/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
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package org.apache.commons.collections4.keyvalue;

import java.io.Serializable;
import java.util.Arrays;

A MultiKey allows multiple map keys to be merged together.

The purpose of this class is to avoid the need to write code to handle
maps of maps. An example might be the need to look up a file name by
key and locale. The typical solution might be nested maps. This class
can be used instead by creating an instance passing in the key and locale.


Example usage:

// populate map with data mapping key+locale to localizedText
Map map = new HashMap();
MultiKey multiKey = new MultiKey(key, locale);
map.put(multiKey, localizedText);
// later retrieve the localized text
MultiKey multiKey = new MultiKey(key, locale);
String localizedText = (String) map.get(multiKey);

Type parameters:
<K> – the type of keys
Since:
3.0/**
 * A <code>MultiKey</code> allows multiple map keys to be merged together.
 * <p>
 * The purpose of this class is to avoid the need to write code to handle
 * maps of maps. An example might be the need to look up a file name by
 * key and locale. The typical solution might be nested maps. This class
 * can be used instead by creating an instance passing in the key and locale.
 * </p>
 * <p>
 * Example usage:
 * </p>
 * <pre>
 * // populate map with data mapping key+locale to localizedText
 * Map map = new HashMap();
 * MultiKey multiKey = new MultiKey(key, locale);
 * map.put(multiKey, localizedText);
 *
 * // later retrieve the localized text
 * MultiKey multiKey = new MultiKey(key, locale);
 * String localizedText = (String) map.get(multiKey);
 * </pre>
 *
 * @param <K> the type of keys
 * @since 3.0
 */
public class MultiKey<K> implements Serializable {
    // This class could implement List, but that would confuse it's purpose

    
Serialisation version /** Serialisation version */
    private static final long serialVersionUID = 4465448607415788805L;

    
The individual keys /** The individual keys */
    private final K[] keys;
    
The cached hashCode /** The cached hashCode */
    private transient int hashCode;

    
Constructor taking two keys.

The keys should be immutable
If they are not then they must not be changed after adding to the MultiKey.
Params:
key1 –  the first key
key2 –  the second key/**
     * Constructor taking two keys.
     * <p>
     * The keys should be immutable
     * If they are not then they must not be changed after adding to the MultiKey.
     *
     * @param key1  the first key
     * @param key2  the second key
     */
    @SuppressWarnings("unchecked")
    public MultiKey(final K key1, final K key2) {
        this((K[]) new Object[] { key1, key2 }, false);
    }

    
Constructor taking three keys.

The keys should be immutable
If they are not then they must not be changed after adding to the MultiKey.
Params:
key1 –  the first key
key2 –  the second key
key3 –  the third key/**
     * Constructor taking three keys.
     * <p>
     * The keys should be immutable
     * If they are not then they must not be changed after adding to the MultiKey.
     *
     * @param key1  the first key
     * @param key2  the second key
     * @param key3  the third key
     */
    @SuppressWarnings("unchecked")
    public MultiKey(final K key1, final K key2, final K key3) {
        this((K[]) new Object[] {key1, key2, key3}, false);
    }

    
Constructor taking four keys.

The keys should be immutable
If they are not then they must not be changed after adding to the MultiKey.
Params:
key1 –  the first key
key2 –  the second key
key3 –  the third key
key4 –  the fourth key/**
     * Constructor taking four keys.
     * <p>
     * The keys should be immutable
     * If they are not then they must not be changed after adding to the MultiKey.
     *
     * @param key1  the first key
     * @param key2  the second key
     * @param key3  the third key
     * @param key4  the fourth key
     */
    @SuppressWarnings("unchecked")
    public MultiKey(final K key1, final K key2, final K key3, final K key4) {
        this((K[]) new Object[] {key1, key2, key3, key4}, false);
    }

    
Constructor taking five keys.

The keys should be immutable
If they are not then they must not be changed after adding to the MultiKey.
Params:
key1 –  the first key
key2 –  the second key
key3 –  the third key
key4 –  the fourth key
key5 –  the fifth key/**
     * Constructor taking five keys.
     * <p>
     * The keys should be immutable
     * If they are not then they must not be changed after adding to the MultiKey.
     *
     * @param key1  the first key
     * @param key2  the second key
     * @param key3  the third key
     * @param key4  the fourth key
     * @param key5  the fifth key
     */
    @SuppressWarnings("unchecked")
    public MultiKey(final K key1, final K key2, final K key3, final K key4, final K key5) {
        this((K[]) new Object[] {key1, key2, key3, key4, key5}, false);
    }

    
Constructor taking an array of keys which is cloned.

The keys should be immutable
If they are not then they must not be changed after adding to the MultiKey.

This is equivalent to new MultiKey(keys, true).
Params:
keys –  the array of keys, not null
Throws:
IllegalArgumentException – if the key array is null/**
     * Constructor taking an array of keys which is cloned.
     * <p>
     * The keys should be immutable
     * If they are not then they must not be changed after adding to the MultiKey.
     * <p>
     * This is equivalent to <code>new MultiKey(keys, true)</code>.
     *
     * @param keys  the array of keys, not null
     * @throws IllegalArgumentException if the key array is null
     */
    public MultiKey(final K[] keys) {
        this(keys, true);
    }

    
Constructor taking an array of keys, optionally choosing whether to clone.

If the array is not cloned, then it must not be modified.

This method is public for performance reasons only, to avoid a clone.
The hashcode is calculated once here in this method.
Therefore, changing the array passed in would not change the hashcode but
would change the equals method, which is a bug.

This is the only fully safe usage of this constructor, as the object array
is never made available in a variable:
new MultiKey(new Object[] {...}, false);


The keys should be immutable
If they are not then they must not be changed after adding to the MultiKey.
Params:
keys –  the array of keys, not null
makeClone –  true to clone the array, false to assign it
Throws:
IllegalArgumentException – if the key array is null
Since:
3.1/**
     * Constructor taking an array of keys, optionally choosing whether to clone.
     * <p>
     * <b>If the array is not cloned, then it must not be modified.</b>
     * <p>
     * This method is public for performance reasons only, to avoid a clone.
     * The hashcode is calculated once here in this method.
     * Therefore, changing the array passed in would not change the hashcode but
     * would change the equals method, which is a bug.
     * <p>
     * This is the only fully safe usage of this constructor, as the object array
     * is never made available in a variable:
     * <pre>
     * new MultiKey(new Object[] {...}, false);
     * </pre>
     * <p>
     * The keys should be immutable
     * If they are not then they must not be changed after adding to the MultiKey.
     *
     * @param keys  the array of keys, not null
     * @param makeClone  true to clone the array, false to assign it
     * @throws IllegalArgumentException if the key array is null
     * @since 3.1
     */
    public MultiKey(final K[] keys, final boolean makeClone) {
        super();
        if (keys == null) {
            throw new IllegalArgumentException("The array of keys must not be null");
        }
        if (makeClone) {
            this.keys = keys.clone();
        } else {
            this.keys = keys;
        }

        calculateHashCode(keys);
    }

    //-----------------------------------------------------------------------
    
Gets a clone of the array of keys.

The keys should be immutable
If they are not then they must not be changed.
Returns:
the individual keys/**
     * Gets a clone of the array of keys.
     * <p>
     * The keys should be immutable
     * If they are not then they must not be changed.
     *
     * @return the individual keys
     */
    public K[] getKeys() {
        return keys.clone();
    }

    
Gets the key at the specified index.

The key should be immutable.
If it is not then it must not be changed.
Params:
index –  the index to retrieve
Throws:
IndexOutOfBoundsException – if the index is invalid
Returns:
the key at the index
Since:
3.1/**
     * Gets the key at the specified index.
     * <p>
     * The key should be immutable.
     * If it is not then it must not be changed.
     *
     * @param index  the index to retrieve
     * @return the key at the index
     * @throws IndexOutOfBoundsException if the index is invalid
     * @since 3.1
     */
    public K getKey(final int index) {
        return keys[index];
    }

    
Gets the size of the list of keys.
Returns:
the size of the list of keys
Since:
3.1/**
     * Gets the size of the list of keys.
     *
     * @return the size of the list of keys
     * @since 3.1
     */
    public int size() {
        return keys.length;
    }

    //-----------------------------------------------------------------------
    
Compares this object to another.

To be equal, the other object must be a MultiKey with the
same number of keys which are also equal.
Params:
other –  the other object to compare to
Returns:
true if equal/**
     * Compares this object to another.
     * <p>
     * To be equal, the other object must be a <code>MultiKey</code> with the
     * same number of keys which are also equal.
     *
     * @param other  the other object to compare to
     * @return true if equal
     */
    @Override
    public boolean equals(final Object other) {
        if (other == this) {
            return true;
        }
        if (other instanceof MultiKey) {
            final MultiKey<?> otherMulti = (MultiKey<?>) other;
            return Arrays.equals(keys, otherMulti.keys);
        }
        return false;
    }

    
Gets the combined hash code that is computed from all the keys.
 This value is computed once and then cached, so elements should not change their hash codes once created (note that this is the same constraint that would be used if the individual keys elements were themselves Map keys. 
Returns:
the hash code/**
     * Gets the combined hash code that is computed from all the keys.
     * <p>
     * This value is computed once and then cached, so elements should not
     * change their hash codes once created (note that this is the same
     * constraint that would be used if the individual keys elements were
     * themselves {@link java.util.Map Map} keys.
     *
     * @return the hash code
     */
    @Override
    public int hashCode() {
        return hashCode;
    }

    
Gets a debugging string version of the key.
Returns:
a debugging string/**
     * Gets a debugging string version of the key.
     *
     * @return a debugging string
     */
    @Override
    public String toString() {
        return "MultiKey" + Arrays.toString(keys);
    }

    
Calculate the hash code of the instance using the provided keys.
Params:
keys – the keys to calculate the hash code for/**
     * Calculate the hash code of the instance using the provided keys.
     * @param keys the keys to calculate the hash code for
     */
    private void calculateHashCode(final Object[] keys)
    {
        int total = 0;
        for (final Object key : keys) {
            if (key != null) {
                total ^= key.hashCode();
            }
        }
        hashCode = total;
    }

    
Recalculate the hash code after deserialization. The hash code of some
keys might have change (hash codes based on the system hash code are
only stable for the same process).
Returns:
the instance with recalculated hash code/**
     * Recalculate the hash code after deserialization. The hash code of some
     * keys might have change (hash codes based on the system hash code are
     * only stable for the same process).
     * @return the instance with recalculated hash code
     */
    protected Object readResolve() {
        calculateHashCode(keys);
        return this;
    }
}