    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.lang.reflect.Array;
    021import java.util.Arrays;
    022import java.util.Objects;
    023
    024/**
    025 * A {@code MultiKey} allows multiple map keys to be merged together.
    026 * <p>
    027 * The purpose of this class is to avoid the need to write code to handle
    028 * maps of maps. An example might be the need to look up a file name by
    029 * key and locale. The typical solution might be nested maps. This class
    030 * can be used instead by creating an instance passing in the key and locale.
    031 * </p>
    032 * <p>
    033 * Example usage:
    034 * </p>
    035 * <pre>
    036 * // populate map with data mapping key+locale to localizedText
    037 * Map map = new HashMap();
    038 * MultiKey multiKey = new MultiKey(key, locale);
    039 * map.put(multiKey, localizedText);
    040 *
    041 * // later retrieve the localized text
    042 * MultiKey multiKey = new MultiKey(key, locale);
    043 * String localizedText = (String) map.get(multiKey);
    044 * </pre>
    045 *
    046 * @param <K> the type of keys
    047 * @since 3.0
    048 */
    049public class MultiKey<K> implements Serializable {
    050    // This class could implement List, but that would confuse its purpose
    051
    052    /** Serialization version */
    053    private static final long serialVersionUID = 4465448607415788805L;
    054
    055    @SuppressWarnings("unchecked")
    056    private static <T> Class<? extends T> getClass(final T value) {
    057        return (Class<? extends T>) (value == null ? Object.class : value.getClass());
    058    }
    059
    060    @SafeVarargs
    061    private static <T> Class<? extends T> getComponentType(final T... values) {
    062        @SuppressWarnings("unchecked")
    063        final Class<? extends T> rootClass = (Class<? extends T>) Object.class;
    064        if (values == null) {
    065            return rootClass;
    066        }
    067        Class<? extends T> prevClass = values.length > 0 ? getClass(values[0]) : rootClass;
    068        for (int i = 1; i < values.length; i++) {
    069            final Class<? extends T> classI = getClass(values[i]);
    070            if (prevClass != classI) {
    071                return rootClass;
    072            }
    073            prevClass = classI;
    074        }
    075        return prevClass;
    076    }
    077
    078    private static <T> T[] newArray(final T key1, final T key2) {
    079        @SuppressWarnings("unchecked")
    080        final T[] array = (T[]) Array.newInstance(getComponentType(key1, key2), 2);
    081        array[0] = key1;
    082        array[1] = key2;
    083        return array;
    084    }
    085
    086    private static <T> T[] newArray(final T key1, final T key2, final T key3) {
    087        @SuppressWarnings("unchecked")
    088        final T[] array = (T[]) Array.newInstance(getComponentType(key1, key2, key3), 3);
    089        array[0] = key1;
    090        array[1] = key2;
    091        array[2] = key3;
    092        return array;
    093    }
    094
    095    private static <T> T[] newArray(final T key1, final T key2, final T key3, final T key4) {
    096        @SuppressWarnings("unchecked")
    097        final T[] array = (T[]) Array.newInstance(getComponentType(key1, key2, key3, key4), 4);
    098        array[0] = key1;
    099        array[1] = key2;
    100        array[2] = key3;
    101        array[3] = key4;
    102        return array;
    103    }
    104
    105    private static <T> T[] newArray(final T key1, final T key2, final T key3, final T key4, final T key5) {
    106        @SuppressWarnings("unchecked")
    107        final T[] array = (T[]) Array.newInstance(getComponentType(key1, key2, key3, key4, key5), 5);
    108        array[0] = key1;
    109        array[1] = key2;
    110        array[2] = key3;
    111        array[3] = key4;
    112        array[4] = key5;
    113        return array;
    114    }
    115
    116    /** The individual keys */
    117    private final K[] keys;
    118
    119    /** The cached hashCode */
    120    private transient int hashCode;
    121
    122    /**
    123     * Constructor taking two keys.
    124     * <p>
    125     * The keys should be immutable.
    126     * If they are not then they must not be changed after adding to the MultiKey.
    127     * </p>
    128     *
    129     * @param key1  the first key
    130     * @param key2  the second key
    131     */
    132    public MultiKey(final K key1, final K key2) {
    133        this(newArray(key1, key2), false);
    134    }
    135
    136    /**
    137     * Constructor taking three keys.
    138     * <p>
    139     * The keys should be immutable
    140     * If they are not then they must not be changed after adding to the MultiKey.
    141     * </p>
    142     *
    143     * @param key1  the first key
    144     * @param key2  the second key
    145     * @param key3  the third key
    146     */
    147    public MultiKey(final K key1, final K key2, final K key3) {
    148        this(newArray(key1, key2, key3), false);
    149    }
    150
    151    /**
    152     * Constructor taking four keys.
    153     * <p>
    154     * The keys should be immutable.
    155     * If they are not then they must not be changed after adding to the MultiKey.
    156     * </p>
    157     *
    158     * @param key1  the first key
    159     * @param key2  the second key
    160     * @param key3  the third key
    161     * @param key4  the fourth key
    162     */
    163    public MultiKey(final K key1, final K key2, final K key3, final K key4) {
    164        this(newArray(key1, key2, key3, key4), false);
    165    }
    166
    167    /**
    168     * Constructor taking five keys.
    169     * <p>
    170     * The keys should be immutable.
    171     * If they are not then they must not be changed after adding to the MultiKey.
    172     * </p>
    173     *
    174     * @param key1  the first key
    175     * @param key2  the second key
    176     * @param key3  the third key
    177     * @param key4  the fourth key
    178     * @param key5  the fifth key
    179     */
    180    public MultiKey(final K key1, final K key2, final K key3, final K key4, final K key5) {
    181        this(newArray(key1, key2, key3, key4, key5), false);
    182    }
    183
    184    /**
    185     * Constructor taking an array of keys which is cloned.
    186     * <p>
    187     * The keys should be immutable.
    188     * If they are not then they must not be changed after adding to the MultiKey.
    189     * </p>
    190     * <p>
    191     * This is equivalent to {@code new MultiKey(keys, true)}.
    192     * </p>
    193     *
    194     * @param keys  the array of keys, not null
    195     * @throws NullPointerException if the key array is null
    196     */
    197    public MultiKey(final K[] keys) {
    198        this(keys, true);
    199    }
    200
    201    /**
    202     * Constructor taking an array of keys, optionally choosing whether to clone.
    203     * <p>
    204     * <strong>If the array is not cloned, then it must not be modified.</strong>
    205     * </p>
    206     * <p>
    207     * This method is public for performance reasons only, to avoid a clone.
    208     * The hash code is calculated once here in this method.
    209     * Therefore, changing the array passed in would not change the hash code but
    210     * would change the equals method, which is a bug.
    211     * </p>
    212     * <p>
    213     * This is the only fully safe usage of this constructor, as the object array
    214     * is never made available in a variable:
    215     * <pre>
    216     * new MultiKey(new Object[] {...}, false);
    217     * </pre>
    218     * <p>
    219     * The keys should be immutable.
    220     * If they are not then they must not be changed after adding to the MultiKey.
    221     * </p>
    222     *
    223     * @param keys  the array of keys, not null
    224     * @param makeClone  true to clone the array, false to assign it
    225     * @throws NullPointerException if the key array is null
    226     * @since 3.1
    227     */
    228    public MultiKey(final K[] keys, final boolean makeClone) {
    229        Objects.requireNonNull(keys, "keys");
    230        this.keys = makeClone ? keys.clone() : keys;
    231        calculateHashCode(keys);
    232    }
    233
    234    /**
    235     * Calculate the hash code of the instance using the provided keys.
    236     * @param keys the keys to calculate the hash code for
    237     */
    238    private void calculateHashCode(final Object[] keys) {
    239        int total = 0;
    240        for (final Object key : keys) {
    241            if (key != null) {
    242                total ^= key.hashCode();
    243            }
    244        }
    245        hashCode = total;
    246    }
    247
    248    /**
    249     * Compares this object to another.
    250     * <p>
    251     * To be equal, the other object must be a {@code MultiKey} with the
    252     * same number of keys which are also equal.
    253     * </p>
    254     *
    255     * @param other  the other object to compare to
    256     * @return true if equal
    257     */
    258    @Override
    259    public boolean equals(final Object other) {
    260        if (other == this) {
    261            return true;
    262        }
    263        if (other instanceof MultiKey) {
    264            final MultiKey<?> otherMulti = (MultiKey<?>) other;
    265            return Arrays.equals(keys, otherMulti.keys);
    266        }
    267        return false;
    268    }
    269
    270    /**
    271     * Gets the key at the specified index.
    272     * <p>
    273     * The key should be immutable.
    274     * If it is not then it must not be changed.
    275     * </p>
    276     *
    277     * @param index  the index to retrieve
    278     * @return the key at the index
    279     * @throws IndexOutOfBoundsException if the index is invalid
    280     * @since 3.1
    281     */
    282    public K getKey(final int index) {
    283        return keys[index];
    284    }
    285
    286    /**
    287     * Gets a clone of the array of keys.
    288     * <p>
    289     * The keys should be immutable
    290     * If they are not then they must not be changed.
    291     * </p>
    292     *
    293     * @return the individual keys
    294     */
    295    public K[] getKeys() {
    296        return keys.clone();
    297    }
    298
    299    /**
    300     * Gets the combined hash code that is computed from all the keys.
    301     * <p>
    302     * This value is computed once and then cached, so elements should not
    303     * change their hash codes once created (note that this is the same
    304     * constraint that would be used if the individual keys elements were
    305     * themselves {@link java.util.Map Map} keys).
    306     * </p>
    307     *
    308     * @return the hash code
    309     */
    310    @Override
    311    public int hashCode() {
    312        return hashCode;
    313    }
    314
    315    /**
    316     * Recalculate the hash code after deserialization. The hash code of some
    317     * keys might have change (hash codes based on the system hash code are
    318     * only stable for the same process).
    319     * @return the instance with recalculated hash code
    320     */
    321    protected Object readResolve() {
    322        calculateHashCode(keys);
    323        return this;
    324    }
    325
    326    /**
    327     * Gets the size of the list of keys.
    328     *
    329     * @return the size of the list of keys
    330     * @since 3.1
    331     */
    332    public int size() {
    333        return keys.length;
    334    }
    335
    336    /**
    337     * Gets a debugging string version of the key.
    338     *
    339     * @return a debugging string
    340     */
    341    @Override
    342    public String toString() {
    343        return "MultiKey" + Arrays.toString(keys);
    344    }
    345}