001/*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.collect;
018
019import static com.google.common.base.Preconditions.checkArgument;
020import static com.google.common.base.Preconditions.checkNotNull;
021import static com.google.common.collect.CollectPreconditions.checkRemove;
022
023import com.google.common.annotations.GwtCompatible;
024import com.google.common.annotations.GwtIncompatible;
025import com.google.common.base.Function;
026import com.google.common.base.Optional;
027import com.google.common.base.Predicate;
028import com.google.common.base.Predicates;
029import com.google.errorprone.annotations.CanIgnoreReturnValue;
030import java.util.Collection;
031import java.util.Comparator;
032import java.util.Iterator;
033import java.util.List;
034import java.util.NoSuchElementException;
035import java.util.Queue;
036import java.util.RandomAccess;
037import java.util.Set;
038import java.util.Spliterator;
039import java.util.function.Consumer;
040import java.util.stream.Stream;
041import javax.annotation.CheckForNull;
042import org.checkerframework.checker.nullness.qual.Nullable;
043
044/**
045 * An assortment of mainly legacy static utility methods that operate on or return objects of type
046 * {@code Iterable}. Except as noted, each method has a corresponding {@link Iterator}-based method
047 * in the {@link Iterators} class.
048 *
049 * <p><b>Java 8 users:</b> several common uses for this class are now more comprehensively addressed
050 * by the new {@link java.util.stream.Stream} library. Read the method documentation below for
051 * comparisons. This class is not being deprecated, but we gently encourage you to migrate to
052 * streams.
053 *
054 * <p><i>Performance notes:</i> Unless otherwise noted, all of the iterables produced in this class
055 * are <i>lazy</i>, which means that their iterators only advance the backing iteration when
056 * absolutely necessary.
057 *
058 * <p>See the Guava User Guide article on <a href=
059 * "https://github.com/google/guava/wiki/CollectionUtilitiesExplained#iterables">{@code
060 * Iterables}</a>.
061 *
062 * @author Kevin Bourrillion
063 * @author Jared Levy
064 * @since 2.0
065 */
066@GwtCompatible(emulated = true)
067@ElementTypesAreNonnullByDefault
068public final class Iterables {
069  private Iterables() {}
070
071  /** Returns an unmodifiable view of {@code iterable}. */
072  public static <T extends @Nullable Object> Iterable<T> unmodifiableIterable(
073      final Iterable<? extends T> iterable) {
074    checkNotNull(iterable);
075    if (iterable instanceof UnmodifiableIterable || iterable instanceof ImmutableCollection) {
076      @SuppressWarnings("unchecked") // Since it's unmodifiable, the covariant cast is safe
077      Iterable<T> result = (Iterable<T>) iterable;
078      return result;
079    }
080    return new UnmodifiableIterable<>(iterable);
081  }
082
083  /**
084   * Simply returns its argument.
085   *
086   * @deprecated no need to use this
087   * @since 10.0
088   */
089  @Deprecated
090  public static <E> Iterable<E> unmodifiableIterable(ImmutableCollection<E> iterable) {
091    return checkNotNull(iterable);
092  }
093
094  private static final class UnmodifiableIterable<T extends @Nullable Object>
095      extends FluentIterable<T> {
096    private final Iterable<? extends T> iterable;
097
098    private UnmodifiableIterable(Iterable<? extends T> iterable) {
099      this.iterable = iterable;
100    }
101
102    @Override
103    public Iterator<T> iterator() {
104      return Iterators.unmodifiableIterator(iterable.iterator());
105    }
106
107    @Override
108    public void forEach(Consumer<? super T> action) {
109      iterable.forEach(action);
110    }
111
112    @SuppressWarnings("unchecked") // safe upcast, assuming no one has a crazy Spliterator subclass
113    @Override
114    public Spliterator<T> spliterator() {
115      return (Spliterator<T>) iterable.spliterator();
116    }
117
118    @Override
119    public String toString() {
120      return iterable.toString();
121    }
122    // no equals and hashCode; it would break the contract!
123  }
124
125  /** Returns the number of elements in {@code iterable}. */
126  public static int size(Iterable<?> iterable) {
127    return (iterable instanceof Collection)
128        ? ((Collection<?>) iterable).size()
129        : Iterators.size(iterable.iterator());
130  }
131
132  /**
133   * Returns {@code true} if {@code iterable} contains any element {@code o} for which {@code
134   * Objects.equals(o, element)} would return {@code true}. Otherwise returns {@code false}, even in
135   * cases where {@link Collection#contains} might throw {@link NullPointerException} or {@link
136   * ClassCastException}.
137   */
138  // <? extends @Nullable Object> instead of <?> because of Kotlin b/189937072, discussed in Joiner.
139  public static boolean contains(
140      Iterable<? extends @Nullable Object> iterable, @CheckForNull Object element) {
141    if (iterable instanceof Collection) {
142      Collection<?> collection = (Collection<?>) iterable;
143      return Collections2.safeContains(collection, element);
144    }
145    return Iterators.contains(iterable.iterator(), element);
146  }
147
148  /**
149   * Removes, from an iterable, every element that belongs to the provided collection.
150   *
151   * <p>This method calls {@link Collection#removeAll} if {@code iterable} is a collection, and
152   * {@link Iterators#removeAll} otherwise.
153   *
154   * @param removeFrom the iterable to (potentially) remove elements from
155   * @param elementsToRemove the elements to remove
156   * @return {@code true} if any element was removed from {@code iterable}
157   */
158  @CanIgnoreReturnValue
159  public static boolean removeAll(Iterable<?> removeFrom, Collection<?> elementsToRemove) {
160    return (removeFrom instanceof Collection)
161        ? ((Collection<?>) removeFrom).removeAll(checkNotNull(elementsToRemove))
162        : Iterators.removeAll(removeFrom.iterator(), elementsToRemove);
163  }
164
165  /**
166   * Removes, from an iterable, every element that does not belong to the provided collection.
167   *
168   * <p>This method calls {@link Collection#retainAll} if {@code iterable} is a collection, and
169   * {@link Iterators#retainAll} otherwise.
170   *
171   * @param removeFrom the iterable to (potentially) remove elements from
172   * @param elementsToRetain the elements to retain
173   * @return {@code true} if any element was removed from {@code iterable}
174   */
175  @CanIgnoreReturnValue
176  public static boolean retainAll(Iterable<?> removeFrom, Collection<?> elementsToRetain) {
177    return (removeFrom instanceof Collection)
178        ? ((Collection<?>) removeFrom).retainAll(checkNotNull(elementsToRetain))
179        : Iterators.retainAll(removeFrom.iterator(), elementsToRetain);
180  }
181
182  /**
183   * Removes, from an iterable, every element that satisfies the provided predicate.
184   *
185   * <p>Removals may or may not happen immediately as each element is tested against the predicate.
186   * The behavior of this method is not specified if {@code predicate} is dependent on {@code
187   * removeFrom}.
188   *
189   * <p><b>Java 8 users:</b> if {@code removeFrom} is a {@link Collection}, use {@code
190   * removeFrom.removeIf(predicate)} instead.
191   *
192   * @param removeFrom the iterable to (potentially) remove elements from
193   * @param predicate a predicate that determines whether an element should be removed
194   * @return {@code true} if any elements were removed from the iterable
195   * @throws UnsupportedOperationException if the iterable does not support {@code remove()}.
196   * @since 2.0
197   */
198  @CanIgnoreReturnValue
199  public static <T extends @Nullable Object> boolean removeIf(
200      Iterable<T> removeFrom, Predicate<? super T> predicate) {
201    if (removeFrom instanceof Collection) {
202      return ((Collection<T>) removeFrom).removeIf(predicate);
203    }
204    return Iterators.removeIf(removeFrom.iterator(), predicate);
205  }
206
207  /** Removes and returns the first matching element, or returns {@code null} if there is none. */
208  @CheckForNull
209  static <T extends @Nullable Object> T removeFirstMatching(
210      Iterable<T> removeFrom, Predicate<? super T> predicate) {
211    checkNotNull(predicate);
212    Iterator<T> iterator = removeFrom.iterator();
213    while (iterator.hasNext()) {
214      T next = iterator.next();
215      if (predicate.apply(next)) {
216        iterator.remove();
217        return next;
218      }
219    }
220    return null;
221  }
222
223  /**
224   * Determines whether two iterables contain equal elements in the same order. More specifically,
225   * this method returns {@code true} if {@code iterable1} and {@code iterable2} contain the same
226   * number of elements and every element of {@code iterable1} is equal to the corresponding element
227   * of {@code iterable2}.
228   */
229  public static boolean elementsEqual(Iterable<?> iterable1, Iterable<?> iterable2) {
230    if (iterable1 instanceof Collection && iterable2 instanceof Collection) {
231      Collection<?> collection1 = (Collection<?>) iterable1;
232      Collection<?> collection2 = (Collection<?>) iterable2;
233      if (collection1.size() != collection2.size()) {
234        return false;
235      }
236    }
237    return Iterators.elementsEqual(iterable1.iterator(), iterable2.iterator());
238  }
239
240  /**
241   * Returns a string representation of {@code iterable}, with the format {@code [e1, e2, ..., en]}
242   * (that is, identical to {@link java.util.Arrays Arrays}{@code
243   * .toString(Iterables.toArray(iterable))}). Note that for <i>most</i> implementations of {@link
244   * Collection}, {@code collection.toString()} also gives the same result, but that behavior is not
245   * generally guaranteed.
246   */
247  public static String toString(Iterable<?> iterable) {
248    return Iterators.toString(iterable.iterator());
249  }
250
251  /**
252   * Returns the single element contained in {@code iterable}.
253   *
254   * <p><b>Java 8 users:</b> the {@code Stream} equivalent to this method is {@code
255   * stream.collect(MoreCollectors.onlyElement())}.
256   *
257   * @throws NoSuchElementException if the iterable is empty
258   * @throws IllegalArgumentException if the iterable contains multiple elements
259   */
260  @ParametricNullness
261  public static <T extends @Nullable Object> T getOnlyElement(Iterable<T> iterable) {
262    return Iterators.getOnlyElement(iterable.iterator());
263  }
264
265  /**
266   * Returns the single element contained in {@code iterable}, or {@code defaultValue} if the
267   * iterable is empty.
268   *
269   * <p><b>Java 8 users:</b> the {@code Stream} equivalent to this method is {@code
270   * stream.collect(MoreCollectors.toOptional()).orElse(defaultValue)}.
271   *
272   * @throws IllegalArgumentException if the iterator contains multiple elements
273   */
274  @ParametricNullness
275  public static <T extends @Nullable Object> T getOnlyElement(
276      Iterable<? extends T> iterable, @ParametricNullness T defaultValue) {
277    return Iterators.getOnlyElement(iterable.iterator(), defaultValue);
278  }
279
280  /**
281   * Copies an iterable's elements into an array.
282   *
283   * @param iterable the iterable to copy
284   * @param type the type of the elements
285   * @return a newly-allocated array into which all the elements of the iterable have been copied
286   */
287  @GwtIncompatible // Array.newInstance(Class, int)
288  /*
289   * If we could express Class<@Nonnull T>, we could generalize the type parameter to <T extends
290   * @Nullable Object>, and then we could accept an Iterable<? extends T> and return a plain T[]
291   * instead of a @Nullable T[].
292   */
293  public static <T> @Nullable T[] toArray(Iterable<? extends @Nullable T> iterable, Class<T> type) {
294    return toArray(iterable, ObjectArrays.newArray(type, 0));
295  }
296
297  static <T extends @Nullable Object> T[] toArray(Iterable<? extends T> iterable, T[] array) {
298    Collection<? extends T> collection = castOrCopyToCollection(iterable);
299    return collection.toArray(array);
300  }
301
302  /**
303   * Copies an iterable's elements into an array.
304   *
305   * @param iterable the iterable to copy
306   * @return a newly-allocated array into which all the elements of the iterable have been copied
307   */
308  static @Nullable Object[] toArray(Iterable<?> iterable) {
309    return castOrCopyToCollection(iterable).toArray();
310  }
311
312  /**
313   * Converts an iterable into a collection. If the iterable is already a collection, it is
314   * returned. Otherwise, an {@link java.util.ArrayList} is created with the contents of the
315   * iterable in the same iteration order.
316   */
317  private static <E extends @Nullable Object> Collection<E> castOrCopyToCollection(
318      Iterable<E> iterable) {
319    return (iterable instanceof Collection)
320        ? (Collection<E>) iterable
321        : Lists.newArrayList(iterable.iterator());
322  }
323
324  /**
325   * Adds all elements in {@code iterable} to {@code collection}.
326   *
327   * @return {@code true} if {@code collection} was modified as a result of this operation.
328   */
329  @CanIgnoreReturnValue
330  public static <T extends @Nullable Object> boolean addAll(
331      Collection<T> addTo, Iterable<? extends T> elementsToAdd) {
332    if (elementsToAdd instanceof Collection) {
333      Collection<? extends T> c = (Collection<? extends T>) elementsToAdd;
334      return addTo.addAll(c);
335    }
336    return Iterators.addAll(addTo, checkNotNull(elementsToAdd).iterator());
337  }
338
339  /**
340   * Returns the number of elements in the specified iterable that equal the specified object. This
341   * implementation avoids a full iteration when the iterable is a {@link Multiset} or {@link Set}.
342   *
343   * <p><b>Java 8 users:</b> In most cases, the {@code Stream} equivalent of this method is {@code
344   * stream.filter(element::equals).count()}. If {@code element} might be null, use {@code
345   * stream.filter(Predicate.isEqual(element)).count()} instead.
346   *
347   * @see java.util.Collections#frequency(Collection, Object) Collections.frequency(Collection,
348   *     Object)
349   */
350  public static int frequency(Iterable<?> iterable, @CheckForNull Object element) {
351    if ((iterable instanceof Multiset)) {
352      return ((Multiset<?>) iterable).count(element);
353    } else if ((iterable instanceof Set)) {
354      return ((Set<?>) iterable).contains(element) ? 1 : 0;
355    }
356    return Iterators.frequency(iterable.iterator(), element);
357  }
358
359  /**
360   * Returns an iterable whose iterators cycle indefinitely over the elements of {@code iterable}.
361   *
362   * <p>That iterator supports {@code remove()} if {@code iterable.iterator()} does. After {@code
363   * remove()} is called, subsequent cycles omit the removed element, which is no longer in {@code
364   * iterable}. The iterator's {@code hasNext()} method returns {@code true} until {@code iterable}
365   * is empty.
366   *
367   * <p><b>Warning:</b> Typical uses of the resulting iterator may produce an infinite loop. You
368   * should use an explicit {@code break} or be certain that you will eventually remove all the
369   * elements.
370   *
371   * <p>To cycle over the iterable {@code n} times, use the following: {@code
372   * Iterables.concat(Collections.nCopies(n, iterable))}
373   *
374   * <p><b>Java 8 users:</b> The {@code Stream} equivalent of this method is {@code
375   * Stream.generate(() -> iterable).flatMap(Streams::stream)}.
376   */
377  public static <T extends @Nullable Object> Iterable<T> cycle(final Iterable<T> iterable) {
378    checkNotNull(iterable);
379    return new FluentIterable<T>() {
380      @Override
381      public Iterator<T> iterator() {
382        return Iterators.cycle(iterable);
383      }
384
385      @Override
386      public Spliterator<T> spliterator() {
387        return Stream.generate(() -> iterable).<T>flatMap(Streams::stream).spliterator();
388      }
389
390      @Override
391      public String toString() {
392        return iterable.toString() + " (cycled)";
393      }
394    };
395  }
396
397  /**
398   * Returns an iterable whose iterators cycle indefinitely over the provided elements.
399   *
400   * <p>After {@code remove} is invoked on a generated iterator, the removed element will no longer
401   * appear in either that iterator or any other iterator created from the same source iterable.
402   * That is, this method behaves exactly as {@code Iterables.cycle(Lists.newArrayList(elements))}.
403   * The iterator's {@code hasNext} method returns {@code true} until all of the original elements
404   * have been removed.
405   *
406   * <p><b>Warning:</b> Typical uses of the resulting iterator may produce an infinite loop. You
407   * should use an explicit {@code break} or be certain that you will eventually remove all the
408   * elements.
409   *
410   * <p>To cycle over the elements {@code n} times, use the following: {@code
411   * Iterables.concat(Collections.nCopies(n, Arrays.asList(elements)))}
412   *
413   * <p><b>Java 8 users:</b> If passing a single element {@code e}, the {@code Stream} equivalent of
414   * this method is {@code Stream.generate(() -> e)}. Otherwise, put the elements in a collection
415   * and use {@code Stream.generate(() -> collection).flatMap(Collection::stream)}.
416   */
417  @SafeVarargs
418  public static <T extends @Nullable Object> Iterable<T> cycle(T... elements) {
419    return cycle(Lists.newArrayList(elements));
420  }
421
422  /**
423   * Combines two iterables into a single iterable. The returned iterable has an iterator that
424   * traverses the elements in {@code a}, followed by the elements in {@code b}. The source
425   * iterators are not polled until necessary.
426   *
427   * <p>The returned iterable's iterator supports {@code remove()} when the corresponding input
428   * iterator supports it.
429   *
430   * <p><b>Java 8 users:</b> The {@code Stream} equivalent of this method is {@code Stream.concat(a,
431   * b)}.
432   */
433  public static <T extends @Nullable Object> Iterable<T> concat(
434      Iterable<? extends T> a, Iterable<? extends T> b) {
435    return FluentIterable.concat(a, b);
436  }
437
438  /**
439   * Combines three iterables into a single iterable. The returned iterable has an iterator that
440   * traverses the elements in {@code a}, followed by the elements in {@code b}, followed by the
441   * elements in {@code c}. The source iterators are not polled until necessary.
442   *
443   * <p>The returned iterable's iterator supports {@code remove()} when the corresponding input
444   * iterator supports it.
445   *
446   * <p><b>Java 8 users:</b> The {@code Stream} equivalent of this method is {@code
447   * Streams.concat(a, b, c)}.
448   */
449  public static <T extends @Nullable Object> Iterable<T> concat(
450      Iterable<? extends T> a, Iterable<? extends T> b, Iterable<? extends T> c) {
451    return FluentIterable.concat(a, b, c);
452  }
453
454  /**
455   * Combines four iterables into a single iterable. The returned iterable has an iterator that
456   * traverses the elements in {@code a}, followed by the elements in {@code b}, followed by the
457   * elements in {@code c}, followed by the elements in {@code d}. The source iterators are not
458   * polled until necessary.
459   *
460   * <p>The returned iterable's iterator supports {@code remove()} when the corresponding input
461   * iterator supports it.
462   *
463   * <p><b>Java 8 users:</b> The {@code Stream} equivalent of this method is {@code
464   * Streams.concat(a, b, c, d)}.
465   */
466  public static <T extends @Nullable Object> Iterable<T> concat(
467      Iterable<? extends T> a,
468      Iterable<? extends T> b,
469      Iterable<? extends T> c,
470      Iterable<? extends T> d) {
471    return FluentIterable.concat(a, b, c, d);
472  }
473
474  /**
475   * Combines multiple iterables into a single iterable. The returned iterable has an iterator that
476   * traverses the elements of each iterable in {@code inputs}. The input iterators are not polled
477   * until necessary.
478   *
479   * <p>The returned iterable's iterator supports {@code remove()} when the corresponding input
480   * iterator supports it.
481   *
482   * <p><b>Java 8 users:</b> The {@code Stream} equivalent of this method is {@code
483   * Streams.concat(...)}.
484   *
485   * @throws NullPointerException if any of the provided iterables is null
486   */
487  @SafeVarargs
488  public static <T extends @Nullable Object> Iterable<T> concat(Iterable<? extends T>... inputs) {
489    return FluentIterable.concat(inputs);
490  }
491
492  /**
493   * Combines multiple iterables into a single iterable. The returned iterable has an iterator that
494   * traverses the elements of each iterable in {@code inputs}. The input iterators are not polled
495   * until necessary.
496   *
497   * <p>The returned iterable's iterator supports {@code remove()} when the corresponding input
498   * iterator supports it. The methods of the returned iterable may throw {@code
499   * NullPointerException} if any of the input iterators is null.
500   *
501   * <p><b>Java 8 users:</b> The {@code Stream} equivalent of this method is {@code
502   * streamOfStreams.flatMap(s -> s)}.
503   */
504  public static <T extends @Nullable Object> Iterable<T> concat(
505      Iterable<? extends Iterable<? extends T>> inputs) {
506    return FluentIterable.concat(inputs);
507  }
508
509  /**
510   * Divides an iterable into unmodifiable sublists of the given size (the final iterable may be
511   * smaller). For example, partitioning an iterable containing {@code [a, b, c, d, e]} with a
512   * partition size of 3 yields {@code [[a, b, c], [d, e]]} -- an outer iterable containing two
513   * inner lists of three and two elements, all in the original order.
514   *
515   * <p>Iterators returned by the returned iterable do not support the {@link Iterator#remove()}
516   * method. The returned lists implement {@link RandomAccess}, whether or not the input list does.
517   *
518   * <p><b>Note:</b> The current implementation eagerly allocates storage for {@code size} elements.
519   * As a consequence, passing values like {@code Integer.MAX_VALUE} can lead to {@link
520   * OutOfMemoryError}.
521   *
522   * <p><b>Note:</b> if {@code iterable} is a {@link List}, use {@link Lists#partition(List, int)}
523   * instead.
524   *
525   * @param iterable the iterable to return a partitioned view of
526   * @param size the desired size of each partition (the last may be smaller)
527   * @return an iterable of unmodifiable lists containing the elements of {@code iterable} divided
528   *     into partitions
529   * @throws IllegalArgumentException if {@code size} is nonpositive
530   */
531  public static <T extends @Nullable Object> Iterable<List<T>> partition(
532      final Iterable<T> iterable, final int size) {
533    checkNotNull(iterable);
534    checkArgument(size > 0);
535    return new FluentIterable<List<T>>() {
536      @Override
537      public Iterator<List<T>> iterator() {
538        return Iterators.partition(iterable.iterator(), size);
539      }
540    };
541  }
542
543  /**
544   * Divides an iterable into unmodifiable sublists of the given size, padding the final iterable
545   * with null values if necessary. For example, partitioning an iterable containing {@code [a, b,
546   * c, d, e]} with a partition size of 3 yields {@code [[a, b, c], [d, e, null]]} -- an outer
547   * iterable containing two inner lists of three elements each, all in the original order.
548   *
549   * <p>Iterators returned by the returned iterable do not support the {@link Iterator#remove()}
550   * method.
551   *
552   * @param iterable the iterable to return a partitioned view of
553   * @param size the desired size of each partition
554   * @return an iterable of unmodifiable lists containing the elements of {@code iterable} divided
555   *     into partitions (the final iterable may have trailing null elements)
556   * @throws IllegalArgumentException if {@code size} is nonpositive
557   */
558  public static <T extends @Nullable Object> Iterable<List<@Nullable T>> paddedPartition(
559      final Iterable<T> iterable, final int size) {
560    checkNotNull(iterable);
561    checkArgument(size > 0);
562    return new FluentIterable<List<@Nullable T>>() {
563      @Override
564      public Iterator<List<@Nullable T>> iterator() {
565        return Iterators.paddedPartition(iterable.iterator(), size);
566      }
567    };
568  }
569
570  /**
571   * Returns a view of {@code unfiltered} containing all elements that satisfy the input predicate
572   * {@code retainIfTrue}. The returned iterable's iterator does not support {@code remove()}.
573   *
574   * <p><b>{@code Stream} equivalent:</b> {@link Stream#filter}.
575   */
576  public static <T extends @Nullable Object> Iterable<T> filter(
577      final Iterable<T> unfiltered, final Predicate<? super T> retainIfTrue) {
578    checkNotNull(unfiltered);
579    checkNotNull(retainIfTrue);
580    return new FluentIterable<T>() {
581      @Override
582      public Iterator<T> iterator() {
583        return Iterators.filter(unfiltered.iterator(), retainIfTrue);
584      }
585
586      @Override
587      public void forEach(Consumer<? super T> action) {
588        checkNotNull(action);
589        unfiltered.forEach(
590            (@ParametricNullness T a) -> {
591              if (retainIfTrue.test(a)) {
592                action.accept(a);
593              }
594            });
595      }
596
597      @Override
598      public Spliterator<T> spliterator() {
599        return CollectSpliterators.filter(unfiltered.spliterator(), retainIfTrue);
600      }
601    };
602  }
603
604  /**
605   * Returns a view of {@code unfiltered} containing all elements that are of the type {@code
606   * desiredType}. The returned iterable's iterator does not support {@code remove()}.
607   *
608   * <p><b>{@code Stream} equivalent:</b> {@code stream.filter(type::isInstance).map(type::cast)}.
609   * This does perform a little more work than necessary, so another option is to insert an
610   * unchecked cast at some later point:
611   *
612   * <pre>
613   * {@code @SuppressWarnings("unchecked") // safe because of ::isInstance check
614   * ImmutableList<NewType> result =
615   *     (ImmutableList) stream.filter(NewType.class::isInstance).collect(toImmutableList());}
616   * </pre>
617   */
618  @SuppressWarnings("unchecked")
619  @GwtIncompatible // Class.isInstance
620  public static <T> Iterable<T> filter(final Iterable<?> unfiltered, final Class<T> desiredType) {
621    checkNotNull(unfiltered);
622    checkNotNull(desiredType);
623    return (Iterable<T>) filter(unfiltered, Predicates.instanceOf(desiredType));
624  }
625
626  /**
627   * Returns {@code true} if any element in {@code iterable} satisfies the predicate.
628   *
629   * <p><b>{@code Stream} equivalent:</b> {@link Stream#anyMatch}.
630   */
631  public static <T extends @Nullable Object> boolean any(
632      Iterable<T> iterable, Predicate<? super T> predicate) {
633    return Iterators.any(iterable.iterator(), predicate);
634  }
635
636  /**
637   * Returns {@code true} if every element in {@code iterable} satisfies the predicate. If {@code
638   * iterable} is empty, {@code true} is returned.
639   *
640   * <p><b>{@code Stream} equivalent:</b> {@link Stream#allMatch}.
641   */
642  public static <T extends @Nullable Object> boolean all(
643      Iterable<T> iterable, Predicate<? super T> predicate) {
644    return Iterators.all(iterable.iterator(), predicate);
645  }
646
647  /**
648   * Returns the first element in {@code iterable} that satisfies the given predicate; use this
649   * method only when such an element is known to exist. If it is possible that <i>no</i> element
650   * will match, use {@link #tryFind} or {@link #find(Iterable, Predicate, Object)} instead.
651   *
652   * <p><b>{@code Stream} equivalent:</b> {@code stream.filter(predicate).findFirst().get()}
653   *
654   * @throws NoSuchElementException if no element in {@code iterable} matches the given predicate
655   */
656  @ParametricNullness
657  public static <T extends @Nullable Object> T find(
658      Iterable<T> iterable, Predicate<? super T> predicate) {
659    return Iterators.find(iterable.iterator(), predicate);
660  }
661
662  /**
663   * Returns the first element in {@code iterable} that satisfies the given predicate, or {@code
664   * defaultValue} if none found. Note that this can usually be handled more naturally using {@code
665   * tryFind(iterable, predicate).or(defaultValue)}.
666   *
667   * <p><b>{@code Stream} equivalent:</b> {@code
668   * stream.filter(predicate).findFirst().orElse(defaultValue)}
669   *
670   * @since 7.0
671   */
672  // The signature we really want here is...
673  //
674  // <T extends @Nullable Object> @JointlyNullable T find(
675  //     Iterable<? extends T> iterable,
676  //     Predicate<? super T> predicate,
677  //     @JointlyNullable T defaultValue);
678  //
679  // ...where "@JointlyNullable" is similar to @PolyNull but slightly different:
680  //
681  // - @PolyNull means "@Nullable or @Nonnull"
682  //   (That would be unsound for an input Iterable<@Nullable Foo>. So, if we wanted to use
683  //   @PolyNull, we would have to restrict this method to non-null <T>. But it has users who pass
684  //   iterables with null elements.)
685  //
686  // - @JointlyNullable means "@Nullable or no annotation"
687  @CheckForNull
688  public static <T extends @Nullable Object> T find(
689      Iterable<? extends T> iterable,
690      Predicate<? super T> predicate,
691      @CheckForNull T defaultValue) {
692    return Iterators.find(iterable.iterator(), predicate, defaultValue);
693  }
694
695  /**
696   * Returns an {@link Optional} containing the first element in {@code iterable} that satisfies the
697   * given predicate, if such an element exists.
698   *
699   * <p><b>Warning:</b> avoid using a {@code predicate} that matches {@code null}. If {@code null}
700   * is matched in {@code iterable}, a NullPointerException will be thrown.
701   *
702   * <p><b>{@code Stream} equivalent:</b> {@code stream.filter(predicate).findFirst()}
703   *
704   * @since 11.0
705   */
706  public static <T> Optional<T> tryFind(Iterable<T> iterable, Predicate<? super T> predicate) {
707    return Iterators.tryFind(iterable.iterator(), predicate);
708  }
709
710  /**
711   * Returns the index in {@code iterable} of the first element that satisfies the provided {@code
712   * predicate}, or {@code -1} if the Iterable has no such elements.
713   *
714   * <p>More formally, returns the lowest index {@code i} such that {@code
715   * predicate.apply(Iterables.get(iterable, i))} returns {@code true}, or {@code -1} if there is no
716   * such index.
717   *
718   * @since 2.0
719   */
720  public static <T extends @Nullable Object> int indexOf(
721      Iterable<T> iterable, Predicate<? super T> predicate) {
722    return Iterators.indexOf(iterable.iterator(), predicate);
723  }
724
725  /**
726   * Returns a view containing the result of applying {@code function} to each element of {@code
727   * fromIterable}.
728   *
729   * <p>The returned iterable's iterator supports {@code remove()} if {@code fromIterable}'s
730   * iterator does. After a successful {@code remove()} call, {@code fromIterable} no longer
731   * contains the corresponding element.
732   *
733   * <p>If the input {@code Iterable} is known to be a {@code List} or other {@code Collection},
734   * consider {@link Lists#transform} and {@link Collections2#transform}.
735   *
736   * <p><b>{@code Stream} equivalent:</b> {@link Stream#map}
737   */
738  public static <F extends @Nullable Object, T extends @Nullable Object> Iterable<T> transform(
739      final Iterable<F> fromIterable, final Function<? super F, ? extends T> function) {
740    checkNotNull(fromIterable);
741    checkNotNull(function);
742    return new FluentIterable<T>() {
743      @Override
744      public Iterator<T> iterator() {
745        return Iterators.transform(fromIterable.iterator(), function);
746      }
747
748      @Override
749      public void forEach(Consumer<? super T> action) {
750        checkNotNull(action);
751        fromIterable.forEach((F f) -> action.accept(function.apply(f)));
752      }
753
754      @Override
755      public Spliterator<T> spliterator() {
756        return CollectSpliterators.map(fromIterable.spliterator(), function);
757      }
758    };
759  }
760
761  /**
762   * Returns the element at the specified position in an iterable.
763   *
764   * <p><b>{@code Stream} equivalent:</b> {@code stream.skip(position).findFirst().get()} (throws
765   * {@code NoSuchElementException} if out of bounds)
766   *
767   * @param position position of the element to return
768   * @return the element at the specified position in {@code iterable}
769   * @throws IndexOutOfBoundsException if {@code position} is negative or greater than or equal to
770   *     the size of {@code iterable}
771   */
772  @ParametricNullness
773  public static <T extends @Nullable Object> T get(Iterable<T> iterable, int position) {
774    checkNotNull(iterable);
775    return (iterable instanceof List)
776        ? ((List<T>) iterable).get(position)
777        : Iterators.get(iterable.iterator(), position);
778  }
779
780  /**
781   * Returns the element at the specified position in an iterable or a default value otherwise.
782   *
783   * <p><b>{@code Stream} equivalent:</b> {@code
784   * stream.skip(position).findFirst().orElse(defaultValue)} (returns the default value if the index
785   * is out of bounds)
786   *
787   * @param position position of the element to return
788   * @param defaultValue the default value to return if {@code position} is greater than or equal to
789   *     the size of the iterable
790   * @return the element at the specified position in {@code iterable} or {@code defaultValue} if
791   *     {@code iterable} contains fewer than {@code position + 1} elements.
792   * @throws IndexOutOfBoundsException if {@code position} is negative
793   * @since 4.0
794   */
795  @ParametricNullness
796  public static <T extends @Nullable Object> T get(
797      Iterable<? extends T> iterable, int position, @ParametricNullness T defaultValue) {
798    checkNotNull(iterable);
799    Iterators.checkNonnegative(position);
800    if (iterable instanceof List) {
801      List<? extends T> list = Lists.cast(iterable);
802      return (position < list.size()) ? list.get(position) : defaultValue;
803    } else {
804      Iterator<? extends T> iterator = iterable.iterator();
805      Iterators.advance(iterator, position);
806      return Iterators.getNext(iterator, defaultValue);
807    }
808  }
809
810  /**
811   * Returns the first element in {@code iterable} or {@code defaultValue} if the iterable is empty.
812   * The {@link Iterators} analog to this method is {@link Iterators#getNext}.
813   *
814   * <p>If no default value is desired (and the caller instead wants a {@link
815   * NoSuchElementException} to be thrown), it is recommended that {@code
816   * iterable.iterator().next()} is used instead.
817   *
818   * <p>To get the only element in a single-element {@code Iterable}, consider using {@link
819   * #getOnlyElement(Iterable)} or {@link #getOnlyElement(Iterable, Object)} instead.
820   *
821   * <p><b>{@code Stream} equivalent:</b> {@code stream.findFirst().orElse(defaultValue)}
822   *
823   * @param defaultValue the default value to return if the iterable is empty
824   * @return the first element of {@code iterable} or the default value
825   * @since 7.0
826   */
827  @ParametricNullness
828  public static <T extends @Nullable Object> T getFirst(
829      Iterable<? extends T> iterable, @ParametricNullness T defaultValue) {
830    return Iterators.getNext(iterable.iterator(), defaultValue);
831  }
832
833  /**
834   * Returns the last element of {@code iterable}. If {@code iterable} is a {@link List} with {@link
835   * RandomAccess} support, then this operation is guaranteed to be {@code O(1)}.
836   *
837   * <p><b>{@code Stream} equivalent:</b> {@link Streams#findLast Streams.findLast(stream).get()}
838   *
839   * @return the last element of {@code iterable}
840   * @throws NoSuchElementException if the iterable is empty
841   */
842  @ParametricNullness
843  public static <T extends @Nullable Object> T getLast(Iterable<T> iterable) {
844    // TODO(kevinb): Support a concurrently modified collection?
845    if (iterable instanceof List) {
846      List<T> list = (List<T>) iterable;
847      if (list.isEmpty()) {
848        throw new NoSuchElementException();
849      }
850      return getLastInNonemptyList(list);
851    }
852
853    return Iterators.getLast(iterable.iterator());
854  }
855
856  /**
857   * Returns the last element of {@code iterable} or {@code defaultValue} if the iterable is empty.
858   * If {@code iterable} is a {@link List} with {@link RandomAccess} support, then this operation is
859   * guaranteed to be {@code O(1)}.
860   *
861   * <p><b>{@code Stream} equivalent:</b> {@code Streams.findLast(stream).orElse(defaultValue)}
862   *
863   * @param defaultValue the value to return if {@code iterable} is empty
864   * @return the last element of {@code iterable} or the default value
865   * @since 3.0
866   */
867  @ParametricNullness
868  public static <T extends @Nullable Object> T getLast(
869      Iterable<? extends T> iterable, @ParametricNullness T defaultValue) {
870    if (iterable instanceof Collection) {
871      Collection<? extends T> c = (Collection<? extends T>) iterable;
872      if (c.isEmpty()) {
873        return defaultValue;
874      } else if (iterable instanceof List) {
875        return getLastInNonemptyList(Lists.cast(iterable));
876      }
877    }
878
879    return Iterators.getLast(iterable.iterator(), defaultValue);
880  }
881
882  @ParametricNullness
883  private static <T extends @Nullable Object> T getLastInNonemptyList(List<T> list) {
884    return list.get(list.size() - 1);
885  }
886
887  /**
888   * Returns a view of {@code iterable} that skips its first {@code numberToSkip} elements. If
889   * {@code iterable} contains fewer than {@code numberToSkip} elements, the returned iterable skips
890   * all of its elements.
891   *
892   * <p>Modifications to the underlying {@link Iterable} before a call to {@code iterator()} are
893   * reflected in the returned iterator. That is, the iterator skips the first {@code numberToSkip}
894   * elements that exist when the {@code Iterator} is created, not when {@code skip()} is called.
895   *
896   * <p>The returned iterable's iterator supports {@code remove()} if the iterator of the underlying
897   * iterable supports it. Note that it is <i>not</i> possible to delete the last skipped element by
898   * immediately calling {@code remove()} on that iterator, as the {@code Iterator} contract states
899   * that a call to {@code remove()} before a call to {@code next()} will throw an {@link
900   * IllegalStateException}.
901   *
902   * <p><b>{@code Stream} equivalent:</b> {@link Stream#skip}
903   *
904   * @since 3.0
905   */
906  public static <T extends @Nullable Object> Iterable<T> skip(
907      final Iterable<T> iterable, final int numberToSkip) {
908    checkNotNull(iterable);
909    checkArgument(numberToSkip >= 0, "number to skip cannot be negative");
910
911    return new FluentIterable<T>() {
912      @Override
913      public Iterator<T> iterator() {
914        if (iterable instanceof List) {
915          final List<T> list = (List<T>) iterable;
916          int toSkip = Math.min(list.size(), numberToSkip);
917          return list.subList(toSkip, list.size()).iterator();
918        }
919        final Iterator<T> iterator = iterable.iterator();
920
921        Iterators.advance(iterator, numberToSkip);
922
923        /*
924         * We can't just return the iterator because an immediate call to its
925         * remove() method would remove one of the skipped elements instead of
926         * throwing an IllegalStateException.
927         */
928        return new Iterator<T>() {
929          boolean atStart = true;
930
931          @Override
932          public boolean hasNext() {
933            return iterator.hasNext();
934          }
935
936          @Override
937          @ParametricNullness
938          public T next() {
939            T result = iterator.next();
940            atStart = false; // not called if next() fails
941            return result;
942          }
943
944          @Override
945          public void remove() {
946            checkRemove(!atStart);
947            iterator.remove();
948          }
949        };
950      }
951
952      @Override
953      public Spliterator<T> spliterator() {
954        if (iterable instanceof List) {
955          final List<T> list = (List<T>) iterable;
956          int toSkip = Math.min(list.size(), numberToSkip);
957          return list.subList(toSkip, list.size()).spliterator();
958        } else {
959          return Streams.stream(iterable).skip(numberToSkip).spliterator();
960        }
961      }
962    };
963  }
964
965  /**
966   * Returns a view of {@code iterable} containing its first {@code limitSize} elements. If {@code
967   * iterable} contains fewer than {@code limitSize} elements, the returned view contains all of its
968   * elements. The returned iterable's iterator supports {@code remove()} if {@code iterable}'s
969   * iterator does.
970   *
971   * <p><b>{@code Stream} equivalent:</b> {@link Stream#limit}
972   *
973   * @param iterable the iterable to limit
974   * @param limitSize the maximum number of elements in the returned iterable
975   * @throws IllegalArgumentException if {@code limitSize} is negative
976   * @since 3.0
977   */
978  public static <T extends @Nullable Object> Iterable<T> limit(
979      final Iterable<T> iterable, final int limitSize) {
980    checkNotNull(iterable);
981    checkArgument(limitSize >= 0, "limit is negative");
982    return new FluentIterable<T>() {
983      @Override
984      public Iterator<T> iterator() {
985        return Iterators.limit(iterable.iterator(), limitSize);
986      }
987
988      @Override
989      public Spliterator<T> spliterator() {
990        return Streams.stream(iterable).limit(limitSize).spliterator();
991      }
992    };
993  }
994
995  /**
996   * Returns a view of the supplied iterable that wraps each generated {@link Iterator} through
997   * {@link Iterators#consumingIterator(Iterator)}.
998   *
999   * <p>Note: If {@code iterable} is a {@link Queue}, the returned iterable will instead use {@link
1000   * Queue#isEmpty} and {@link Queue#remove()}, since {@link Queue}'s iteration order is undefined.
1001   * Calling {@link Iterator#hasNext()} on a generated iterator from the returned iterable may cause
1002   * an item to be immediately dequeued for return on a subsequent call to {@link Iterator#next()}.
1003   *
1004   * <p>Whether the input {@code iterable} is a {@link Queue} or not, the returned {@code Iterable}
1005   * is not thread-safe.
1006   *
1007   * @param iterable the iterable to wrap
1008   * @return a view of the supplied iterable that wraps each generated iterator through {@link
1009   *     Iterators#consumingIterator(Iterator)}; for queues, an iterable that generates iterators
1010   *     that return and consume the queue's elements in queue order
1011   * @see Iterators#consumingIterator(Iterator)
1012   * @since 2.0
1013   */
1014  public static <T extends @Nullable Object> Iterable<T> consumingIterable(
1015      final Iterable<T> iterable) {
1016    checkNotNull(iterable);
1017
1018    return new FluentIterable<T>() {
1019      @Override
1020      public Iterator<T> iterator() {
1021        return (iterable instanceof Queue)
1022            ? new ConsumingQueueIterator<>((Queue<T>) iterable)
1023            : Iterators.consumingIterator(iterable.iterator());
1024      }
1025
1026      @Override
1027      public String toString() {
1028        return "Iterables.consumingIterable(...)";
1029      }
1030    };
1031  }
1032
1033  // Methods only in Iterables, not in Iterators
1034
1035  /**
1036   * Determines if the given iterable contains no elements.
1037   *
1038   * <p>There is no precise {@link Iterator} equivalent to this method, since one can only ask an
1039   * iterator whether it has any elements <i>remaining</i> (which one does using {@link
1040   * Iterator#hasNext}).
1041   *
1042   * <p><b>{@code Stream} equivalent:</b> {@code !stream.findAny().isPresent()}
1043   *
1044   * @return {@code true} if the iterable contains no elements
1045   */
1046  public static boolean isEmpty(Iterable<?> iterable) {
1047    if (iterable instanceof Collection) {
1048      return ((Collection<?>) iterable).isEmpty();
1049    }
1050    return !iterable.iterator().hasNext();
1051  }
1052
1053  /**
1054   * Returns an iterable over the merged contents of all given {@code iterables}. Equivalent entries
1055   * will not be de-duplicated.
1056   *
1057   * <p>Callers must ensure that the source {@code iterables} are in non-descending order as this
1058   * method does not sort its input.
1059   *
1060   * <p>For any equivalent elements across all {@code iterables}, it is undefined which element is
1061   * returned first.
1062   *
1063   * @since 11.0
1064   */
1065  public static <T extends @Nullable Object> Iterable<T> mergeSorted(
1066      final Iterable<? extends Iterable<? extends T>> iterables,
1067      final Comparator<? super T> comparator) {
1068    checkNotNull(iterables, "iterables");
1069    checkNotNull(comparator, "comparator");
1070    Iterable<T> iterable =
1071        new FluentIterable<T>() {
1072          @Override
1073          public Iterator<T> iterator() {
1074            return Iterators.mergeSorted(
1075                Iterables.transform(iterables, Iterable::iterator), comparator);
1076          }
1077        };
1078    return new UnmodifiableIterable<>(iterable);
1079  }
1080}