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}