MultiKey.java
/*
* 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.lang.reflect.Array;
import java.util.Arrays;
import java.util.Objects;
/**
* A {@code MultiKey} 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 its purpose
/** Serialisation version */
private static final long serialVersionUID = 4465448607415788805L;
@SuppressWarnings("unchecked")
private static <T> Class<? extends T> getClass(final T value) {
return (Class<? extends T>) (value == null ? Object.class : value.getClass());
}
private static <T> Class<? extends T> getComponentType(final T... values) {
@SuppressWarnings("unchecked")
final Class<? extends T> rootClass = (Class<? extends T>) Object.class;
if (values == null) {
return rootClass;
}
Class<? extends T> prevClass = values.length > 0 ? getClass(values[0]) : rootClass;
for (int i = 1; i < values.length; i++) {
final Class<? extends T> classI = getClass(values[i]);
if (prevClass != classI) {
return rootClass;
}
prevClass = classI;
}
return prevClass;
}
private static <T> T[] newArray(final T key1, final T key2) {
@SuppressWarnings("unchecked")
final T[] array = (T[]) Array.newInstance(getComponentType(key1, key2), 2);
array[0] = key1;
array[1] = key2;
return array;
}
private static <T> T[] newArray(final T key1, final T key2, final T key3) {
@SuppressWarnings("unchecked")
final T[] array = (T[]) Array.newInstance(getComponentType(key1, key2, key3), 3);
array[0] = key1;
array[1] = key2;
array[2] = key3;
return array;
}
private static <T> T[] newArray(final T key1, final T key2, final T key3, final T key4) {
@SuppressWarnings("unchecked")
final T[] array = (T[]) Array.newInstance(getComponentType(key1, key2, key3, key4), 4);
array[0] = key1;
array[1] = key2;
array[2] = key3;
array[3] = key4;
return array;
}
private static <T> T[] newArray(final T key1, final T key2, final T key3, final T key4, final T key5) {
@SuppressWarnings("unchecked")
final T[] array = (T[]) Array.newInstance(getComponentType(key1, key2, key3, key4, key5), 5);
array[0] = key1;
array[1] = key2;
array[2] = key3;
array[3] = key4;
array[4] = key5;
return array;
}
/** 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
*/
public MultiKey(final K key1, final K key2) {
this(newArray(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
*/
public MultiKey(final K key1, final K key2, final K key3) {
this(newArray(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
*/
public MultiKey(final K key1, final K key2, final K key3, final K key4) {
this(newArray(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
*/
public MultiKey(final K key1, final K key2, final K key3, final K key4, final K key5) {
this(newArray(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)}.
*
* @param keys the array of keys, not null
* @throws NullPointerException 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 hash code is calculated once here in this method.
* Therefore, changing the array passed in would not change the hash code 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 NullPointerException if the key array is null
* @since 3.1
*/
public MultiKey(final K[] keys, final boolean makeClone) {
Objects.requireNonNull(keys, "keys");
this.keys = makeClone ? keys.clone() : keys;
calculateHashCode(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;
}
/**
* Compares this object to another.
* <p>
* To be equal, the other object must be a {@code MultiKey} 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 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 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 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;
}
/**
* 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;
}
/**
* 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;
}
/**
* Gets a debugging string version of the key.
*
* @return a debugging string
*/
@Override
public String toString() {
return "MultiKey" + Arrays.toString(keys);
}
}