Java tutorial
/* * 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. */ package org.apache.commons.collections4.keyvalue; import java.io.Serializable; import java.util.Arrays; /** * 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> * Example usage: * <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> * * @since 3.0 * @version $Id: MultiKey.java 1477753 2013-04-30 18:24:24Z tn $ */ public class MultiKey<K> implements Serializable { // This class could implement List, but that would confuse it's purpose /** Serialisation version */ private static final long serialVersionUID = 4465448607415788805L; /** The individual keys */ private final K[] keys; /** The cached hashCode */ private transient int hashCode; /** * 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. * <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. * <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. * <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. * <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. * <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. * <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. * <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. * * @return the size of the list of keys * @since 3.1 */ public int size() { return keys.length; } //----------------------------------------------------------------------- /** * 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. * <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. * * @return a debugging string */ @Override public String toString() { return "MultiKey" + Arrays.toString(keys); } /** * 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). * @return the instance with recalculated hash code */ private Object readResolve() { calculateHashCode(keys); return this; } }