CollectionUtils.java
package org.codehaus.plexus.util;
/*
* Copyright The Codehaus Foundation.
*
* Licensed 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.
*/
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
/**
* @author <a href="mailto:olamy@codehaus.org">olamy</a>
*
*/
public class CollectionUtils {
// ----------------------------------------------------------------------
// Static methods that can probably be moved to a real util class.
// ----------------------------------------------------------------------
/**
* Take a dominant and recessive Map and merge the key:value pairs where the recessive Map may add key:value pairs
* to the dominant Map but may not override any existing key:value pairs. If we have two Maps, a dominant and
* recessive, and their respective keys are as follows: dominantMapKeys = { a, b, c, d, e, f } recessiveMapKeys = {
* a, b, c, x, y, z } Then the result should be the following: resultantKeys = { a, b, c, d, e, f, x, y, z }
*
* @param dominantMap Dominant Map.
* @param recessiveMap Recessive Map.
* @param <K> type
* @param <V> type
* @return The result map with combined dominant and recessive values.
*/
public static <K, V> Map<K, V> mergeMaps(Map<K, V> dominantMap, Map<K, V> recessiveMap) {
if (dominantMap == null && recessiveMap == null) {
return null;
}
if (dominantMap != null && recessiveMap == null) {
return dominantMap;
}
if (dominantMap == null) {
return recessiveMap;
}
Map<K, V> result = new HashMap<>();
// Grab the keys from the dominant and recessive maps.
Set<K> dominantMapKeys = dominantMap.keySet();
Set<K> recessiveMapKeys = recessiveMap.keySet();
// Create the set of keys that will be contributed by the
// recessive Map by subtracting the intersection of keys
// from the recessive Map's keys.
Collection<K> contributingRecessiveKeys = CollectionUtils.subtract(
recessiveMapKeys, CollectionUtils.intersection(dominantMapKeys, recessiveMapKeys));
result.putAll(dominantMap);
// Now take the keys we just found and extract the values from
// the recessiveMap and put the key:value pairs into the dominantMap.
for (K key : contributingRecessiveKeys) {
result.put(key, recessiveMap.get(key));
}
return result;
}
/**
* Take a series of <code>Map</code>s and merge them where the ordering of the array from 0..n is the dominant
* order.
*
* @param maps An array of Maps to merge.
* @param <K> type
* @param <V> type
* @return Map The result Map produced after the merging process.
*/
public static <K, V> Map<K, V> mergeMaps(Map<K, V>[] maps) {
Map<K, V> result;
if (maps.length == 0) {
result = null;
} else if (maps.length == 1) {
result = maps[0];
} else {
result = mergeMaps(maps[0], maps[1]);
for (int i = 2; i < maps.length; i++) {
result = mergeMaps(result, maps[i]);
}
}
return result;
}
/**
* <p>
* Returns a {@link Collection} containing the intersection of the given {@link Collection}s.
* </p>
* The cardinality of each element in the returned {@link Collection} will be equal to the minimum of the
* cardinality of that element in the two given {@link Collection}s.
*
* @param a The first collection
* @param b The second collection
* @param <E> the type
* @see Collection#retainAll
* @return The intersection of a and b, never null
*/
public static <E> Collection<E> intersection(final Collection<E> a, final Collection<E> b) {
ArrayList<E> list = new ArrayList<>();
Map<E, Integer> mapa = getCardinalityMap(a);
Map<E, Integer> mapb = getCardinalityMap(b);
Set<E> elts = new HashSet<>(a);
elts.addAll(b);
for (E obj : elts) {
for (int i = 0, m = Math.min(getFreq(obj, mapa), getFreq(obj, mapb)); i < m; i++) {
list.add(obj);
}
}
return list;
}
/**
* Returns a {@link Collection} containing <code>a - b</code>. The cardinality of each element <i>e</i> in
* the returned {@link Collection} will be the cardinality of <i>e</i> in <i>a</i> minus the cardinality of <i>e</i>
* in <i>b</i>, or zero, whichever is greater.
*
* @param a The start collection
* @param b The collection that will be subtracted
* @param <T> the type
* @see Collection#removeAll
* @return The result of the subtraction
*/
public static <T> Collection<T> subtract(final Collection<T> a, final Collection<T> b) {
ArrayList<T> list = new ArrayList<>(a);
for (T aB : b) {
list.remove(aB);
}
return list;
}
/**
* Returns a {@link Map} mapping each unique element in the given {@link Collection} to an {@link Integer}
* representing the number of occurrences of that element in the {@link Collection}. An entry that maps to
* <code>null</code> indicates that the element does not appear in the given {@link Collection}.
*
* @param col The collection to count cardinalities for
* @param <E> the type
* @return A map of counts, indexed on each element in the collection
*/
public static <E> Map<E, Integer> getCardinalityMap(final Collection<E> col) {
HashMap<E, Integer> count = new HashMap<>();
for (E obj : col) {
Integer c = count.get(obj);
if (null == c) {
count.put(obj, 1);
} else {
count.put(obj, c + 1);
}
}
return count;
}
public static <E> List<E> iteratorToList(Iterator<E> it) {
if (it == null) {
throw new NullPointerException("it cannot be null.");
}
List<E> list = new ArrayList<E>();
while (it.hasNext()) {
list.add(it.next());
}
return list;
}
// ----------------------------------------------------------------------
//
// ----------------------------------------------------------------------
private static <E> int getFreq(final E obj, final Map<E, Integer> freqMap) {
try {
Integer o = freqMap.get(obj);
if (o != null) // minimize NullPointerExceptions
{
return o;
}
} catch (NullPointerException | NoSuchElementException ignore) {
}
return 0;
}
}