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