    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 *      https://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;
    024import java.util.Objects;
    025
    026import org.apache.commons.collections4.Factory;
    027import org.apache.commons.collections4.Transformer;
    028import org.apache.commons.collections4.functors.FactoryTransformer;
    029
    030/**
    031 * Decorates another {@code Map} to create objects in the map on demand.
    032 * <p>
    033 * When the {@link #get(Object)} method is called with a key that does not
    034 * exist in the map, the factory is used to create the object. The created
    035 * object will be added to the map using the requested key.
    036 * </p>
    037 * <p>
    038 * For instance:
    039 * </p>
    040 * <pre>
    041 * Factory&lt;Date&gt; factory = new Factory&lt;Date&gt;() {
    042 *     public Date create() {
    043 *         return new Date();
    044 *     }
    045 * }
    046 * Map&lt;String, Date&gt; lazy = LazyMap.lazyMap(new HashMap&lt;String, Date&gt;(), factory);
    047 * Date date = lazy.get("NOW");
    048 * </pre>
    049 *
    050 * <p>
    051 * After the above code is executed, {@code date} will refer to
    052 * a new {@code Date} instance. Furthermore, that {@code Date}
    053 * instance is mapped to the "NOW" key in the map.
    054 * </p>
    055 * <p>
    056 * <strong>Note that LazyMap is not synchronized and is not thread-safe.</strong>
    057 * If you wish to use this map from multiple threads concurrently, you must use
    058 * appropriate synchronization. The simplest approach is to wrap this map
    059 * using {@link java.util.Collections#synchronizedMap(Map)}. This class may throw
    060 * exceptions when accessed by concurrent threads without synchronization.
    061 * </p>
    062 * <p>
    063 * This class is Serializable from Commons Collections 3.1.
    064 * </p>
    065 *
    066 * @param <K> The type of the keys in this map
    067 * @param <V> The type of the values in this map
    068 * @since 3.0
    069 */
    070public class LazyMap<K, V> extends AbstractMapDecorator<K, V> implements Serializable {
    071
    072    /** Serialization version */
    073    private static final long serialVersionUID = 7990956402564206740L;
    074
    075    /**
    076     * Factory method to create a lazily instantiated map.
    077     *
    078     * @param <K>  the key type
    079     * @param <V>  the value type
    080     * @param map  The map to decorate, must not be null
    081     * @param factory  The factory to use, must not be null
    082     * @return A new lazy map
    083     * @throws NullPointerException if map or factory is null
    084     * @since 4.0
    085     */
    086    public static <K, V> LazyMap<K, V> lazyMap(final Map<K, V> map, final Factory<? extends V> factory) {
    087        return new LazyMap<>(map, factory);
    088    }
    089
    090    /**
    091     * Factory method to create a lazily instantiated map.
    092     *
    093     * @param <K>  the key type
    094     * @param <V>  the value type
    095     * @param map  The map to decorate, must not be null
    096     * @param factory  The factory to use, must not be null
    097     * @return A new lazy map
    098     * @throws NullPointerException if map or factory is null
    099     * @since 4.0
    100     */
    101    public static <V, K> LazyMap<K, V> lazyMap(final Map<K, V> map, final Transformer<? super K, ? extends V> factory) {
    102        return new LazyMap<>(map, factory);
    103    }
    104
    105    /** The factory to use to construct elements */
    106    protected final Transformer<? super K, ? extends V> factory;
    107
    108    /**
    109     * Constructor that wraps (not copies).
    110     *
    111     * @param map  The map to decorate, must not be null
    112     * @param factory  The factory to use, must not be null
    113     * @throws NullPointerException if map or factory is null
    114     */
    115    protected LazyMap(final Map<K, V> map, final Factory<? extends V> factory) {
    116        super(map);
    117        this.factory = FactoryTransformer.factoryTransformer(Objects.requireNonNull(factory, "factory"));
    118    }
    119
    120    /**
    121     * Constructor that wraps (not copies).
    122     *
    123     * @param map  The map to decorate, must not be null
    124     * @param factory  The factory to use, must not be null
    125     * @throws NullPointerException if map or factory is null
    126     */
    127    protected LazyMap(final Map<K, V> map, final Transformer<? super K, ? extends V> factory) {
    128        super(map);
    129        this.factory = Objects.requireNonNull(factory, "factory");
    130    }
    131
    132    @Override
    133    public V get(final Object key) {
    134        // create value for key if key is not currently in the map
    135        if (!map.containsKey(key)) {
    136            @SuppressWarnings("unchecked")
    137            final K castKey = (K) key;
    138            final V value = factory.apply(castKey);
    139            map.put(castKey, value);
    140            return value;
    141        }
    142        return map.get(key);
    143    }
    144
    145    /**
    146     * Deserializes the map in using a custom routine.
    147     *
    148     * @param in  The input stream
    149     * @throws IOException Thrown if an error occurs while reading from the stream
    150     * @throws ClassNotFoundException if an object read from the stream cannot 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     * Serializes this object to an ObjectOutputStream.
    161     *
    162     * @param out The target ObjectOutputStream.
    163     * @throws IOException thrown when an I/O errors occur writing to the target stream.
    164     * @since 3.1
    165     */
    166    private void writeObject(final ObjectOutputStream out) throws IOException {
    167        out.defaultWriteObject();
    168        out.writeObject(map);
    169    }
    170
    171    // no need to wrap keySet, entrySet or values as they are views of
    172    // existing map entries - you can't do a map-style get on them.
    173}