    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.HashMap;
    024import java.util.Map;
    025
    026import org.apache.commons.collections4.Factory;
    027import org.apache.commons.collections4.Transformer;
    028import org.apache.commons.collections4.functors.ConstantTransformer;
    029import org.apache.commons.collections4.functors.FactoryTransformer;
    030
    031/**
    032 * Decorates another <code>Map</code> returning a default value if the map
    033 * does not contain the requested key.
    034 * <p>
    035 * When the {@link #get(Object)} method is called with a key that does not
    036 * exist in the map, this map will return the default value specified in
    037 * the constructor/factory. Only the get method is altered, so the
    038 * {@link Map#containsKey(Object)} can be used to determine if a key really
    039 * is in the map or not.
    040 * </p>
    041 * <p>
    042 * The defaulted value is not added to the map.
    043 * Compare this behaviour with {@link LazyMap}, which does add the value
    044 * to the map (via a Transformer).
    045 * </p>
    046 * <p>
    047 * For instance:
    048 * </p>
    049 * <pre>
    050 * Map map = new DefaultedMap("NULL");
    051 * Object obj = map.get("Surname");
    052 * // obj == "NULL"
    053 * </pre>
    054 * <p>
    055 * After the above code is executed the map is still empty.
    056 * </p>
    057 * <p>
    058 * <strong>Note that DefaultedMap is not synchronized and is not thread-safe.</strong>
    059 * If you wish to use this map from multiple threads concurrently, you must use
    060 * appropriate synchronization. The simplest approach is to wrap this map
    061 * using {@link java.util.Collections#synchronizedMap(Map)}. This class may throw
    062 * exceptions when accessed by concurrent threads without synchronization.
    063 * </p>
    064 *
    065 * @param <K> the type of the keys in this map
    066 * @param <V> the type of the values in this map
    067 *
    068 * @since 3.2
    069 * @see LazyMap
    070 */
    071public class DefaultedMap<K, V> extends AbstractMapDecorator<K, V> implements Serializable {
    072
    073    /** Serialization version */
    074    private static final long serialVersionUID = 19698628745827L;
    075
    076    /** The transformer to use if the map does not contain a key */
    077    private final Transformer<? super K, ? extends V> value;
    078
    079    //-----------------------------------------------------------------------
    080    /**
    081     * Factory method to create a defaulting map.
    082     * <p>
    083     * The value specified is returned when a missing key is found.
    084     *
    085     * @param <K>  the key type
    086     * @param <V>  the value type
    087     * @param map  the map to decorate, must not be null
    088     * @param defaultValue  the default value to return when the key is not found
    089     * @return a new defaulting map
    090     * @throws NullPointerException if map is null
    091     * @since 4.0
    092     */
    093    public static <K, V> DefaultedMap<K, V> defaultedMap(final Map<K, V> map, final V defaultValue) {
    094        return new DefaultedMap<>(map, ConstantTransformer.constantTransformer(defaultValue));
    095    }
    096
    097    /**
    098     * Factory method to create a defaulting map.
    099     * <p>
    100     * The factory specified is called when a missing key is found.
    101     * The result will be returned as the result of the map get(key) method.
    102     *
    103     * @param <K>  the key type
    104     * @param <V>  the value type
    105     * @param map  the map to decorate, must not be null
    106     * @param factory  the factory to use to create entries, must not be null
    107     * @return a new defaulting map
    108     * @throws NullPointerException if map or factory is null
    109     * @since 4.0
    110     */
    111    public static <K, V> DefaultedMap<K, V> defaultedMap(final Map<K, V> map, final Factory<? extends V> factory) {
    112        if (factory == null) {
    113            throw new IllegalArgumentException("Factory must not be null");
    114        }
    115        return new DefaultedMap<>(map, FactoryTransformer.factoryTransformer(factory));
    116    }
    117
    118    /**
    119     * Factory method to create a defaulting map.
    120     * <p>
    121     * The transformer specified is called when a missing key is found.
    122     * The key is passed to the transformer as the input, and the result
    123     * will be returned as the result of the map get(key) method.
    124     *
    125     * @param <K>  the key type
    126     * @param <V>  the value type
    127     * @param map  the map to decorate, must not be null
    128     * @param transformer  the transformer to use as a factory to create entries, must not be null
    129     * @return a new defaulting map
    130     * @throws NullPointerException if map or factory is null
    131     * @since 4.0
    132     */
    133    public static <K, V> Map<K, V> defaultedMap(final Map<K, V> map,
    134                                                final Transformer<? super K, ? extends V> transformer) {
    135        if (transformer == null) {
    136           throw new IllegalArgumentException("Transformer must not be null");
    137       }
    138       return new DefaultedMap<>(map, transformer);
    139    }
    140
    141    //-----------------------------------------------------------------------
    142    /**
    143     * Constructs a new empty <code>DefaultedMap</code> that decorates
    144     * a <code>HashMap</code>.
    145     * <p>
    146     * The object passed in will be returned by the map whenever an
    147     * unknown key is requested.
    148     *
    149     * @param defaultValue  the default value to return when the key is not found
    150     */
    151    public DefaultedMap(final V defaultValue) {
    152        this(ConstantTransformer.constantTransformer(defaultValue));
    153    }
    154
    155    /**
    156     * Constructs a new empty <code>DefaultedMap</code> that decorates a <code>HashMap</code>.
    157     *
    158     * @param defaultValueTransformer transformer to use to generate missing values.
    159     */
    160    public DefaultedMap(final Transformer<? super K, ? extends V> defaultValueTransformer) {
    161        this(new HashMap<K, V>(), defaultValueTransformer);
    162    }
    163
    164    /**
    165     * Constructor that wraps (not copies).
    166     *
    167     * @param map  the map to decorate, must not be null
    168     * @param defaultValueTransformer  the value transformer to use
    169     * @throws NullPointerException if map or transformer is null
    170     */
    171    protected DefaultedMap(final Map<K, V> map, final Transformer<? super K, ? extends V> defaultValueTransformer) {
    172        super(map);
    173        if (defaultValueTransformer == null) {
    174            throw new NullPointerException("Transformer must not be null.");
    175        }
    176        this.value = defaultValueTransformer;
    177    }
    178
    179    //-----------------------------------------------------------------------
    180    /**
    181     * Write the map out using a custom routine.
    182     *
    183     * @param out  the output stream
    184     * @throws IOException if an error occurs while writing to the stream
    185     */
    186    private void writeObject(final ObjectOutputStream out) throws IOException {
    187        out.defaultWriteObject();
    188        out.writeObject(map);
    189    }
    190
    191    /**
    192     * Read the map in using a custom routine.
    193     *
    194     * @param in  the input stream
    195     * @throws IOException if an error occurs while reading from the stream
    196     * @throws ClassNotFoundException if an object read from the stream can not be loaded
    197     */
    198    @SuppressWarnings("unchecked")
    199    private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
    200        in.defaultReadObject();
    201        map = (Map<K, V>) in.readObject();
    202    }
    203
    204    //-----------------------------------------------------------------------
    205    @Override
    206    @SuppressWarnings("unchecked")
    207    public V get(final Object key) {
    208        V v;
    209        return (((v = map.get(key)) != null) || map.containsKey(key))
    210          ? v
    211          : value.transform((K) key);
    212    }
    213
    214    // no need to wrap keySet, entrySet or values as they are views of
    215    // existing map entries - you can't do a map-style get on them.
    216}