com.google.common.collect.Collections2 Maven / Gradle / Ivy
/*
* Copyright (C) 2008 The Guava Authors
*
* 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.
*/
package com.google.common.collect;
import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.CollectPreconditions.checkNonnegative;
import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Function;
import com.google.common.base.Predicate;
import com.google.common.base.Predicates;
import com.google.common.math.IntMath;
import com.google.common.primitives.Ints;
import java.util.AbstractCollection;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import org.checkerframework.checker.nullness.compatqual.NullableDecl;
/**
* Provides static methods for working with {@code Collection} instances.
*
* Java 8 users: several common uses for this class are now more comprehensively addressed
* by the new {@link java.util.stream.Stream} library. Read the method documentation below for
* comparisons. These methods are not being deprecated, but we gently encourage you to migrate to
* streams.
*
* @author Chris Povirk
* @author Mike Bostock
* @author Jared Levy
* @since 2.0
*/
@GwtCompatible
public final class Collections2 {
private Collections2() {}
/**
* Returns the elements of {@code unfiltered} that satisfy a predicate. The returned collection is
* a live view of {@code unfiltered}; changes to one affect the other.
*
*
The resulting collection's iterator does not support {@code remove()}, but all other
* collection methods are supported. When given an element that doesn't satisfy the predicate, the
* collection's {@code add()} and {@code addAll()} methods throw an {@link
* IllegalArgumentException}. When methods such as {@code removeAll()} and {@code clear()} are
* called on the filtered collection, only elements that satisfy the filter will be removed from
* the underlying collection.
*
*
The returned collection isn't threadsafe or serializable, even if {@code unfiltered} is.
*
*
Many of the filtered collection's methods, such as {@code size()}, iterate across every
* element in the underlying collection and determine which elements satisfy the filter. When a
* live view is not needed, it may be faster to copy {@code Iterables.filter(unfiltered,
* predicate)} and use the copy.
*
*
Warning: {@code predicate} must be consistent with equals, as documented at
* {@link Predicate#apply}. Do not provide a predicate such as {@code
* Predicates.instanceOf(ArrayList.class)}, which is inconsistent with equals. (See {@link
* Iterables#filter(Iterable, Class)} for related functionality.)
*
*
{@code Stream} equivalent: {@link java.util.stream.Stream#filter Stream.filter}.
*/
// TODO(kevinb): how can we omit that Iterables link when building gwt
// javadoc?
public static Collection filter(Collection unfiltered, Predicate predicate) {
if (unfiltered instanceof FilteredCollection) {
// Support clear(), removeAll(), and retainAll() when filtering a filtered
// collection.
return ((FilteredCollection) unfiltered).createCombined(predicate);
}
return new FilteredCollection(checkNotNull(unfiltered), checkNotNull(predicate));
}
/**
* Delegates to {@link Collection#contains}. Returns {@code false} if the {@code contains} method
* throws a {@code ClassCastException} or {@code NullPointerException}.
*/
static boolean safeContains(Collection collection, @NullableDecl Object object) {
checkNotNull(collection);
try {
return collection.contains(object);
} catch (ClassCastException | NullPointerException e) {
return false;
}
}
/**
* Delegates to {@link Collection#remove}. Returns {@code false} if the {@code remove} method
* throws a {@code ClassCastException} or {@code NullPointerException}.
*/
static boolean safeRemove(Collection collection, @NullableDecl Object object) {
checkNotNull(collection);
try {
return collection.remove(object);
} catch (ClassCastException | NullPointerException e) {
return false;
}
}
static class FilteredCollection extends AbstractCollection {
final Collection unfiltered;
final Predicate predicate;
FilteredCollection(Collection unfiltered, Predicate predicate) {
this.unfiltered = unfiltered;
this.predicate = predicate;
}
FilteredCollection createCombined(Predicate newPredicate) {
return new FilteredCollection(unfiltered, Predicates.and(predicate, newPredicate));
// . above needed to compile in JDK 5
}
@Override
public boolean add(E element) {
checkArgument(predicate.apply(element));
return unfiltered.add(element);
}
@Override
public boolean addAll(Collection collection) {
for (E element : collection) {
checkArgument(predicate.apply(element));
}
return unfiltered.addAll(collection);
}
@Override
public void clear() {
Iterables.removeIf(unfiltered, predicate);
}
@Override
public boolean contains(@NullableDecl Object element) {
if (safeContains(unfiltered, element)) {
@SuppressWarnings("unchecked") // element is in unfiltered, so it must be an E
E e = (E) element;
return predicate.apply(e);
}
return false;
}
@Override
public boolean containsAll(Collection collection) {
return containsAllImpl(this, collection);
}
@Override
public boolean isEmpty() {
return !Iterables.any(unfiltered, predicate);
}
@Override
public Iterator iterator() {
return Iterators.filter(unfiltered.iterator(), predicate);
}
@Override
public boolean remove(Object element) {
return contains(element) && unfiltered.remove(element);
}
@Override
public boolean removeAll(final Collection collection) {
boolean changed = false;
Iterator itr = unfiltered.iterator();
while (itr.hasNext()) {
E e = itr.next();
if (predicate.apply(e) && collection.contains(e)) {
itr.remove();
changed = true;
}
}
return changed;
}
@Override
public boolean retainAll(final Collection collection) {
boolean changed = false;
Iterator itr = unfiltered.iterator();
while (itr.hasNext()) {
E e = itr.next();
if (predicate.apply(e) && !collection.contains(e)) {
itr.remove();
changed = true;
}
}
return changed;
}
@Override
public int size() {
int size = 0;
for (E e : unfiltered) {
if (predicate.apply(e)) {
size++;
}
}
return size;
}
@Override
public Object[] toArray() {
// creating an ArrayList so filtering happens once
return Lists.newArrayList(iterator()).toArray();
}
@Override
public T[] toArray(T[] array) {
return Lists.newArrayList(iterator()).toArray(array);
}
}
/**
* Returns a collection that applies {@code function} to each element of {@code fromCollection}.
* The returned collection is a live view of {@code fromCollection}; changes to one affect the
* other.
*
* The returned collection's {@code add()} and {@code addAll()} methods throw an {@link
* UnsupportedOperationException}. All other collection methods are supported, as long as {@code
* fromCollection} supports them.
*
*
The returned collection isn't threadsafe or serializable, even if {@code fromCollection} is.
*
*
When a live view is not needed, it may be faster to copy the transformed collection
* and use the copy.
*
*
If the input {@code Collection} is known to be a {@code List}, consider {@link
* Lists#transform}. If only an {@code Iterable} is available, use {@link Iterables#transform}.
*
*
{@code Stream} equivalent: {@link java.util.stream.Stream#map Stream.map}.
*/
public static Collection transform(
Collection fromCollection, Function function) {
return new TransformedCollection<>(fromCollection, function);
}
static class TransformedCollection extends AbstractCollection {
final Collection fromCollection;
final Function function;
TransformedCollection(Collection fromCollection, Function function) {
this.fromCollection = checkNotNull(fromCollection);
this.function = checkNotNull(function);
}
@Override
public void clear() {
fromCollection.clear();
}
@Override
public boolean isEmpty() {
return fromCollection.isEmpty();
}
@Override
public Iterator iterator() {
return Iterators.transform(fromCollection.iterator(), function);
}
@Override
public int size() {
return fromCollection.size();
}
}
/**
* Returns {@code true} if the collection {@code self} contains all of the elements in the
* collection {@code c}.
*
* This method iterates over the specified collection {@code c}, checking each element returned
* by the iterator in turn to see if it is contained in the specified collection {@code self}. If
* all elements are so contained, {@code true} is returned, otherwise {@code false}.
*
* @param self a collection which might contain all elements in {@code c}
* @param c a collection whose elements might be contained by {@code self}
*/
static boolean containsAllImpl(Collection self, Collection c) {
for (Object o : c) {
if (!self.contains(o)) {
return false;
}
}
return true;
}
/** An implementation of {@link Collection#toString()}. */
static String toStringImpl(final Collection collection) {
StringBuilder sb = newStringBuilderForCollection(collection.size()).append('[');
boolean first = true;
for (Object o : collection) {
if (!first) {
sb.append(", ");
}
first = false;
if (o == collection) {
sb.append("(this Collection)");
} else {
sb.append(o);
}
}
return sb.append(']').toString();
}
/** Returns best-effort-sized StringBuilder based on the given collection size. */
static StringBuilder newStringBuilderForCollection(int size) {
checkNonnegative(size, "size");
return new StringBuilder((int) Math.min(size * 8L, Ints.MAX_POWER_OF_TWO));
}
/** Used to avoid http://bugs.sun.com/view_bug.do?bug_id=6558557 */
static Collection cast(Iterable iterable) {
return (Collection) iterable;
}
/**
* Returns a {@link Collection} of all the permutations of the specified {@link Iterable}.
*
* Notes: This is an implementation of the algorithm for Lexicographical Permutations
* Generation, described in Knuth's "The Art of Computer Programming", Volume 4, Chapter 7,
* Section 7.2.1.2. The iteration order follows the lexicographical order. This means that the
* first permutation will be in ascending order, and the last will be in descending order.
*
*
Duplicate elements are considered equal. For example, the list [1, 1] will have only one
* permutation, instead of two. This is why the elements have to implement {@link Comparable}.
*
*
An empty iterable has only one permutation, which is an empty list.
*
*
This method is equivalent to {@code Collections2.orderedPermutations(list,
* Ordering.natural())}.
*
* @param elements the original iterable whose elements have to be permuted.
* @return an immutable {@link Collection} containing all the different permutations of the
* original iterable.
* @throws NullPointerException if the specified iterable is null or has any null elements.
* @since 12.0
*/
@Beta
public static > Collection> orderedPermutations(
Iterable elements) {
return orderedPermutations(elements, Ordering.natural());
}
/**
* Returns a {@link Collection} of all the permutations of the specified {@link Iterable} using
* the specified {@link Comparator} for establishing the lexicographical ordering.
*
* Examples:
*
*
{@code
* for (List perm : orderedPermutations(asList("b", "c", "a"))) {
* println(perm);
* }
* // -> ["a", "b", "c"]
* // -> ["a", "c", "b"]
* // -> ["b", "a", "c"]
* // -> ["b", "c", "a"]
* // -> ["c", "a", "b"]
* // -> ["c", "b", "a"]
*
* for (List perm : orderedPermutations(asList(1, 2, 2, 1))) {
* println(perm);
* }
* // -> [1, 1, 2, 2]
* // -> [1, 2, 1, 2]
* // -> [1, 2, 2, 1]
* // -> [2, 1, 1, 2]
* // -> [2, 1, 2, 1]
* // -> [2, 2, 1, 1]
* }
*
* Notes: This is an implementation of the algorithm for Lexicographical Permutations
* Generation, described in Knuth's "The Art of Computer Programming", Volume 4, Chapter 7,
* Section 7.2.1.2. The iteration order follows the lexicographical order. This means that the
* first permutation will be in ascending order, and the last will be in descending order.
*
*
Elements that compare equal are considered equal and no new permutations are created by
* swapping them.
*
*
An empty iterable has only one permutation, which is an empty list.
*
* @param elements the original iterable whose elements have to be permuted.
* @param comparator a comparator for the iterable's elements.
* @return an immutable {@link Collection} containing all the different permutations of the
* original iterable.
* @throws NullPointerException If the specified iterable is null, has any null elements, or if
* the specified comparator is null.
* @since 12.0
*/
@Beta
public static Collection> orderedPermutations(
Iterable elements, Comparator comparator) {
return new OrderedPermutationCollection(elements, comparator);
}
private static final class OrderedPermutationCollection extends AbstractCollection> {
final ImmutableList inputList;
final Comparator comparator;
final int size;
OrderedPermutationCollection(Iterable input, Comparator comparator) {
this.inputList = ImmutableList.sortedCopyOf(comparator, input);
this.comparator = comparator;
this.size = calculateSize(inputList, comparator);
}
/**
* The number of permutations with repeated elements is calculated as follows:
*
*
* - For an empty list, it is 1 (base case).
*
- When r numbers are added to a list of n-r elements, the number of permutations is
* increased by a factor of (n choose r).
*
*/
private static int calculateSize(
List sortedInputList, Comparator comparator) {
int permutations = 1;
int n = 1;
int r = 1;
while (n < sortedInputList.size()) {
int comparison = comparator.compare(sortedInputList.get(n - 1), sortedInputList.get(n));
if (comparison < 0) {
// We move to the next non-repeated element.
permutations = IntMath.saturatedMultiply(permutations, IntMath.binomial(n, r));
r = 0;
if (permutations == Integer.MAX_VALUE) {
return Integer.MAX_VALUE;
}
}
n++;
r++;
}
return IntMath.saturatedMultiply(permutations, IntMath.binomial(n, r));
}
@Override
public int size() {
return size;
}
@Override
public boolean isEmpty() {
return false;
}
@Override
public Iterator> iterator() {
return new OrderedPermutationIterator(inputList, comparator);
}
@Override
public boolean contains(@NullableDecl Object obj) {
if (obj instanceof List) {
List list = (List) obj;
return isPermutation(inputList, list);
}
return false;
}
@Override
public String toString() {
return "orderedPermutationCollection(" + inputList + ")";
}
}
private static final class OrderedPermutationIterator extends AbstractIterator> {
@NullableDecl List nextPermutation;
final Comparator comparator;
OrderedPermutationIterator(List list, Comparator comparator) {
this.nextPermutation = Lists.newArrayList(list);
this.comparator = comparator;
}
@Override
protected List computeNext() {
if (nextPermutation == null) {
return endOfData();
}
ImmutableList next = ImmutableList.copyOf(nextPermutation);
calculateNextPermutation();
return next;
}
void calculateNextPermutation() {
int j = findNextJ();
if (j == -1) {
nextPermutation = null;
return;
}
int l = findNextL(j);
Collections.swap(nextPermutation, j, l);
int n = nextPermutation.size();
Collections.reverse(nextPermutation.subList(j + 1, n));
}
int findNextJ() {
for (int k = nextPermutation.size() - 2; k >= 0; k--) {
if (comparator.compare(nextPermutation.get(k), nextPermutation.get(k + 1)) < 0) {
return k;
}
}
return -1;
}
int findNextL(int j) {
E ak = nextPermutation.get(j);
for (int l = nextPermutation.size() - 1; l > j; l--) {
if (comparator.compare(ak, nextPermutation.get(l)) < 0) {
return l;
}
}
throw new AssertionError("this statement should be unreachable");
}
}
/**
* Returns a {@link Collection} of all the permutations of the specified {@link Collection}.
*
* Notes: This is an implementation of the Plain Changes algorithm for permutations
* generation, described in Knuth's "The Art of Computer Programming", Volume 4, Chapter 7,
* Section 7.2.1.2.
*
*
If the input list contains equal elements, some of the generated permutations will be equal.
*
*
An empty collection has only one permutation, which is an empty list.
*
* @param elements the original collection whose elements have to be permuted.
* @return an immutable {@link Collection} containing all the different permutations of the
* original collection.
* @throws NullPointerException if the specified collection is null or has any null elements.
* @since 12.0
*/
@Beta
public static Collection> permutations(Collection elements) {
return new PermutationCollection(ImmutableList.copyOf(elements));
}
private static final class PermutationCollection extends AbstractCollection> {
final ImmutableList inputList;
PermutationCollection(ImmutableList input) {
this.inputList = input;
}
@Override
public int size() {
return IntMath.factorial(inputList.size());
}
@Override
public boolean isEmpty() {
return false;
}
@Override
public Iterator> iterator() {
return new PermutationIterator(inputList);
}
@Override
public boolean contains(@NullableDecl Object obj) {
if (obj instanceof List) {
List list = (List) obj;
return isPermutation(inputList, list);
}
return false;
}
@Override
public String toString() {
return "permutations(" + inputList + ")";
}
}
private static class PermutationIterator extends AbstractIterator> {
final List list;
final int[] c;
final int[] o;
int j;
PermutationIterator(List list) {
this.list = new ArrayList(list);
int n = list.size();
c = new int[n];
o = new int[n];
Arrays.fill(c, 0);
Arrays.fill(o, 1);
j = Integer.MAX_VALUE;
}
@Override
protected List computeNext() {
if (j <= 0) {
return endOfData();
}
ImmutableList next = ImmutableList.copyOf(list);
calculateNextPermutation();
return next;
}
void calculateNextPermutation() {
j = list.size() - 1;
int s = 0;
// Handle the special case of an empty list. Skip the calculation of the
// next permutation.
if (j == -1) {
return;
}
while (true) {
int q = c[j] + o[j];
if (q < 0) {
switchDirection();
continue;
}
if (q == j + 1) {
if (j == 0) {
break;
}
s++;
switchDirection();
continue;
}
Collections.swap(list, j - c[j] + s, j - q + s);
c[j] = q;
break;
}
}
void switchDirection() {
o[j] = -o[j];
j--;
}
}
/** Returns {@code true} if the second list is a permutation of the first. */
private static boolean isPermutation(List first, List second) {
if (first.size() != second.size()) {
return false;
}
ObjectCountHashMap firstCounts = counts(first);
ObjectCountHashMap secondCounts = counts(second);
if (first.size() != second.size()) {
return false;
}
for (int i = 0; i < first.size(); i++) {
if (firstCounts.getValue(i) != secondCounts.get(firstCounts.getKey(i))) {
return false;
}
}
return true;
}
private static ObjectCountHashMap counts(Collection collection) {
ObjectCountHashMap map = new ObjectCountHashMap<>();
for (E e : collection) {
map.put(e, map.get(e) + 1);
}
return map;
}
}