above code is executed, <code>date</code> will refer to
    048 * a new <code>Date</code> instance. Furthermore, that <code>Date</code>
    049 * instance is mapped to the "NOW" key in the map.
    050 * <p>
    051 * <strong>Note that LazyMap is not synchronized and is not thread-safe.</strong>
    052 * If you wish to use this map from multiple threads concurrently, you must use
    053 * appropriate synchronization. The simplest approach is to wrap this map
    054 * using {@link java.util.Collections#synchronizedMap(Map)}. This class may throw
    055 * exceptions when accessed by concurrent threads without synchronization.
    056 * <p>
    057 * This class is Serializable from Commons Collections 3.1.
    058 *
    059 * @param <K> the type of the keys in this map
    060 * @param <V> the type of the values in this map
    061 * @since 3.0
    062 */
    063public class LazyMap<K, V> extends AbstractMapDecorator<K, V> implements Serializable {
    064
    065    /** Serialization version */
    066    private static final long serialVersionUID = 7990956402564206740L;
    067
    068    /** The factory to use to construct elements */
    069    protected final Transformer<? super K, ? extends V> factory;
    070
    071    /**
    072     * Factory method to create a lazily instantiated map.
    073     *
    074     * @param <K>  the key type
    075     * @param <V>  the value type
    076     * @param map  the map to decorate, must not be null
    077     * @param factory  the factory to use, must not be null
    078     * @return a new lazy map
    079     * @throws NullPointerException if map or factory is null
    080     * @since 4.0
    081     */
    082    public static <K, V> LazyMap<K, V> lazyMap(final Map<K, V> map, final Factory< ? extends V> factory) {
    083        return new LazyMap<>(map, factory);
    084    }
    085
    086    /**
    087     * Factory method to create a lazily instantiated map.
    088     *
    089     * @param <K>  the key type
    090     * @param <V>  the value type
    091     * @param map  the map to decorate, must not be null
    092     * @param factory  the factory to use, must not be null
    093     * @return a new lazy map
    094     * @throws NullPointerException if map or factory is null
    095     * @since 4.0
    096     */
    097    public static <V, K> LazyMap<K, V> lazyMap(final Map<K, V> map, final Transformer<? super K, ? extends V> factory) {
    098        return new LazyMap<>(map, factory);
    099    }
    100
    101    //-----------------------------------------------------------------------
    102    /**
    103     * Constructor that wraps (not copies).
    104     *
    105     * @param map  the map to decorate, must not be null
    106     * @param factory  the factory to use, must not be null
    107     * @throws NullPointerException if map or factory is null
    108     */
    109    protected LazyMap(final Map<K,V> map, final Factory<? extends V> factory) {
    110        super(map);
    111        if (factory == null) {
    112            throw new NullPointerException("Factory must not be null");
    113        }
    114        this.factory = FactoryTransformer.factoryTransformer(factory);
    115    }
    116
    117    /**
    118     * Constructor that wraps (not copies).
    119     *
    120     * @param map  the map to decorate, must not be null
    121     * @param factory  the factory to use, must not be null
    122     * @throws NullPointerException if map or factory is null
    123     */
    124    protected LazyMap(final Map<K,V> map, final Transformer<? super K, ? extends V> factory) {
    125        super(map);
    126        if (factory == null) {
    127            throw new NullPointerException("Factory must not be null");
    128        }
    129        this.factory = factory;
    130    }
    131
    132    //-----------------------------------------------------------------------
    133    /**
    134     * Write the map out using a custom routine.
    135     *
    136     * @param out  the output stream
    137     * @throws IOException if an error occurs while writing to the stream
    138     * @since 3.1
    139     */
    140    private void writeObject(final ObjectOutputStream out) throws IOException {
    141        out.defaultWriteObject();
    142        out.writeObject(map);
    143    }
    144
    145    /**
    146     * Read the map in using a custom routine.
    147     *
    148     * @param in  the input stream
    149     * @throws IOException if an error occurs while reading from the stream
    150     * @throws ClassNotFoundException if an object read from the stream can not be loaded
    151     * @since 3.1
    152     */
    153    @SuppressWarnings("unchecked")
    154    private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
    155        in.defaultReadObject();
    156        map = (Map<K, V>) in.readObject();
    157    }
    158
    159    //-----------------------------------------------------------------------
    160    @Override
    161    public V get(final Object key) {
    162        // create value for key if key is not currently in the map
    163        if (map.containsKey(key) == false) {
    164            @SuppressWarnings("unchecked")
    165            final K castKey = (K) key;
    166            final V value = factory.transform(castKey);
    167            map.put(castKey, value);
    168            return value;
    169        }
    170        return map.get(key);
    171    }
    172
    173    // no need to wrap keySet, entrySet or values as they are views of
    174    // existing map entries - you can't do a map-style get on them.
    175}