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.Beta;
024import com.google.common.annotations.GwtCompatible;
025import com.google.common.annotations.GwtIncompatible;
026import com.google.common.base.Function;
027import com.google.common.base.Optional;
028import com.google.common.base.Predicate;
029import com.google.common.base.Predicates;
030import com.google.errorprone.annotations.CanIgnoreReturnValue;
031import java.util.Collection;
032import java.util.Comparator;
033import java.util.Iterator;
034import java.util.List;
035import java.util.NoSuchElementException;
036import java.util.Queue;
037import java.util.RandomAccess;
038import java.util.Set;
039import java.util.Spliterator;
040import java.util.function.Consumer;
041import java.util.stream.Stream;
042import javax.annotation.CheckForNull;
043import org.checkerframework.checker.nullness.qual.Nullable;
044
045/**
046 * An assortment of mainly legacy static utility methods that operate on or return objects of type
047 * {@code Iterable}. Except as noted, each method has a corresponding {@link Iterator}-based method
048 * in the {@link Iterators} class.
049 *
050 * <p><b>Java 8 users:</b> several common uses for this class are now more comprehensively addressed
051 * by the new {@link java.util.stream.Stream} library. Read the method documentation below for
052 * comparisons. This class is not being deprecated, but we gently encourage you to migrate to
053 * streams.
054 *
055 * <p><i>Performance notes:</i> Unless otherwise noted, all of the iterables produced in this class
056 * are <i>lazy</i>, which means that their iterators only advance the backing iteration when
057 * absolutely necessary.
058 *
059 * <p>See the Guava User Guide article on <a href=
060 * "https://github.com/google/guava/wiki/CollectionUtilitiesExplained#iterables"> {@code
061 * Iterables}</a>.
062 *
063 * @author Kevin Bourrillion
064 * @author Jared Levy
065 * @since 2.0
066 */
067@GwtCompatible(emulated = true)
068@ElementTypesAreNonnullByDefault
069public final class Iterables {
070  private Iterables() {}
071
072  /** Returns an unmodifiable view of {@code iterable}. */
073  public static <T extends @Nullable Object> Iterable<T> unmodifiableIterable(
074      final Iterable<? extends T> iterable) {
075    checkNotNull(iterable);
076    if (iterable instanceof UnmodifiableIterable || iterable instanceof ImmutableCollection) {
077      @SuppressWarnings("unchecked") // Since it's unmodifiable, the covariant cast is safe
078      Iterable<T> result = (Iterable<T>) iterable;
079      return result;
080    }
081    return new UnmodifiableIterable<>(iterable);
082  }
083
084  /**
085   * Simply returns its argument.
086   *
087   * @deprecated no need to use this
088   * @since 10.0
089   */
090  @Deprecated
091  public static <E> Iterable<E> unmodifiableIterable(ImmutableCollection<E> iterable) {
092    return checkNotNull(iterable);
093  }
094
095  private static final class UnmodifiableIterable<T extends @Nullable Object>
096      extends FluentIterable<T> {
097    private final Iterable<? extends T> iterable;
098
099    private UnmodifiableIterable(Iterable<? extends T> iterable) {
100      this.iterable = iterable;
101    }
102
103    @Override
104    public Iterator<T> iterator() {
105      return Iterators.unmodifiableIterator(iterable.iterator());
106    }
107
108    @Override
109    public void forEach(Consumer<? super T> action) {
110      iterable.forEach(action);
111    }
112
113    @SuppressWarnings("unchecked") // safe upcast, assuming no one has a crazy Spliterator subclass
114    @Override
115    public Spliterator<T> spliterator() {
116      return (Spliterator<T>) iterable.spliterator();
117    }
118
119    @Override
120    public String toString() {
121      return iterable.toString();
122    }
123    // no equals and hashCode; it would break the contract!
124  }
125
126  /** Returns the number of elements in {@code iterable}. */
127  public static int size(Iterable<?> iterable) {
128    return (iterable instanceof Collection)
129        ? ((Collection<?>) iterable).size()
130        : Iterators.size(iterable.iterator());
131  }
132
133  /**
134   * Returns {@code true} if {@code iterable} contains any element {@code o} for which {@code
135   * Objects.equals(o, element)} would return {@code true}. Otherwise returns {@code false}, even in
136   * cases where {@link Collection#contains} might throw {@link NullPointerException} or {@link
137   * ClassCastException}.
138   */
139  // <? extends @Nullable Object> instead of <?> because of Kotlin b/189937072, discussed in Joiner.
140  public static boolean contains(
141      Iterable<? extends @Nullable Object> iterable, @CheckForNull Object element) {
142    if (iterable instanceof Collection) {
143      Collection<?> collection = (Collection<?>) iterable;
144      return Collections2.safeContains(collection, element);
145    }
146    return Iterators.contains(iterable.iterator(), element);
147  }
148
149  /**
150   * Removes, from an iterable, every element that belongs to the provided collection.
151   *
152   * <p>This method calls {@link Collection#removeAll} if {@code iterable} is a collection, and
153   * {@link Iterators#removeAll} otherwise.
154   *
155   * @param removeFrom the iterable to (potentially) remove elements from
156   * @param elementsToRemove the elements to remove
157   * @return {@code true} if any element was removed from {@code iterable}
158   */
159  @CanIgnoreReturnValue
160  public static boolean removeAll(Iterable<?> removeFrom, Collection<?> elementsToRemove) {
161    return (removeFrom instanceof Collection)
162        ? ((Collection<?>) removeFrom).removeAll(checkNotNull(elementsToRemove))
163        : Iterators.removeAll(removeFrom.iterator(), elementsToRemove);
164  }
165
166  /**
167   * Removes, from an iterable, every element that does not belong to the provided collection.
168   *
169   * <p>This method calls {@link Collection#retainAll} if {@code iterable} is a collection, and
170   * {@link Iterators#retainAll} otherwise.
171   *
172   * @param removeFrom the iterable to (potentially) remove elements from
173   * @param elementsToRetain the elements to retain
174   * @return {@code true} if any element was removed from {@code iterable}
175   */
176  @CanIgnoreReturnValue
177  public static boolean retainAll(Iterable<?> removeFrom, Collection<?> elementsToRetain) {
178    return (removeFrom instanceof Collection)
179        ? ((Collection<?>) removeFrom).retainAll(checkNotNull(elementsToRetain))
180        : Iterators.retainAll(removeFrom.iterator(), elementsToRetain);
181  }
182
183  /**
184   * Removes, from an iterable, every element that satisfies the provided predicate.
185   *
186   * <p>Removals may or may not happen immediately as each element is tested against the predicate.
187   * The behavior of this method is not specified if {@code predicate} is dependent on {@code
188   * removeFrom}.
189   *
190   * <p><b>Java 8 users:</b> if {@code removeFrom} is a {@link Collection}, use {@code
191   * removeFrom.removeIf(predicate)} instead.
192   *
193   * @param removeFrom the iterable to (potentially) remove elements from
194   * @param predicate a predicate that determines whether an element should be removed
195   * @return {@code true} if any elements were removed from the iterable
196   * @throws UnsupportedOperationException if the iterable does not support {@code remove()}.
197   * @since 2.0
198   */
199  @CanIgnoreReturnValue
200  public static <T extends @Nullable Object> boolean removeIf(
201      Iterable<T> removeFrom, Predicate<? super T> predicate) {
202    if (removeFrom instanceof Collection) {
203      return ((Collection<T>) removeFrom).removeIf(predicate);
204    }
205    return Iterators.removeIf(removeFrom.iterator(), predicate);
206  }
207
208  /** Removes and returns the first matching element, or returns {@code null} if there is none. */
209  @CheckForNull
210  static <T extends @Nullable Object> T removeFirstMatching(
211      Iterable<T> removeFrom, Predicate<? super T> predicate) {
212    checkNotNull(predicate);
213    Iterator<T> iterator = removeFrom.iterator();
214    while (iterator.hasNext()) {
215      T next = iterator.next();
216      if (predicate.apply(next)) {
217        iterator.remove();
218        return next;
219      }
220    }
221    return null;
222  }
223
224  /**
225   * Determines whether two iterables contain equal elements in the same order. More specifically,
226   * this method returns {@code true} if {@code iterable1} and {@code iterable2} contain the same
227   * number of elements and every element of {@code iterable1} is equal to the corresponding element
228   * of {@code iterable2}.
229   */
230  public static boolean elementsEqual(Iterable<?> iterable1, Iterable<?> iterable2) {
231    if (iterable1 instanceof Collection && iterable2 instanceof Collection) {
232      Collection<?> collection1 = (Collection<?>) iterable1;
233      Collection<?> collection2 = (Collection<?>) iterable2;
234      if (collection1.size() != collection2.size()) {
235        return false;
236      }
237    }
238    return Iterators.elementsEqual(iterable1.iterator(), iterable2.iterator());
239  }
240
241  /**
242   * Returns a string representation of {@code iterable}, with the format {@code [e1, e2, ..., en]}
243   * (that is, identical to {@link java.util.Arrays Arrays}{@code
244   * .toString(Iterables.toArray(iterable))}). Note that for <i>most</i> implementations of {@link
245   * Collection}, {@code collection.toString()} also gives the same result, but that behavior is not
246   * generally guaranteed.
247   */
248  public static String toString(Iterable<?> iterable) {
249    return Iterators.toString(iterable.iterator());
250  }
251
252  /**
253   * Returns the single element contained in {@code iterable}.
254   *
255   * <p><b>Java 8 users:</b> the {@code Stream} equivalent to this method is {@code
256   * stream.collect(MoreCollectors.onlyElement())}.
257   *
258   * @throws NoSuchElementException if the iterable is empty
259   * @throws IllegalArgumentException if the iterable contains multiple elements
260   */
261  @ParametricNullness
262  public static <T extends @Nullable Object> T getOnlyElement(Iterable<T> iterable) {
263    return Iterators.getOnlyElement(iterable.iterator());
264  }
265
266  /**
267   * Returns the single element contained in {@code iterable}, or {@code defaultValue} if the
268   * iterable is empty.
269   *
270   * <p><b>Java 8 users:</b> the {@code Stream} equivalent to this method is {@code
271   * stream.collect(MoreCollectors.toOptional()).orElse(defaultValue)}.
272   *
273   * @throws IllegalArgumentException if the iterator contains multiple elements
274   */
275  @ParametricNullness
276  public static <T extends @Nullable Object> T getOnlyElement(
277      Iterable<? extends T> iterable, @ParametricNullness T defaultValue) {
278    return Iterators.getOnlyElement(iterable.iterator(), defaultValue);
279  }
280
281  /**
282   * Copies an iterable's elements into an array.
283   *
284   * @param iterable the iterable to copy
285   * @param type the type of the elements
286   * @return a newly-allocated array into which all the elements of the iterable have been copied
287   */
288  @GwtIncompatible // Array.newInstance(Class, int)
289  /*
290   * If we could express Class<@Nonnull T>, we could generalize the type parameter to <T extends
291   * @Nullable Object>, and then we could accept an Iterable<? extends T> and return a plain T[]
292   * instead of a @Nullable T[].
293   */
294  public static <T> @Nullable T[] toArray(Iterable<? extends @Nullable T> iterable, Class<T> type) {
295    return toArray(iterable, ObjectArrays.newArray(type, 0));
296  }
297
298  static <T extends @Nullable Object> T[] toArray(Iterable<? extends T> iterable, T[] array) {
299    Collection<? extends T> collection = castOrCopyToCollection(iterable);
300    return collection.toArray(array);
301  }
302
303  /**
304   * Copies an iterable's elements into an array.
305   *
306   * @param iterable the iterable to copy
307   * @return a newly-allocated array into which all the elements of the iterable have been copied
308   */
309  static @Nullable Object[] toArray(Iterable<?> iterable) {
310    return castOrCopyToCollection(iterable).toArray();
311  }
312
313  /**
314   * Converts an iterable into a collection. If the iterable is already a collection, it is
315   * returned. Otherwise, an {@link java.util.ArrayList} is created with the contents of the
316   * iterable in the same iteration order.
317   */
318  private static <E extends @Nullable Object> Collection<E> castOrCopyToCollection(
319      Iterable<E> iterable) {
320    return (iterable instanceof Collection)
321        ? (Collection<E>) iterable
322        : Lists.newArrayList(iterable.iterator());
323  }
324
325  /**
326   * Adds all elements in {@code iterable} to {@code collection}.
327   *
328   * @return {@code true} if {@code collection} was modified as a result of this operation.
329   */
330  @CanIgnoreReturnValue
331  public static <T extends @Nullable Object> boolean addAll(
332      Collection<T> addTo, Iterable<? extends T> elementsToAdd) {
333    if (elementsToAdd instanceof Collection) {
334      Collection<? extends T> c = (Collection<? extends T>) elementsToAdd;
335      return addTo.addAll(c);
336    }
337    return Iterators.addAll(addTo, checkNotNull(elementsToAdd).iterator());
338  }
339
340  /**
341   * Returns the number of elements in the specified iterable that equal the specified object. This
342   * implementation avoids a full iteration when the iterable is a {@link Multiset} or {@link Set}.
343   *
344   * <p><b>Java 8 users:</b> In most cases, the {@code Stream} equivalent of this method is {@code
345   * stream.filter(element::equals).count()}. If {@code element} might be null, use {@code
346   * stream.filter(Predicate.isEqual(element)).count()} instead.
347   *
348   * @see java.util.Collections#frequency(Collection, Object) Collections.frequency(Collection,
349   *     Object)
350   */
351  public static int frequency(Iterable<?> iterable, @CheckForNull Object element) {
352    if ((iterable instanceof Multiset)) {
353      return ((Multiset<?>) iterable).count(element);
354    } else if ((iterable instanceof Set)) {
355      return ((Set<?>) iterable).contains(element) ? 1 : 0;
356    }
357    return Iterators.frequency(iterable.iterator(), element);
358  }
359
360  /**
361   * Returns an iterable whose iterators cycle indefinitely over the elements of {@code iterable}.
362   *
363   * <p>That iterator supports {@code remove()} if {@code iterable.iterator()} does. After {@code
364   * remove()} is called, subsequent cycles omit the removed element, which is no longer in {@code
365   * iterable}. The iterator's {@code hasNext()} method returns {@code true} until {@code iterable}
366   * is empty.
367   *
368   * <p><b>Warning:</b> Typical uses of the resulting iterator may produce an infinite loop. You
369   * should use an explicit {@code break} or be certain that you will eventually remove all the
370   * elements.
371   *
372   * <p>To cycle over the iterable {@code n} times, use the following: {@code
373   * Iterables.concat(Collections.nCopies(n, iterable))}
374   *
375   * <p><b>Java 8 users:</b> The {@code Stream} equivalent of this method is {@code
376   * Stream.generate(() -> iterable).flatMap(Streams::stream)}.
377   */
378  public static <T extends @Nullable Object> Iterable<T> cycle(final Iterable<T> iterable) {
379    checkNotNull(iterable);
380    return new FluentIterable<T>() {
381      @Override
382      public Iterator<T> iterator() {
383        return Iterators.cycle(iterable);
384      }
385
386      @Override
387      public Spliterator<T> spliterator() {
388        return Stream.generate(() -> iterable).<T>flatMap(Streams::stream).spliterator();
389      }
390
391      @Override
392      public String toString() {
393        return iterable.toString() + " (cycled)";
394      }
395    };
396  }
397
398  /**
399   * Returns an iterable whose iterators cycle indefinitely over the provided elements.
400   *
401   * <p>After {@code remove} is invoked on a generated iterator, the removed element will no longer
402   * appear in either that iterator or any other iterator created from the same source iterable.
403   * That is, this method behaves exactly as {@code Iterables.cycle(Lists.newArrayList(elements))}.
404   * The iterator's {@code hasNext} method returns {@code true} until all of the original elements
405   * have been removed.
406   *
407   * <p><b>Warning:</b> Typical uses of the resulting iterator may produce an infinite loop. You
408   * should use an explicit {@code break} or be certain that you will eventually remove all the
409   * elements.
410   *
411   * <p>To cycle over the elements {@code n} times, use the following: {@code
412   * Iterables.concat(Collections.nCopies(n, Arrays.asList(elements)))}
413   *
414   * <p><b>Java 8 users:</b> If passing a single element {@code e}, the {@code Stream} equivalent of
415   * this method is {@code Stream.generate(() -> e)}. Otherwise, put the elements in a collection
416   * and use {@code Stream.generate(() -> collection).flatMap(Collection::stream)}.
417   */
418  @SafeVarargs
419  public static <T extends @Nullable Object> Iterable<T> cycle(T... elements) {
420    return cycle(Lists.newArrayList(elements));
421  }
422
423  /**
424   * Combines two iterables into a single iterable. The returned iterable has an iterator that
425   * traverses the elements in {@code a}, followed by the elements in {@code b}. The source
426   * iterators are not polled until necessary.
427   *
428   * <p>The returned iterable's iterator supports {@code remove()} when the corresponding input
429   * iterator supports it.
430   *
431   * <p><b>Java 8 users:</b> The {@code Stream} equivalent of this method is {@code Stream.concat(a,
432   * b)}.
433   */
434  public static <T extends @Nullable Object> Iterable<T> concat(
435      Iterable<? extends T> a, Iterable<? extends T> b) {
436    return FluentIterable.concat(a, b);
437  }
438
439  /**
440   * Combines three iterables into a single iterable. The returned iterable has an iterator that
441   * traverses the elements in {@code a}, followed by the elements in {@code b}, followed by the
442   * elements in {@code c}. The source iterators are not polled until necessary.
443   *
444   * <p>The returned iterable's iterator supports {@code remove()} when the corresponding input
445   * iterator supports it.
446   *
447   * <p><b>Java 8 users:</b> The {@code Stream} equivalent of this method is {@code
448   * Streams.concat(a, b, c)}.
449   */
450  public static <T extends @Nullable Object> Iterable<T> concat(
451      Iterable<? extends T> a, Iterable<? extends T> b, Iterable<? extends T> c) {
452    return FluentIterable.concat(a, b, c);
453  }
454
455  /**
456   * Combines four iterables into a single iterable. The returned iterable has an iterator that
457   * traverses the elements in {@code a}, followed by the elements in {@code b}, followed by the
458   * elements in {@code c}, followed by the elements in {@code d}. The source iterators are not
459   * polled until necessary.
460   *
461   * <p>The returned iterable's iterator supports {@code remove()} when the corresponding input
462   * iterator supports it.
463   *
464   * <p><b>Java 8 users:</b> The {@code Stream} equivalent of this method is {@code
465   * Streams.concat(a, b, c, d)}.
466   */
467  public static <T extends @Nullable Object> Iterable<T> concat(
468      Iterable<? extends T> a,
469      Iterable<? extends T> b,
470      Iterable<? extends T> c,
471      Iterable<? extends T> d) {
472    return FluentIterable.concat(a, b, c, d);
473  }
474
475  /**
476   * Combines multiple iterables into a single iterable. The returned iterable has an iterator that
477   * traverses the elements of each iterable in {@code inputs}. The input iterators are not polled
478   * until necessary.
479   *
480   * <p>The returned iterable's iterator supports {@code remove()} when the corresponding input
481   * iterator supports it.
482   *
483   * <p><b>Java 8 users:</b> The {@code Stream} equivalent of this method is {@code
484   * Streams.concat(...)}.
485   *
486   * @throws NullPointerException if any of the provided iterables is null
487   */
488  @SafeVarargs
489  public static <T extends @Nullable Object> Iterable<T> concat(Iterable<? extends T>... inputs) {
490    return FluentIterable.concat(inputs);
491  }
492
493  /**
494   * Combines multiple iterables into a single iterable. The returned iterable has an iterator that
495   * traverses the elements of each iterable in {@code inputs}. The input iterators are not polled
496   * until necessary.
497   *
498   * <p>The returned iterable's iterator supports {@code remove()} when the corresponding input
499   * iterator supports it. The methods of the returned iterable may throw {@code
500   * NullPointerException} if any of the input iterators is null.
501   *
502   * <p><b>Java 8 users:</b> The {@code Stream} equivalent of this method is {@code
503   * streamOfStreams.flatMap(s -> s)}.
504   */
505  public static <T extends @Nullable Object> Iterable<T> concat(
506      Iterable<? extends Iterable<? extends T>> inputs) {
507    return FluentIterable.concat(inputs);
508  }
509
510  /**
511   * Divides an iterable into unmodifiable sublists of the given size (the final iterable may be
512   * smaller). For example, partitioning an iterable containing {@code [a, b, c, d, e]} with a
513   * partition size of 3 yields {@code [[a, b, c], [d, e]]} -- an outer iterable containing two
514   * inner lists of three and two elements, all in the original order.
515   *
516   * <p>Iterators returned by the returned iterable do not support the {@link Iterator#remove()}
517   * method. The returned lists implement {@link RandomAccess}, whether or not the input list does.
518   *
519   * <p><b>Note:</b> if {@code iterable} is a {@link List}, use {@link Lists#partition(List, int)}
520   * instead.
521   *
522   * @param iterable the iterable to return a partitioned view of
523   * @param size the desired size of each partition (the last may be smaller)
524   * @return an iterable of unmodifiable lists containing the elements of {@code iterable} divided
525   *     into partitions
526   * @throws IllegalArgumentException if {@code size} is nonpositive
527   */
528  public static <T extends @Nullable Object> Iterable<List<T>> partition(
529      final Iterable<T> iterable, final int size) {
530    checkNotNull(iterable);
531    checkArgument(size > 0);
532    return new FluentIterable<List<T>>() {
533      @Override
534      public Iterator<List<T>> iterator() {
535        return Iterators.partition(iterable.iterator(), size);
536      }
537    };
538  }
539
540  /**
541   * Divides an iterable into unmodifiable sublists of the given size, padding the final iterable
542   * with null values if necessary. For example, partitioning an iterable containing {@code [a, b,
543   * c, d, e]} with a partition size of 3 yields {@code [[a, b, c], [d, e, null]]} -- an outer
544   * iterable containing two inner lists of three elements each, all in the original order.
545   *
546   * <p>Iterators returned by the returned iterable do not support the {@link Iterator#remove()}
547   * method.
548   *
549   * @param iterable the iterable to return a partitioned view of
550   * @param size the desired size of each partition
551   * @return an iterable of unmodifiable lists containing the elements of {@code iterable} divided
552   *     into partitions (the final iterable may have trailing null elements)
553   * @throws IllegalArgumentException if {@code size} is nonpositive
554   */
555  public static <T extends @Nullable Object> Iterable<List<@Nullable T>> paddedPartition(
556      final Iterable<T> iterable, final int size) {
557    checkNotNull(iterable);
558    checkArgument(size > 0);
559    return new FluentIterable<List<@Nullable T>>() {
560      @Override
561      public Iterator<List<@Nullable T>> iterator() {
562        return Iterators.paddedPartition(iterable.iterator(), size);
563      }
564    };
565  }
566
567  /**
568   * Returns a view of {@code unfiltered} containing all elements that satisfy the input predicate
569   * {@code retainIfTrue}. The returned iterable's iterator does not support {@code remove()}.
570   *
571   * <p><b>{@code Stream} equivalent:</b> {@link Stream#filter}.
572   */
573  public static <T extends @Nullable Object> Iterable<T> filter(
574      final Iterable<T> unfiltered, final Predicate<? super T> retainIfTrue) {
575    checkNotNull(unfiltered);
576    checkNotNull(retainIfTrue);
577    return new FluentIterable<T>() {
578      @Override
579      public Iterator<T> iterator() {
580        return Iterators.filter(unfiltered.iterator(), retainIfTrue);
581      }
582
583      @Override
584      public void forEach(Consumer<? super T> action) {
585        checkNotNull(action);
586        unfiltered.forEach(
587            (@ParametricNullness T a) -> {
588              if (retainIfTrue.test(a)) {
589                action.accept(a);
590              }
591            });
592      }
593
594      @Override
595      public Spliterator<T> spliterator() {
596        return CollectSpliterators.filter(unfiltered.spliterator(), retainIfTrue);
597      }
598    };
599  }
600
601  /**
602   * Returns a view of {@code unfiltered} containing all elements that are of the type {@code
603   * desiredType}. The returned iterable's iterator does not support {@code remove()}.
604   *
605   * <p><b>{@code Stream} equivalent:</b> {@code stream.filter(type::isInstance).map(type::cast)}.
606   * This does perform a little more work than necessary, so another option is to insert an
607   * unchecked cast at some later point:
608   *
609   * <pre>
610   * {@code @SuppressWarnings("unchecked") // safe because of ::isInstance check
611   * ImmutableList<NewType> result =
612   *     (ImmutableList) stream.filter(NewType.class::isInstance).collect(toImmutableList());}
613   * </pre>
614   */
615  @SuppressWarnings("unchecked")
616  @GwtIncompatible // Class.isInstance
617  public static <T> Iterable<T> filter(final Iterable<?> unfiltered, final Class<T> desiredType) {
618    checkNotNull(unfiltered);
619    checkNotNull(desiredType);
620    return (Iterable<T>) filter(unfiltered, Predicates.instanceOf(desiredType));
621  }
622
623  /**
624   * Returns {@code true} if any element in {@code iterable} satisfies the predicate.
625   *
626   * <p><b>{@code Stream} equivalent:</b> {@link Stream#anyMatch}.
627   */
628  public static <T extends @Nullable Object> boolean any(
629      Iterable<T> iterable, Predicate<? super T> predicate) {
630    return Iterators.any(iterable.iterator(), predicate);
631  }
632
633  /**
634   * Returns {@code true} if every element in {@code iterable} satisfies the predicate. If {@code
635   * iterable} is empty, {@code true} is returned.
636   *
637   * <p><b>{@code Stream} equivalent:</b> {@link Stream#allMatch}.
638   */
639  public static <T extends @Nullable Object> boolean all(
640      Iterable<T> iterable, Predicate<? super T> predicate) {
641    return Iterators.all(iterable.iterator(), predicate);
642  }
643
644  /**
645   * Returns the first element in {@code iterable} that satisfies the given predicate; use this
646   * method only when such an element is known to exist. If it is possible that <i>no</i> element
647   * will match, use {@link #tryFind} or {@link #find(Iterable, Predicate, Object)} instead.
648   *
649   * <p><b>{@code Stream} equivalent:</b> {@code stream.filter(predicate).findFirst().get()}
650   *
651   * @throws NoSuchElementException if no element in {@code iterable} matches the given predicate
652   */
653  @ParametricNullness
654  public static <T extends @Nullable Object> T find(
655      Iterable<T> iterable, Predicate<? super T> predicate) {
656    return Iterators.find(iterable.iterator(), predicate);
657  }
658
659  /**
660   * Returns the first element in {@code iterable} that satisfies the given predicate, or {@code
661   * defaultValue} if none found. Note that this can usually be handled more naturally using {@code
662   * tryFind(iterable, predicate).or(defaultValue)}.
663   *
664   * <p><b>{@code Stream} equivalent:</b> {@code
665   * stream.filter(predicate).findFirst().orElse(defaultValue)}
666   *
667   * @since 7.0
668   */
669  // The signature we really want here is...
670  //
671  // <T extends @Nullable Object> @JointlyNullable T find(
672  //     Iterable<? extends T> iterable,
673  //     Predicate<? super T> predicate,
674  //     @JointlyNullable T defaultValue);
675  //
676  // ...where "@JointlyNullable" is similar to @PolyNull but slightly different:
677  //
678  // - @PolyNull means "@Nullable or @Nonnull"
679  //   (That would be unsound for an input Iterable<@Nullable Foo>. So, if we wanted to use
680  //   @PolyNull, we would have to restrict this method to non-null <T>. But it has users who pass
681  //   iterables with null elements.)
682  //
683  // - @JointlyNullable means "@Nullable or no annotation"
684  @CheckForNull
685  public static <T extends @Nullable Object> T find(
686      Iterable<? extends T> iterable,
687      Predicate<? super T> predicate,
688      @CheckForNull T defaultValue) {
689    return Iterators.find(iterable.iterator(), predicate, defaultValue);
690  }
691
692  /**
693   * Returns an {@link Optional} containing the first element in {@code iterable} that satisfies the
694   * given predicate, if such an element exists.
695   *
696   * <p><b>Warning:</b> avoid using a {@code predicate} that matches {@code null}. If {@code null}
697   * is matched in {@code iterable}, a NullPointerException will be thrown.
698   *
699   * <p><b>{@code Stream} equivalent:</b> {@code stream.filter(predicate).findFirst()}
700   *
701   * @since 11.0
702   */
703  public static <T> Optional<T> tryFind(Iterable<T> iterable, Predicate<? super T> predicate) {
704    return Iterators.tryFind(iterable.iterator(), predicate);
705  }
706
707  /**
708   * Returns the index in {@code iterable} of the first element that satisfies the provided {@code
709   * predicate}, or {@code -1} if the Iterable has no such elements.
710   *
711   * <p>More formally, returns the lowest index {@code i} such that {@code
712   * predicate.apply(Iterables.get(iterable, i))} returns {@code true}, or {@code -1} if there is no
713   * such index.
714   *
715   * @since 2.0
716   */
717  public static <T extends @Nullable Object> int indexOf(
718      Iterable<T> iterable, Predicate<? super T> predicate) {
719    return Iterators.indexOf(iterable.iterator(), predicate);
720  }
721
722  /**
723   * Returns a view containing the result of applying {@code function} to each element of {@code
724   * fromIterable}.
725   *
726   * <p>The returned iterable's iterator supports {@code remove()} if {@code fromIterable}'s
727   * iterator does. After a successful {@code remove()} call, {@code fromIterable} no longer
728   * contains the corresponding element.
729   *
730   * <p>If the input {@code Iterable} is known to be a {@code List} or other {@code Collection},
731   * consider {@link Lists#transform} and {@link Collections2#transform}.
732   *
733   * <p><b>{@code Stream} equivalent:</b> {@link Stream#map}
734   */
735  public static <F extends @Nullable Object, T extends @Nullable Object> Iterable<T> transform(
736      final Iterable<F> fromIterable, final Function<? super F, ? extends T> function) {
737    checkNotNull(fromIterable);
738    checkNotNull(function);
739    return new FluentIterable<T>() {
740      @Override
741      public Iterator<T> iterator() {
742        return Iterators.transform(fromIterable.iterator(), function);
743      }
744
745      @Override
746      public void forEach(Consumer<? super T> action) {
747        checkNotNull(action);
748        fromIterable.forEach((F f) -> action.accept(function.apply(f)));
749      }
750
751      @Override
752      public Spliterator<T> spliterator() {
753        return CollectSpliterators.map(fromIterable.spliterator(), function);
754      }
755    };
756  }
757
758  /**
759   * Returns the element at the specified position in an iterable.
760   *
761   * <p><b>{@code Stream} equivalent:</b> {@code stream.skip(position).findFirst().get()} (throws
762   * {@code NoSuchElementException} if out of bounds)
763   *
764   * @param position position of the element to return
765   * @return the element at the specified position in {@code iterable}
766   * @throws IndexOutOfBoundsException if {@code position} is negative or greater than or equal to
767   *     the size of {@code iterable}
768   */
769  @ParametricNullness
770  public static <T extends @Nullable Object> T get(Iterable<T> iterable, int position) {
771    checkNotNull(iterable);
772    return (iterable instanceof List)
773        ? ((List<T>) iterable).get(position)
774        : Iterators.get(iterable.iterator(), position);
775  }
776
777  /**
778   * Returns the element at the specified position in an iterable or a default value otherwise.
779   *
780   * <p><b>{@code Stream} equivalent:</b> {@code
781   * stream.skip(position).findFirst().orElse(defaultValue)} (returns the default value if the index
782   * is out of bounds)
783   *
784   * @param position position of the element to return
785   * @param defaultValue the default value to return if {@code position} is greater than or equal to
786   *     the size of the iterable
787   * @return the element at the specified position in {@code iterable} or {@code defaultValue} if
788   *     {@code iterable} contains fewer than {@code position + 1} elements.
789   * @throws IndexOutOfBoundsException if {@code position} is negative
790   * @since 4.0
791   */
792  @ParametricNullness
793  public static <T extends @Nullable Object> T get(
794      Iterable<? extends T> iterable, int position, @ParametricNullness T defaultValue) {
795    checkNotNull(iterable);
796    Iterators.checkNonnegative(position);
797    if (iterable instanceof List) {
798      List<? extends T> list = Lists.cast(iterable);
799      return (position < list.size()) ? list.get(position) : defaultValue;
800    } else {
801      Iterator<? extends T> iterator = iterable.iterator();
802      Iterators.advance(iterator, position);
803      return Iterators.getNext(iterator, defaultValue);
804    }
805  }
806
807  /**
808   * Returns the first element in {@code iterable} or {@code defaultValue} if the iterable is empty.
809   * The {@link Iterators} analog to this method is {@link Iterators#getNext}.
810   *
811   * <p>If no default value is desired (and the caller instead wants a {@link
812   * NoSuchElementException} to be thrown), it is recommended that {@code
813   * iterable.iterator().next()} is used instead.
814   *
815   * <p>To get the only element in a single-element {@code Iterable}, consider using {@link
816   * #getOnlyElement(Iterable)} or {@link #getOnlyElement(Iterable, Object)} instead.
817   *
818   * <p><b>{@code Stream} equivalent:</b> {@code stream.findFirst().orElse(defaultValue)}
819   *
820   * @param defaultValue the default value to return if the iterable is empty
821   * @return the first element of {@code iterable} or the default value
822   * @since 7.0
823   */
824  @ParametricNullness
825  public static <T extends @Nullable Object> T getFirst(
826      Iterable<? extends T> iterable, @ParametricNullness T defaultValue) {
827    return Iterators.getNext(iterable.iterator(), defaultValue);
828  }
829
830  /**
831   * Returns the last element of {@code iterable}. If {@code iterable} is a {@link List} with {@link
832   * RandomAccess} support, then this operation is guaranteed to be {@code O(1)}.
833   *
834   * <p><b>{@code Stream} equivalent:</b> {@link Streams#findLast Streams.findLast(stream).get()}
835   *
836   * @return the last element of {@code iterable}
837   * @throws NoSuchElementException if the iterable is empty
838   */
839  @ParametricNullness
840  public static <T extends @Nullable Object> T getLast(Iterable<T> iterable) {
841    // TODO(kevinb): Support a concurrently modified collection?
842    if (iterable instanceof List) {
843      List<T> list = (List<T>) iterable;
844      if (list.isEmpty()) {
845        throw new NoSuchElementException();
846      }
847      return getLastInNonemptyList(list);
848    }
849
850    return Iterators.getLast(iterable.iterator());
851  }
852
853  /**
854   * Returns the last element of {@code iterable} or {@code defaultValue} if the iterable is empty.
855   * If {@code iterable} is a {@link List} with {@link RandomAccess} support, then this operation is
856   * guaranteed to be {@code O(1)}.
857   *
858   * <p><b>{@code Stream} equivalent:</b> {@code Streams.findLast(stream).orElse(defaultValue)}
859   *
860   * @param defaultValue the value to return if {@code iterable} is empty
861   * @return the last element of {@code iterable} or the default value
862   * @since 3.0
863   */
864  @ParametricNullness
865  public static <T extends @Nullable Object> T getLast(
866      Iterable<? extends T> iterable, @ParametricNullness T defaultValue) {
867    if (iterable instanceof Collection) {
868      Collection<? extends T> c = (Collection<? extends T>) iterable;
869      if (c.isEmpty()) {
870        return defaultValue;
871      } else if (iterable instanceof List) {
872        return getLastInNonemptyList(Lists.cast(iterable));
873      }
874    }
875
876    return Iterators.getLast(iterable.iterator(), defaultValue);
877  }
878
879  @ParametricNullness
880  private static <T extends @Nullable Object> T getLastInNonemptyList(List<T> list) {
881    return list.get(list.size() - 1);
882  }
883
884  /**
885   * Returns a view of {@code iterable} that skips its first {@code numberToSkip} elements. If
886   * {@code iterable} contains fewer than {@code numberToSkip} elements, the returned iterable skips
887   * all of its elements.
888   *
889   * <p>Modifications to the underlying {@link Iterable} before a call to {@code iterator()} are
890   * reflected in the returned iterator. That is, the iterator skips the first {@code numberToSkip}
891   * elements that exist when the {@code Iterator} is created, not when {@code skip()} is called.
892   *
893   * <p>The returned iterable's iterator supports {@code remove()} if the iterator of the underlying
894   * iterable supports it. Note that it is <i>not</i> possible to delete the last skipped element by
895   * immediately calling {@code remove()} on that iterator, as the {@code Iterator} contract states
896   * that a call to {@code remove()} before a call to {@code next()} will throw an {@link
897   * IllegalStateException}.
898   *
899   * <p><b>{@code Stream} equivalent:</b> {@link Stream#skip}
900   *
901   * @since 3.0
902   */
903  public static <T extends @Nullable Object> Iterable<T> skip(
904      final Iterable<T> iterable, final int numberToSkip) {
905    checkNotNull(iterable);
906    checkArgument(numberToSkip >= 0, "number to skip cannot be negative");
907
908    return new FluentIterable<T>() {
909      @Override
910      public Iterator<T> iterator() {
911        if (iterable instanceof List) {
912          final List<T> list = (List<T>) iterable;
913          int toSkip = Math.min(list.size(), numberToSkip);
914          return list.subList(toSkip, list.size()).iterator();
915        }
916        final Iterator<T> iterator = iterable.iterator();
917
918        Iterators.advance(iterator, numberToSkip);
919
920        /*
921         * We can't just return the iterator because an immediate call to its
922         * remove() method would remove one of the skipped elements instead of
923         * throwing an IllegalStateException.
924         */
925        return new Iterator<T>() {
926          boolean atStart = true;
927
928          @Override
929          public boolean hasNext() {
930            return iterator.hasNext();
931          }
932
933          @Override
934          @ParametricNullness
935          public T next() {
936            T result = iterator.next();
937            atStart = false; // not called if next() fails
938            return result;
939          }
940
941          @Override
942          public void remove() {
943            checkRemove(!atStart);
944            iterator.remove();
945          }
946        };
947      }
948
949      @Override
950      public Spliterator<T> spliterator() {
951        if (iterable instanceof List) {
952          final List<T> list = (List<T>) iterable;
953          int toSkip = Math.min(list.size(), numberToSkip);
954          return list.subList(toSkip, list.size()).spliterator();
955        } else {
956          return Streams.stream(iterable).skip(numberToSkip).spliterator();
957        }
958      }
959    };
960  }
961
962  /**
963   * Returns a view of {@code iterable} containing its first {@code limitSize} elements. If {@code
964   * iterable} contains fewer than {@code limitSize} elements, the returned view contains all of its
965   * elements. The returned iterable's iterator supports {@code remove()} if {@code iterable}'s
966   * iterator does.
967   *
968   * <p><b>{@code Stream} equivalent:</b> {@link Stream#limit}
969   *
970   * @param iterable the iterable to limit
971   * @param limitSize the maximum number of elements in the returned iterable
972   * @throws IllegalArgumentException if {@code limitSize} is negative
973   * @since 3.0
974   */
975  public static <T extends @Nullable Object> Iterable<T> limit(
976      final Iterable<T> iterable, final int limitSize) {
977    checkNotNull(iterable);
978    checkArgument(limitSize >= 0, "limit is negative");
979    return new FluentIterable<T>() {
980      @Override
981      public Iterator<T> iterator() {
982        return Iterators.limit(iterable.iterator(), limitSize);
983      }
984
985      @Override
986      public Spliterator<T> spliterator() {
987        return Streams.stream(iterable).limit(limitSize).spliterator();
988      }
989    };
990  }
991
992  /**
993   * Returns a view of the supplied iterable that wraps each generated {@link Iterator} through
994   * {@link Iterators#consumingIterator(Iterator)}.
995   *
996   * <p>Note: If {@code iterable} is a {@link Queue}, the returned iterable will get entries from
997   * {@link Queue#remove()} since {@link Queue}'s iteration order is undefined. Calling {@link
998   * Iterator#hasNext()} on a generated iterator from the returned iterable may cause an item to be
999   * immediately dequeued for return on a subsequent call to {@link Iterator#next()}.
1000   *
1001   * @param iterable the iterable to wrap
1002   * @return a view of the supplied iterable that wraps each generated iterator through {@link
1003   *     Iterators#consumingIterator(Iterator)}; for queues, an iterable that generates iterators
1004   *     that return and consume the queue's elements in queue order
1005   * @see Iterators#consumingIterator(Iterator)
1006   * @since 2.0
1007   */
1008  public static <T extends @Nullable Object> Iterable<T> consumingIterable(
1009      final Iterable<T> iterable) {
1010    checkNotNull(iterable);
1011
1012    return new FluentIterable<T>() {
1013      @Override
1014      public Iterator<T> iterator() {
1015        return (iterable instanceof Queue)
1016            ? new ConsumingQueueIterator<>((Queue<T>) iterable)
1017            : Iterators.consumingIterator(iterable.iterator());
1018      }
1019
1020      @Override
1021      public String toString() {
1022        return "Iterables.consumingIterable(...)";
1023      }
1024    };
1025  }
1026
1027  // Methods only in Iterables, not in Iterators
1028
1029  /**
1030   * Determines if the given iterable contains no elements.
1031   *
1032   * <p>There is no precise {@link Iterator} equivalent to this method, since one can only ask an
1033   * iterator whether it has any elements <i>remaining</i> (which one does using {@link
1034   * Iterator#hasNext}).
1035   *
1036   * <p><b>{@code Stream} equivalent:</b> {@code !stream.findAny().isPresent()}
1037   *
1038   * @return {@code true} if the iterable contains no elements
1039   */
1040  public static boolean isEmpty(Iterable<?> iterable) {
1041    if (iterable instanceof Collection) {
1042      return ((Collection<?>) iterable).isEmpty();
1043    }
1044    return !iterable.iterator().hasNext();
1045  }
1046
1047  /**
1048   * Returns an iterable over the merged contents of all given {@code iterables}. Equivalent entries
1049   * will not be de-duplicated.
1050   *
1051   * <p>Callers must ensure that the source {@code iterables} are in non-descending order as this
1052   * method does not sort its input.
1053   *
1054   * <p>For any equivalent elements across all {@code iterables}, it is undefined which element is
1055   * returned first.
1056   *
1057   * @since 11.0
1058   */
1059  @Beta
1060  public static <T extends @Nullable Object> Iterable<T> mergeSorted(
1061      final Iterable<? extends Iterable<? extends T>> iterables,
1062      final Comparator<? super T> comparator) {
1063    checkNotNull(iterables, "iterables");
1064    checkNotNull(comparator, "comparator");
1065    Iterable<T> iterable =
1066        new FluentIterable<T>() {
1067          @Override
1068          public Iterator<T> iterator() {
1069            return Iterators.mergeSorted(
1070                Iterables.transform(iterables, Iterables.<T>toIterator()), comparator);
1071          }
1072        };
1073    return new UnmodifiableIterable<>(iterable);
1074  }
1075
1076  // TODO(user): Is this the best place for this? Move to fluent functions?
1077  // Useful as a public method?
1078  static <T extends @Nullable Object>
1079      Function<Iterable<? extends T>, Iterator<? extends T>> toIterator() {
1080    return new Function<Iterable<? extends T>, Iterator<? extends T>>() {
1081      @Override
1082      public Iterator<? extends T> apply(Iterable<? extends T> iterable) {
1083        return iterable.iterator();
1084      }
1085    };
1086  }
1087}