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 org.checkerframework.checker.nullness.qual.NonNull;
039import org.checkerframework.checker.nullness.qual.Nullable;
040
041/**
042 * An assortment of mainly legacy static utility methods that operate on or return objects of type
043 * {@code Iterable}. Except as noted, each method has a corresponding {@link Iterator}-based method
044 * in the {@link Iterators} class.
045 *
046 * <p><b>Java 8+ users:</b> several common uses for this class are now more comprehensively
047 * addressed by the new {@link java.util.stream.Stream} library. Read the method documentation below
048 * for comparisons. This class is not being deprecated, but we gently encourage you to migrate to
049 * streams.
050 *
051 * <p><i>Performance notes:</i> Unless otherwise noted, all of the iterables produced in this class
052 * are <i>lazy</i>, which means that their iterators only advance the backing iteration when
053 * absolutely necessary.
054 *
055 * <p>See the Guava User Guide article on <a href=
056 * "https://github.com/google/guava/wiki/CollectionUtilitiesExplained#iterables">{@code
057 * Iterables}</a>.
058 *
059 * @author Kevin Bourrillion
060 * @author Jared Levy
061 * @since 2.0
062 */
063@GwtCompatible(emulated = true)
064public final class Iterables {
065  private Iterables() {}
066
067  /** Returns an unmodifiable view of {@code iterable}. */
068  public static <T extends @Nullable Object> Iterable<T> unmodifiableIterable(
069      final Iterable<? extends T> iterable) {
070    checkNotNull(iterable);
071    if (iterable instanceof UnmodifiableIterable || iterable instanceof ImmutableCollection) {
072      @SuppressWarnings("unchecked") // Since it's unmodifiable, the covariant cast is safe
073      Iterable<T> result = (Iterable<T>) iterable;
074      return result;
075    }
076    return new UnmodifiableIterable<>(iterable);
077  }
078
079  /**
080   * Simply returns its argument.
081   *
082   * @deprecated no need to use this
083   * @since 10.0
084   */
085  @Deprecated
086  public static <E> Iterable<E> unmodifiableIterable(ImmutableCollection<E> iterable) {
087    return checkNotNull(iterable);
088  }
089
090  private static final class UnmodifiableIterable<T extends @Nullable Object>
091      extends FluentIterable<T> {
092    private final Iterable<? extends T> iterable;
093
094    private UnmodifiableIterable(Iterable<? extends T> iterable) {
095      this.iterable = iterable;
096    }
097
098    @Override
099    public Iterator<T> iterator() {
100      return Iterators.unmodifiableIterator(iterable.iterator());
101    }
102
103    @Override
104    public String toString() {
105      return iterable.toString();
106    }
107    // no equals and hashCode; it would break the contract!
108  }
109
110  /** Returns the number of elements in {@code iterable}. */
111  public static int size(Iterable<?> iterable) {
112    return (iterable instanceof Collection)
113        ? ((Collection<?>) iterable).size()
114        : Iterators.size(iterable.iterator());
115  }
116
117  /**
118   * Returns {@code true} if {@code iterable} contains any element {@code o} for which {@code
119   * Objects.equals(o, element)} would return {@code true}. Otherwise returns {@code false}, even in
120   * cases where {@link Collection#contains} might throw {@link NullPointerException} or {@link
121   * ClassCastException}.
122   */
123  // <? extends @Nullable Object> instead of <?> because of Kotlin b/189937072, discussed in Joiner.
124  public static boolean contains(
125      Iterable<? extends @Nullable Object> iterable, @Nullable Object element) {
126    if (iterable instanceof Collection) {
127      Collection<?> collection = (Collection<?>) iterable;
128      return Collections2.safeContains(collection, element);
129    }
130    return Iterators.contains(iterable.iterator(), element);
131  }
132
133  /**
134   * Removes, from an iterable, every element that belongs to the provided collection.
135   *
136   * <p>This method calls {@link Collection#removeAll} if {@code iterable} is a collection, and
137   * {@link Iterators#removeAll} otherwise.
138   *
139   * @param removeFrom the iterable to (potentially) remove elements from
140   * @param elementsToRemove the elements to remove
141   * @return {@code true} if any element was removed from {@code iterable}
142   */
143  @CanIgnoreReturnValue
144  public static boolean removeAll(Iterable<?> removeFrom, Collection<?> elementsToRemove) {
145    return (removeFrom instanceof Collection)
146        ? ((Collection<?>) removeFrom).removeAll(checkNotNull(elementsToRemove))
147        : Iterators.removeAll(removeFrom.iterator(), elementsToRemove);
148  }
149
150  /**
151   * Removes, from an iterable, every element that does not belong to the provided collection.
152   *
153   * <p>This method calls {@link Collection#retainAll} if {@code iterable} is a collection, and
154   * {@link Iterators#retainAll} otherwise.
155   *
156   * @param removeFrom the iterable to (potentially) remove elements from
157   * @param elementsToRetain the elements to retain
158   * @return {@code true} if any element was removed from {@code iterable}
159   */
160  @CanIgnoreReturnValue
161  public static boolean retainAll(Iterable<?> removeFrom, Collection<?> elementsToRetain) {
162    return (removeFrom instanceof Collection)
163        ? ((Collection<?>) removeFrom).retainAll(checkNotNull(elementsToRetain))
164        : Iterators.retainAll(removeFrom.iterator(), elementsToRetain);
165  }
166
167  /**
168   * Removes, from an iterable, every element that satisfies the provided predicate.
169   *
170   * <p>Removals may or may not happen immediately as each element is tested against the predicate.
171   * The behavior of this method is not specified if {@code predicate} is dependent on {@code
172   * removeFrom}.
173   *
174   * <p><b>Java 8+ users:</b> if {@code removeFrom} is a {@link Collection}, use {@code
175   * removeFrom.removeIf(predicate)} instead.
176   *
177   * @param removeFrom the iterable to (potentially) remove elements from
178   * @param predicate a predicate that determines whether an element should be removed
179   * @return {@code true} if any elements were removed from the iterable
180   * @throws UnsupportedOperationException if the iterable does not support {@code remove()}.
181   * @since 2.0
182   */
183  @CanIgnoreReturnValue
184  public static <T extends @Nullable Object> boolean removeIf(
185      Iterable<T> removeFrom, Predicate<? super T> predicate) {
186    if (removeFrom instanceof RandomAccess && removeFrom instanceof List) {
187      return removeIfFromRandomAccessList((List<T>) removeFrom, checkNotNull(predicate));
188    }
189    return Iterators.removeIf(removeFrom.iterator(), predicate);
190  }
191
192  private static <T extends @Nullable Object> boolean removeIfFromRandomAccessList(
193      List<T> list, Predicate<? super T> predicate) {
194    // Note: Not all random access lists support set(). Additionally, it's possible
195    // for a list to reject setting an element, such as when the list does not permit
196    // duplicate elements. For both of those cases,  we need to fall back to a slower
197    // implementation.
198    int from = 0;
199    int to = 0;
200
201    for (; from < list.size(); from++) {
202      T element = list.get(from);
203      if (!predicate.apply(element)) {
204        if (from > to) {
205          try {
206            list.set(to, element);
207          } catch (UnsupportedOperationException e) {
208            slowRemoveIfForRemainingElements(list, predicate, to, from);
209            return true;
210          } catch (IllegalArgumentException e) {
211            slowRemoveIfForRemainingElements(list, predicate, to, from);
212            return true;
213          }
214        }
215        to++;
216      }
217    }
218
219    // Clear the tail of any remaining items
220    list.subList(to, list.size()).clear();
221    return from != to;
222  }
223
224  private static <T extends @Nullable Object> void slowRemoveIfForRemainingElements(
225      List<T> list, Predicate<? super T> predicate, int to, int from) {
226    // Here we know that:
227    // * (to < from) and that both are valid indices.
228    // * Everything with (index < to) should be kept.
229    // * Everything with (to <= index < from) should be removed.
230    // * The element with (index == from) should be kept.
231    // * Everything with (index > from) has not been checked yet.
232
233    // Check from the end of the list backwards (minimize expected cost of
234    // moving elements when remove() is called). Stop before 'from' because
235    // we already know that should be kept.
236    for (int n = list.size() - 1; n > from; n--) {
237      if (predicate.apply(list.get(n))) {
238        list.remove(n);
239      }
240    }
241    // And now remove everything in the range [to, from) (going backwards).
242    for (int n = from - 1; n >= to; n--) {
243      list.remove(n);
244    }
245  }
246
247  /** Removes and returns the first matching element, or returns {@code null} if there is none. */
248  static <T extends @Nullable Object> @Nullable T removeFirstMatching(
249      Iterable<T> removeFrom, Predicate<? super T> predicate) {
250    checkNotNull(predicate);
251    Iterator<T> iterator = removeFrom.iterator();
252    while (iterator.hasNext()) {
253      T next = iterator.next();
254      if (predicate.apply(next)) {
255        iterator.remove();
256        return next;
257      }
258    }
259    return null;
260  }
261
262  /**
263   * Determines whether two iterables contain equal elements in the same order. More specifically,
264   * this method returns {@code true} if {@code iterable1} and {@code iterable2} contain the same
265   * number of elements and every element of {@code iterable1} is equal to the corresponding element
266   * of {@code iterable2}.
267   */
268  public static boolean elementsEqual(Iterable<?> iterable1, Iterable<?> iterable2) {
269    if (iterable1 instanceof Collection && iterable2 instanceof Collection) {
270      Collection<?> collection1 = (Collection<?>) iterable1;
271      Collection<?> collection2 = (Collection<?>) iterable2;
272      if (collection1.size() != collection2.size()) {
273        return false;
274      }
275    }
276    return Iterators.elementsEqual(iterable1.iterator(), iterable2.iterator());
277  }
278
279  /**
280   * Returns a string representation of {@code iterable}, with the format {@code [e1, e2, ..., en]}
281   * (that is, identical to {@link java.util.Arrays Arrays}{@code
282   * .toString(Iterables.toArray(iterable))}). Note that for <i>most</i> implementations of {@link
283   * Collection}, {@code collection.toString()} also gives the same result, but that behavior is not
284   * generally guaranteed.
285   */
286  public static String toString(Iterable<?> iterable) {
287    return Iterators.toString(iterable.iterator());
288  }
289
290  /**
291   * Returns the single element contained in {@code iterable}.
292   *
293   * <p><b>Java 8+ users:</b> the {@code Stream} equivalent to this method is {@code
294   * stream.collect(MoreCollectors.onlyElement())}.
295   *
296   * @throws NoSuchElementException if the iterable is empty
297   * @throws IllegalArgumentException if the iterable contains multiple elements
298   */
299  @ParametricNullness
300  public static <T extends @Nullable Object> T getOnlyElement(Iterable<T> iterable) {
301    return Iterators.getOnlyElement(iterable.iterator());
302  }
303
304  /**
305   * Returns the single element contained in {@code iterable}, or {@code defaultValue} if the
306   * iterable is empty.
307   *
308   * <p><b>Java 8+ users:</b> the {@code Stream} equivalent to this method is {@code
309   * stream.collect(MoreCollectors.toOptional()).orElse(defaultValue)}.
310   *
311   * @throws IllegalArgumentException if the iterator contains multiple elements
312   */
313  @ParametricNullness
314  public static <T extends @Nullable Object> T getOnlyElement(
315      Iterable<? extends T> iterable, @ParametricNullness T defaultValue) {
316    return Iterators.getOnlyElement(iterable.iterator(), defaultValue);
317  }
318
319  /**
320   * Copies an iterable's elements into an array.
321   *
322   * @param iterable the iterable to copy
323   * @param type the type of the elements
324   * @return a newly-allocated array into which all the elements of the iterable have been copied
325   */
326  @GwtIncompatible // Array.newInstance(Class, int)
327  public static <T extends @Nullable Object> T[] toArray(
328      Iterable<? extends T> iterable, Class<@NonNull T> type) {
329    return toArray(iterable, ObjectArrays.newArray(type, 0));
330  }
331
332  static <T extends @Nullable Object> T[] toArray(Iterable<? extends T> iterable, T[] array) {
333    Collection<? extends T> collection = castOrCopyToCollection(iterable);
334    return collection.toArray(array);
335  }
336
337  /**
338   * Copies an iterable's elements into an array.
339   *
340   * @param iterable the iterable to copy
341   * @return a newly-allocated array into which all the elements of the iterable have been copied
342   */
343  static @Nullable Object[] toArray(Iterable<?> iterable) {
344    return castOrCopyToCollection(iterable).toArray();
345  }
346
347  /**
348   * Converts an iterable into a collection. If the iterable is already a collection, it is
349   * returned. Otherwise, an {@link java.util.ArrayList} is created with the contents of the
350   * iterable in the same iteration order.
351   */
352  private static <E extends @Nullable Object> Collection<E> castOrCopyToCollection(
353      Iterable<E> iterable) {
354    return (iterable instanceof Collection)
355        ? (Collection<E>) iterable
356        : Lists.newArrayList(iterable.iterator());
357  }
358
359  /**
360   * Adds all elements in {@code iterable} to {@code collection}.
361   *
362   * @return {@code true} if {@code collection} was modified as a result of this operation.
363   */
364  @CanIgnoreReturnValue
365  public static <T extends @Nullable Object> boolean addAll(
366      Collection<T> addTo, Iterable<? extends T> elementsToAdd) {
367    if (elementsToAdd instanceof Collection) {
368      Collection<? extends T> c = (Collection<? extends T>) elementsToAdd;
369      return addTo.addAll(c);
370    }
371    return Iterators.addAll(addTo, checkNotNull(elementsToAdd).iterator());
372  }
373
374  /**
375   * Returns the number of elements in the specified iterable that equal the specified object. This
376   * implementation avoids a full iteration when the iterable is a {@link Multiset} or {@link Set}.
377   *
378   * <p><b>Java 8+ users:</b> In most cases, the {@code Stream} equivalent of this method is {@code
379   * stream.filter(element::equals).count()}. If {@code element} might be null, use {@code
380   * stream.filter(Predicate.isEqual(element)).count()} instead.
381   *
382   * @see java.util.Collections#frequency(Collection, Object) Collections.frequency(Collection,
383   *     Object)
384   */
385  public static int frequency(Iterable<?> iterable, @Nullable Object element) {
386    if ((iterable instanceof Multiset)) {
387      return ((Multiset<?>) iterable).count(element);
388    } else if ((iterable instanceof Set)) {
389      return ((Set<?>) iterable).contains(element) ? 1 : 0;
390    }
391    return Iterators.frequency(iterable.iterator(), element);
392  }
393
394  /**
395   * Returns an iterable whose iterators cycle indefinitely over the elements of {@code iterable}.
396   *
397   * <p>That iterator supports {@code remove()} if {@code iterable.iterator()} does. After {@code
398   * remove()} is called, subsequent cycles omit the removed element, which is no longer in {@code
399   * iterable}. The iterator's {@code hasNext()} method returns {@code true} until {@code iterable}
400   * is empty.
401   *
402   * <p><b>Warning:</b> Typical uses of the resulting iterator may produce an infinite loop. You
403   * should use an explicit {@code break} or be certain that you will eventually remove all the
404   * elements.
405   *
406   * <p>To cycle over the iterable {@code n} times, use the following: {@code
407   * Iterables.concat(Collections.nCopies(n, iterable))}
408   *
409   * <p><b>Java 8+ users:</b> The {@code Stream} equivalent of this method is {@code
410   * Stream.generate(() -> iterable).flatMap(Streams::stream)}.
411   */
412  public static <T extends @Nullable Object> Iterable<T> cycle(final Iterable<T> iterable) {
413    checkNotNull(iterable);
414    return new FluentIterable<T>() {
415      @Override
416      public Iterator<T> iterator() {
417        return Iterators.cycle(iterable);
418      }
419
420      @Override
421      public String toString() {
422        return iterable.toString() + " (cycled)";
423      }
424    };
425  }
426
427  /**
428   * Returns an iterable whose iterators cycle indefinitely over the provided elements.
429   *
430   * <p>After {@code remove} is invoked on a generated iterator, the removed element will no longer
431   * appear in either that iterator or any other iterator created from the same source iterable.
432   * That is, this method behaves exactly as {@code Iterables.cycle(Lists.newArrayList(elements))}.
433   * The iterator's {@code hasNext} method returns {@code true} until all of the original elements
434   * have been removed.
435   *
436   * <p><b>Warning:</b> Typical uses of the resulting iterator may produce an infinite loop. You
437   * should use an explicit {@code break} or be certain that you will eventually remove all the
438   * elements.
439   *
440   * <p>To cycle over the elements {@code n} times, use the following: {@code
441   * Iterables.concat(Collections.nCopies(n, Arrays.asList(elements)))}
442   *
443   * <p><b>Java 8+ users:</b> If passing a single element {@code e}, the {@code Stream} equivalent
444   * of this method is {@code Stream.generate(() -> e)}. Otherwise, put the elements in a collection
445   * and use {@code Stream.generate(() -> collection).flatMap(Collection::stream)}.
446   */
447  @SafeVarargs
448  public static <T extends @Nullable Object> Iterable<T> cycle(T... elements) {
449    return cycle(Lists.newArrayList(elements));
450  }
451
452  /**
453   * Combines two iterables into a single iterable. The returned iterable has an iterator that
454   * traverses the elements in {@code a}, followed by the elements in {@code b}. The source
455   * iterators are not polled until necessary.
456   *
457   * <p>The returned iterable's iterator supports {@code remove()} when the corresponding input
458   * iterator supports it.
459   *
460   * <p><b>Java 8+ users:</b> The {@code Stream} equivalent of this method is {@code
461   * Stream.concat(a, b)}.
462   */
463  public static <T extends @Nullable Object> Iterable<T> concat(
464      Iterable<? extends T> a, Iterable<? extends T> b) {
465    return FluentIterable.concat(a, b);
466  }
467
468  /**
469   * Combines three iterables into a single iterable. The returned iterable has an iterator that
470   * traverses the elements in {@code a}, followed by the elements in {@code b}, followed by the
471   * elements in {@code c}. The source iterators are not polled until necessary.
472   *
473   * <p>The returned iterable's iterator supports {@code remove()} when the corresponding input
474   * iterator supports it.
475   *
476   * <p><b>Java 8+ users:</b> The {@code Stream} equivalent of this method is {@code
477   * Streams.concat(a, b, c)}.
478   */
479  public static <T extends @Nullable Object> Iterable<T> concat(
480      Iterable<? extends T> a, Iterable<? extends T> b, Iterable<? extends T> c) {
481    return FluentIterable.concat(a, b, c);
482  }
483
484  /**
485   * Combines four iterables into a single iterable. The returned iterable has an iterator that
486   * traverses the elements in {@code a}, followed by the elements in {@code b}, followed by the
487   * elements in {@code c}, followed by the elements in {@code d}. The source iterators are not
488   * polled until necessary.
489   *
490   * <p>The returned iterable's iterator supports {@code remove()} when the corresponding input
491   * iterator supports it.
492   *
493   * <p><b>Java 8+ users:</b> The {@code Stream} equivalent of this method is {@code
494   * Streams.concat(a, b, c, d)}.
495   */
496  public static <T extends @Nullable Object> Iterable<T> concat(
497      Iterable<? extends T> a,
498      Iterable<? extends T> b,
499      Iterable<? extends T> c,
500      Iterable<? extends T> d) {
501    return FluentIterable.concat(a, b, c, d);
502  }
503
504  /**
505   * Combines multiple iterables into a single iterable. The returned iterable has an iterator that
506   * traverses the elements of each iterable in {@code inputs}. The input iterators are not polled
507   * until necessary.
508   *
509   * <p>The returned iterable's iterator supports {@code remove()} when the corresponding input
510   * iterator supports it.
511   *
512   * <p><b>Java 8+ users:</b> The {@code Stream} equivalent of this method is {@code
513   * Streams.concat(...)}.
514   *
515   * @throws NullPointerException if any of the provided iterables is null
516   */
517  @SafeVarargs
518  public static <T extends @Nullable Object> Iterable<T> concat(Iterable<? extends T>... inputs) {
519    return FluentIterable.concat(inputs);
520  }
521
522  /**
523   * Combines multiple iterables into a single iterable. The returned iterable has an iterator that
524   * traverses the elements of each iterable in {@code inputs}. The input iterators are not polled
525   * until necessary.
526   *
527   * <p>The returned iterable's iterator supports {@code remove()} when the corresponding input
528   * iterator supports it. The methods of the returned iterable may throw {@code
529   * NullPointerException} if any of the input iterators is null.
530   *
531   * <p><b>Java 8+ users:</b> The {@code Stream} equivalent of this method is {@code
532   * streamOfStreams.flatMap(s -> s)}.
533   */
534  public static <T extends @Nullable Object> Iterable<T> concat(
535      Iterable<? extends Iterable<? extends T>> inputs) {
536    return FluentIterable.concat(inputs);
537  }
538
539  /**
540   * Divides an iterable into unmodifiable sublists of the given size (the final iterable may be
541   * smaller). For example, partitioning an iterable containing {@code [a, b, c, d, e]} with a
542   * partition size of 3 yields {@code [[a, b, c], [d, e]]} -- an outer iterable containing two
543   * inner lists of three and two elements, all in the original order.
544   *
545   * <p>Iterators returned by the returned iterable do not support the {@link Iterator#remove()}
546   * method. The returned lists implement {@link RandomAccess}, whether or not the input list does.
547   *
548   * <p><b>Note:</b> The current implementation eagerly allocates storage for {@code size} elements.
549   * As a consequence, passing values like {@code Integer.MAX_VALUE} can lead to {@link
550   * OutOfMemoryError}.
551   *
552   * <p><b>Note:</b> if {@code iterable} is a {@link List}, use {@link Lists#partition(List, int)}
553   * instead.
554   *
555   * @param iterable the iterable to return a partitioned view of
556   * @param size the desired size of each partition (the last may be smaller)
557   * @return an iterable of unmodifiable lists containing the elements of {@code iterable} divided
558   *     into partitions
559   * @throws IllegalArgumentException if {@code size} is nonpositive
560   */
561  public static <T extends @Nullable Object> Iterable<List<T>> partition(
562      final Iterable<T> iterable, final int size) {
563    checkNotNull(iterable);
564    checkArgument(size > 0);
565    return new FluentIterable<List<T>>() {
566      @Override
567      public Iterator<List<T>> iterator() {
568        return Iterators.partition(iterable.iterator(), size);
569      }
570    };
571  }
572
573  /**
574   * Divides an iterable into unmodifiable sublists of the given size, padding the final iterable
575   * with null values if necessary. For example, partitioning an iterable containing {@code [a, b,
576   * c, d, e]} with a partition size of 3 yields {@code [[a, b, c], [d, e, null]]} -- an outer
577   * iterable containing two inner lists of three elements each, all in the original order.
578   *
579   * <p>Iterators returned by the returned iterable do not support the {@link Iterator#remove()}
580   * method.
581   *
582   * @param iterable the iterable to return a partitioned view of
583   * @param size the desired size of each partition
584   * @return an iterable of unmodifiable lists containing the elements of {@code iterable} divided
585   *     into partitions (the final iterable may have trailing null elements)
586   * @throws IllegalArgumentException if {@code size} is nonpositive
587   */
588  public static <T extends @Nullable Object> Iterable<List<@Nullable T>> paddedPartition(
589      final Iterable<T> iterable, final int size) {
590    checkNotNull(iterable);
591    checkArgument(size > 0);
592    return new FluentIterable<List<@Nullable T>>() {
593      @Override
594      public Iterator<List<@Nullable T>> iterator() {
595        return Iterators.paddedPartition(iterable.iterator(), size);
596      }
597    };
598  }
599
600  /**
601   * Returns a view of {@code unfiltered} containing all elements that satisfy the input predicate
602   * {@code retainIfTrue}. The returned iterable's iterator does not support {@code remove()}.
603   *
604   * <p><b>{@code Stream} equivalent:</b> {@link Stream#filter}.
605   */
606  public static <T extends @Nullable Object> Iterable<T> filter(
607      final Iterable<T> unfiltered, final Predicate<? super T> retainIfTrue) {
608    checkNotNull(unfiltered);
609    checkNotNull(retainIfTrue);
610    return new FluentIterable<T>() {
611      @Override
612      public Iterator<T> iterator() {
613        return Iterators.filter(unfiltered.iterator(), retainIfTrue);
614      }
615    };
616  }
617
618  /**
619   * Returns a view of {@code unfiltered} containing all elements that are of the type {@code
620   * desiredType}. The returned iterable's iterator does not support {@code remove()}.
621   *
622   * <p><b>{@code Stream} equivalent:</b> {@code stream.filter(type::isInstance).map(type::cast)}.
623   * This does perform a little more work than necessary, so another option is to insert an
624   * unchecked cast at some later point:
625   *
626   * <pre>
627   * {@code @SuppressWarnings("unchecked") // safe because of ::isInstance check
628   * ImmutableList<NewType> result =
629   *     (ImmutableList) stream.filter(NewType.class::isInstance).collect(toImmutableList());}
630   * </pre>
631   */
632  @SuppressWarnings("unchecked")
633  @GwtIncompatible // Class.isInstance
634  public static <T> Iterable<T> filter(final Iterable<?> unfiltered, final Class<T> desiredType) {
635    checkNotNull(unfiltered);
636    checkNotNull(desiredType);
637    return (Iterable<T>) filter(unfiltered, Predicates.instanceOf(desiredType));
638  }
639
640  /**
641   * Returns {@code true} if any element in {@code iterable} satisfies the predicate.
642   *
643   * <p><b>{@code Stream} equivalent:</b> {@link Stream#anyMatch}.
644   */
645  public static <T extends @Nullable Object> boolean any(
646      Iterable<T> iterable, Predicate<? super T> predicate) {
647    return Iterators.any(iterable.iterator(), predicate);
648  }
649
650  /**
651   * Returns {@code true} if every element in {@code iterable} satisfies the predicate. If {@code
652   * iterable} is empty, {@code true} is returned.
653   *
654   * <p><b>{@code Stream} equivalent:</b> {@link Stream#allMatch}.
655   */
656  public static <T extends @Nullable Object> boolean all(
657      Iterable<T> iterable, Predicate<? super T> predicate) {
658    return Iterators.all(iterable.iterator(), predicate);
659  }
660
661  /**
662   * Returns the first element in {@code iterable} that satisfies the given predicate; use this
663   * method only when such an element is known to exist. If it is possible that <i>no</i> element
664   * will match, use {@link #tryFind} or {@link #find(Iterable, Predicate, Object)} instead.
665   *
666   * <p><b>{@code Stream} equivalent:</b> {@code stream.filter(predicate).findFirst().get()}
667   *
668   * @throws NoSuchElementException if no element in {@code iterable} matches the given predicate
669   */
670  @ParametricNullness
671  public static <T extends @Nullable Object> T find(
672      Iterable<T> iterable, Predicate<? super T> predicate) {
673    return Iterators.find(iterable.iterator(), predicate);
674  }
675
676  /**
677   * Returns the first element in {@code iterable} that satisfies the given predicate, or {@code
678   * defaultValue} if none found. Note that this can usually be handled more naturally using {@code
679   * tryFind(iterable, predicate).or(defaultValue)}.
680   *
681   * <p><b>{@code Stream} equivalent:</b> {@code
682   * stream.filter(predicate).findFirst().orElse(defaultValue)}
683   *
684   * @since 7.0
685   */
686  // The signature we really want here is...
687  //
688  // <T extends @Nullable Object> @JointlyNullable T find(
689  //     Iterable<? extends T> iterable,
690  //     Predicate<? super T> predicate,
691  //     @JointlyNullable T defaultValue);
692  //
693  // ...where "@JointlyNullable" is similar to @PolyNull but slightly different:
694  //
695  // - @PolyNull means "@Nullable or @Nonnull"
696  //   (That would be unsound for an input Iterable<@Nullable Foo>. So, if we wanted to use
697  //   @PolyNull, we would have to restrict this method to non-null <T>. But it has users who pass
698  //   iterables with null elements.)
699  //
700  // - @JointlyNullable means "@Nullable or no annotation"
701  public static <T extends @Nullable Object> @Nullable T find(
702      Iterable<? extends T> iterable, Predicate<? super T> predicate, @Nullable T defaultValue) {
703    return Iterators.<T>find(iterable.iterator(), predicate, defaultValue);
704  }
705
706  /**
707   * Returns an {@link Optional} containing the first element in {@code iterable} that satisfies the
708   * given predicate, if such an element exists.
709   *
710   * <p><b>Warning:</b> avoid using a {@code predicate} that matches {@code null}. If {@code null}
711   * is matched in {@code iterable}, a NullPointerException will be thrown.
712   *
713   * <p><b>{@code Stream} equivalent:</b> {@code stream.filter(predicate).findFirst()}
714   *
715   * @since 11.0
716   */
717  public static <T> Optional<T> tryFind(Iterable<T> iterable, Predicate<? super T> predicate) {
718    return Iterators.tryFind(iterable.iterator(), predicate);
719  }
720
721  /**
722   * Returns the index in {@code iterable} of the first element that satisfies the provided {@code
723   * predicate}, or {@code -1} if the Iterable has no such elements.
724   *
725   * <p>More formally, returns the lowest index {@code i} such that {@code
726   * predicate.apply(Iterables.get(iterable, i))} returns {@code true}, or {@code -1} if there is no
727   * such index.
728   *
729   * @since 2.0
730   */
731  public static <T extends @Nullable Object> int indexOf(
732      Iterable<T> iterable, Predicate<? super T> predicate) {
733    return Iterators.indexOf(iterable.iterator(), predicate);
734  }
735
736  /**
737   * Returns a view containing the result of applying {@code function} to each element of {@code
738   * fromIterable}.
739   *
740   * <p>The returned iterable's iterator supports {@code remove()} if {@code fromIterable}'s
741   * iterator does. After a successful {@code remove()} call, {@code fromIterable} no longer
742   * contains the corresponding element.
743   *
744   * <p>If the input {@code Iterable} is known to be a {@code List} or other {@code Collection},
745   * consider {@link Lists#transform} and {@link Collections2#transform}.
746   *
747   * <p><b>{@code Stream} equivalent:</b> {@link Stream#map}
748   */
749  public static <F extends @Nullable Object, T extends @Nullable Object> Iterable<T> transform(
750      final Iterable<F> fromIterable, final Function<? super F, ? extends T> function) {
751    checkNotNull(fromIterable);
752    checkNotNull(function);
753    return new FluentIterable<T>() {
754      @Override
755      public Iterator<T> iterator() {
756        return Iterators.transform(fromIterable.iterator(), 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 = (List<? extends T>) 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((List<? extends T>) 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  }
953
954  /**
955   * Returns a view of {@code iterable} containing its first {@code limitSize} elements. If {@code
956   * iterable} contains fewer than {@code limitSize} elements, the returned view contains all of its
957   * elements. The returned iterable's iterator supports {@code remove()} if {@code iterable}'s
958   * iterator does.
959   *
960   * <p><b>{@code Stream} equivalent:</b> {@link Stream#limit}
961   *
962   * @param iterable the iterable to limit
963   * @param limitSize the maximum number of elements in the returned iterable
964   * @throws IllegalArgumentException if {@code limitSize} is negative
965   * @since 3.0
966   */
967  public static <T extends @Nullable Object> Iterable<T> limit(
968      final Iterable<T> iterable, final int limitSize) {
969    checkNotNull(iterable);
970    checkArgument(limitSize >= 0, "limit is negative");
971    return new FluentIterable<T>() {
972      @Override
973      public Iterator<T> iterator() {
974        return Iterators.limit(iterable.iterator(), limitSize);
975      }
976    };
977  }
978
979  /**
980   * Returns a view of the supplied iterable that wraps each generated {@link Iterator} through
981   * {@link Iterators#consumingIterator(Iterator)}.
982   *
983   * <p>Note: If {@code iterable} is a {@link Queue}, the returned iterable will instead use {@link
984   * Queue#isEmpty} and {@link Queue#remove()}, since {@link Queue}'s iteration order is undefined.
985   * Calling {@link Iterator#hasNext()} on a generated iterator from the returned iterable may cause
986   * an item to be immediately dequeued for return on a subsequent call to {@link Iterator#next()}.
987   *
988   * <p>Whether the input {@code iterable} is a {@link Queue} or not, the returned {@code Iterable}
989   * is not thread-safe.
990   *
991   * @param iterable the iterable to wrap
992   * @return a view of the supplied iterable that wraps each generated iterator through {@link
993   *     Iterators#consumingIterator(Iterator)}; for queues, an iterable that generates iterators
994   *     that return and consume the queue's elements in queue order
995   * @see Iterators#consumingIterator(Iterator)
996   * @since 2.0
997   */
998  public static <T extends @Nullable Object> Iterable<T> consumingIterable(
999      final Iterable<T> iterable) {
1000    checkNotNull(iterable);
1001
1002    return new FluentIterable<T>() {
1003      @Override
1004      public Iterator<T> iterator() {
1005        return (iterable instanceof Queue)
1006            ? new ConsumingQueueIterator<>((Queue<T>) iterable)
1007            : Iterators.consumingIterator(iterable.iterator());
1008      }
1009
1010      @Override
1011      public String toString() {
1012        return "Iterables.consumingIterable(...)";
1013      }
1014    };
1015  }
1016
1017  // Methods only in Iterables, not in Iterators
1018
1019  /**
1020   * Determines if the given iterable contains no elements.
1021   *
1022   * <p>There is no precise {@link Iterator} equivalent to this method, since one can only ask an
1023   * iterator whether it has any elements <i>remaining</i> (which one does using {@link
1024   * Iterator#hasNext}).
1025   *
1026   * <p><b>{@code Stream} equivalent:</b> {@code !stream.findAny().isPresent()}
1027   *
1028   * @return {@code true} if the iterable contains no elements
1029   */
1030  public static boolean isEmpty(Iterable<?> iterable) {
1031    if (iterable instanceof Collection) {
1032      return ((Collection<?>) iterable).isEmpty();
1033    }
1034    return !iterable.iterator().hasNext();
1035  }
1036
1037  /**
1038   * Returns an iterable over the merged contents of all given {@code iterables}. Equivalent entries
1039   * will not be de-duplicated.
1040   *
1041   * <p>Callers must ensure that the source {@code iterables} are in non-descending order as this
1042   * method does not sort its input.
1043   *
1044   * <p>For any equivalent elements across all {@code iterables}, it is undefined which element is
1045   * returned first.
1046   *
1047   * @since 11.0
1048   */
1049  public static <T extends @Nullable Object> Iterable<T> mergeSorted(
1050      final Iterable<? extends Iterable<? extends T>> iterables,
1051      final Comparator<? super T> comparator) {
1052    checkNotNull(iterables, "iterables");
1053    checkNotNull(comparator, "comparator");
1054    Iterable<T> iterable =
1055        new FluentIterable<T>() {
1056          @Override
1057          public Iterator<T> iterator() {
1058            return Iterators.mergeSorted(
1059                Iterables.transform(iterables, Iterable::iterator), comparator);
1060          }
1061        };
1062    return new UnmodifiableIterable<>(iterable);
1063  }
1064}