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.keyvalue;
018
019import java.io.Serializable;
020import java.util.Arrays;
021
022/**
023 * A <code>MultiKey</code> allows multiple map keys to be merged together.
024 * <p>
025 * The purpose of this class is to avoid the need to write code to handle
026 * maps of maps. An example might be the need to look up a file name by
027 * key and locale. The typical solution might be nested maps. This class
028 * can be used instead by creating an instance passing in the key and locale.
029 * </p>
030 * <p>
031 * Example usage:
032 * </p>
033 * <pre>
034 * // populate map with data mapping key+locale to localizedText
035 * Map map = new HashMap();
036 * MultiKey multiKey = new MultiKey(key, locale);
037 * map.put(multiKey, localizedText);
038 *
039 * // later retrieve the localized text
040 * MultiKey multiKey = new MultiKey(key, locale);
041 * String localizedText = (String) map.get(multiKey);
042 * </pre>
043 *
044 * @param <K> the type of keys
045 * @since 3.0
046 */
047public class MultiKey<K> implements Serializable {
048    // This class could implement List, but that would confuse it's purpose
049
050    /** Serialisation version */
051    private static final long serialVersionUID = 4465448607415788805L;
052
053    /** The individual keys */
054    private final K[] keys;
055    /** The cached hashCode */
056    private transient int hashCode;
057
058    /**
059     * Constructor taking two keys.
060     * <p>
061     * The keys should be immutable
062     * If they are not then they must not be changed after adding to the MultiKey.
063     *
064     * @param key1  the first key
065     * @param key2  the second key
066     */
067    @SuppressWarnings("unchecked")
068    public MultiKey(final K key1, final K key2) {
069        this((K[]) new Object[] { key1, key2 }, false);
070    }
071
072    /**
073     * Constructor taking three keys.
074     * <p>
075     * The keys should be immutable
076     * If they are not then they must not be changed after adding to the MultiKey.
077     *
078     * @param key1  the first key
079     * @param key2  the second key
080     * @param key3  the third key
081     */
082    @SuppressWarnings("unchecked")
083    public MultiKey(final K key1, final K key2, final K key3) {
084        this((K[]) new Object[] {key1, key2, key3}, false);
085    }
086
087    /**
088     * Constructor taking four keys.
089     * <p>
090     * The keys should be immutable
091     * If they are not then they must not be changed after adding to the MultiKey.
092     *
093     * @param key1  the first key
094     * @param key2  the second key
095     * @param key3  the third key
096     * @param key4  the fourth key
097     */
098    @SuppressWarnings("unchecked")
099    public MultiKey(final K key1, final K key2, final K key3, final K key4) {
100        this((K[]) new Object[] {key1, key2, key3, key4}, false);
101    }
102
103    /**
104     * Constructor taking five keys.
105     * <p>
106     * The keys should be immutable
107     * If they are not then they must not be changed after adding to the MultiKey.
108     *
109     * @param key1  the first key
110     * @param key2  the second key
111     * @param key3  the third key
112     * @param key4  the fourth key
113     * @param key5  the fifth key
114     */
115    @SuppressWarnings("unchecked")
116    public MultiKey(final K key1, final K key2, final K key3, final K key4, final K key5) {
117        this((K[]) new Object[] {key1, key2, key3, key4, key5}, false);
118    }
119
120    /**
121     * Constructor taking an array of keys which is cloned.
122     * <p>
123     * The keys should be immutable
124     * If they are not then they must not be changed after adding to the MultiKey.
125     * <p>
126     * This is equivalent to <code>new MultiKey(keys, true)</code>.
127     *
128     * @param keys  the array of keys, not null
129     * @throws IllegalArgumentException if the key array is null
130     */
131    public MultiKey(final K[] keys) {
132        this(keys, true);
133    }
134
135    /**
136     * Constructor taking an array of keys, optionally choosing whether to clone.
137     * <p>
138     * <b>If the array is not cloned, then it must not be modified.</b>
139     * <p>
140     * This method is public for performance reasons only, to avoid a clone.
141     * The hashcode is calculated once here in this method.
142     * Therefore, changing the array passed in would not change the hashcode but
143     * would change the equals method, which is a bug.
144     * <p>
145     * This is the only fully safe usage of this constructor, as the object array
146     * is never made available in a variable:
147     * <pre>
148     * new MultiKey(new Object[] {...}, false);
149     * </pre>
150     * <p>
151     * The keys should be immutable
152     * If they are not then they must not be changed after adding to the MultiKey.
153     *
154     * @param keys  the array of keys, not null
155     * @param makeClone  true to clone the array, false to assign it
156     * @throws IllegalArgumentException if the key array is null
157     * @since 3.1
158     */
159    public MultiKey(final K[] keys, final boolean makeClone) {
160        super();
161        if (keys == null) {
162            throw new IllegalArgumentException("The array of keys must not be null");
163        }
164        if (makeClone) {
165            this.keys = keys.clone();
166        } else {
167            this.keys = keys;
168        }
169
170        calculateHashCode(keys);
171    }
172
173    //-----------------------------------------------------------------------
174    /**
175     * Gets a clone of the array of keys.
176     * <p>
177     * The keys should be immutable
178     * If they are not then they must not be changed.
179     *
180     * @return the individual keys
181     */
182    public K[] getKeys() {
183        return keys.clone();
184    }
185
186    /**
187     * Gets the key at the specified index.
188     * <p>
189     * The key should be immutable.
190     * If it is not then it must not be changed.
191     *
192     * @param index  the index to retrieve
193     * @return the key at the index
194     * @throws IndexOutOfBoundsException if the index is invalid
195     * @since 3.1
196     */
197    public K getKey(final int index) {
198        return keys[index];
199    }
200
201    /**
202     * Gets the size of the list of keys.
203     *
204     * @return the size of the list of keys
205     * @since 3.1
206     */
207    public int size() {
208        return keys.length;
209    }
210
211    //-----------------------------------------------------------------------
212    /**
213     * Compares this object to another.
214     * <p>
215     * To be equal, the other object must be a <code>MultiKey</code> with the
216     * same number of keys which are also equal.
217     *
218     * @param other  the other object to compare to
219     * @return true if equal
220     */
221    @Override
222    public boolean equals(final Object other) {
223        if (other == this) {
224            return true;
225        }
226        if (other instanceof MultiKey) {
227            final MultiKey<?> otherMulti = (MultiKey<?>) other;
228            return Arrays.equals(keys, otherMulti.keys);
229        }
230        return false;
231    }
232
233    /**
234     * Gets the combined hash code that is computed from all the keys.
235     * <p>
236     * This value is computed once and then cached, so elements should not
237     * change their hash codes once created (note that this is the same
238     * constraint that would be used if the individual keys elements were
239     * themselves {@link java.util.Map Map} keys.
240     *
241     * @return the hash code
242     */
243    @Override
244    public int hashCode() {
245        return hashCode;
246    }
247
248    /**
249     * Gets a debugging string version of the key.
250     *
251     * @return a debugging string
252     */
253    @Override
254    public String toString() {
255        return "MultiKey" + Arrays.toString(keys);
256    }
257
258    /**
259     * Calculate the hash code of the instance using the provided keys.
260     * @param keys the keys to calculate the hash code for
261     */
262    private void calculateHashCode(final Object[] keys)
263    {
264        int total = 0;
265        for (final Object key : keys) {
266            if (key != null) {
267                total ^= key.hashCode();
268            }
269        }
270        hashCode = total;
271    }
272
273    /**
274     * Recalculate the hash code after deserialization. The hash code of some
275     * keys might have change (hash codes based on the system hash code are
276     * only stable for the same process).
277     * @return the instance with recalculated hash code
278     */
279    protected Object readResolve() {
280        calculateHashCode(keys);
281        return this;
282    }
283}