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