com.google.common.collect.Iterables Maven / Gradle / Ivy
/*
* Copyright (C) 2007 The Guava Authors
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.google.common.collect;
import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.collect.CollectPreconditions.checkRemove;
import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.base.Function;
import com.google.common.base.Optional;
import com.google.common.base.Predicate;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.Queue;
import java.util.RandomAccess;
import java.util.Set;
import javax.annotation.Nullable;
/**
* This class contains static utility methods that operate on or return objects
* of type {@code Iterable}. Except as noted, each method has a corresponding
* {@link Iterator}-based method in the {@link Iterators} class.
*
* Performance notes: Unless otherwise noted, all of the iterables
* produced in this class are lazy, which means that their iterators
* only advance the backing iteration when absolutely necessary.
*
*
See the Guava User Guide article on
* {@code Iterables}.
*
* @author Kevin Bourrillion
* @author Jared Levy
* @since 2.0 (imported from Google Collections Library)
*/
@GwtCompatible(emulated = true)
public final class Iterables {
private Iterables() {}
/** Returns an unmodifiable view of {@code iterable}. */
public static Iterable unmodifiableIterable(
final Iterable iterable) {
checkNotNull(iterable);
if (iterable instanceof UnmodifiableIterable ||
iterable instanceof ImmutableCollection) {
return iterable;
}
return new UnmodifiableIterable(iterable);
}
/**
* Simply returns its argument.
*
* @deprecated no need to use this
* @since 10.0
*/
@Deprecated public static Iterable unmodifiableIterable(
ImmutableCollection iterable) {
return checkNotNull(iterable);
}
private static final class UnmodifiableIterable extends FluentIterable {
private final Iterable iterable;
private UnmodifiableIterable(Iterable iterable) {
this.iterable = iterable;
}
@Override
public Iterator iterator() {
return Iterators.unmodifiableIterator(iterable.iterator());
}
@Override
public String toString() {
return iterable.toString();
}
// no equals and hashCode; it would break the contract!
}
/**
* Returns the number of elements in {@code iterable}.
*/
public static int size(Iterable iterable) {
return (iterable instanceof Collection)
? ((Collection) iterable).size()
: Iterators.size(iterable.iterator());
}
/**
* Returns {@code true} if {@code iterable} contains any object for which {@code equals(element)}
* is true.
*/
public static boolean contains(Iterable iterable, @Nullable Object element) {
if (iterable instanceof Collection) {
Collection collection = (Collection) iterable;
return Collections2.safeContains(collection, element);
}
return Iterators.contains(iterable.iterator(), element);
}
/**
* Removes, from an iterable, every element that belongs to the provided
* collection.
*
* This method calls {@link Collection#removeAll} if {@code iterable} is a
* collection, and {@link Iterators#removeAll} otherwise.
*
* @param removeFrom the iterable to (potentially) remove elements from
* @param elementsToRemove the elements to remove
* @return {@code true} if any element was removed from {@code iterable}
*/
public static boolean removeAll(
Iterable removeFrom, Collection elementsToRemove) {
return (removeFrom instanceof Collection)
? ((Collection) removeFrom).removeAll(checkNotNull(elementsToRemove))
: Iterators.removeAll(removeFrom.iterator(), elementsToRemove);
}
/**
* Removes, from an iterable, every element that does not belong to the
* provided collection.
*
*
This method calls {@link Collection#retainAll} if {@code iterable} is a
* collection, and {@link Iterators#retainAll} otherwise.
*
* @param removeFrom the iterable to (potentially) remove elements from
* @param elementsToRetain the elements to retain
* @return {@code true} if any element was removed from {@code iterable}
*/
public static boolean retainAll(
Iterable removeFrom, Collection elementsToRetain) {
return (removeFrom instanceof Collection)
? ((Collection) removeFrom).retainAll(checkNotNull(elementsToRetain))
: Iterators.retainAll(removeFrom.iterator(), elementsToRetain);
}
/**
* Removes, from an iterable, every element that satisfies the provided
* predicate.
*
* @param removeFrom the iterable to (potentially) remove elements from
* @param predicate a predicate that determines whether an element should
* be removed
* @return {@code true} if any elements were removed from the iterable
*
* @throws UnsupportedOperationException if the iterable does not support
* {@code remove()}.
* @since 2.0
*/
public static boolean removeIf(
Iterable removeFrom, Predicate predicate) {
if (removeFrom instanceof RandomAccess && removeFrom instanceof List) {
return removeIfFromRandomAccessList(
(List) removeFrom, checkNotNull(predicate));
}
return Iterators.removeIf(removeFrom.iterator(), predicate);
}
private static boolean removeIfFromRandomAccessList(
List list, Predicate predicate) {
// Note: Not all random access lists support set() so we need to deal with
// those that don't and attempt the slower remove() based solution.
int from = 0;
int to = 0;
for (; from < list.size(); from++) {
T element = list.get(from);
if (!predicate.apply(element)) {
if (from > to) {
try {
list.set(to, element);
} catch (UnsupportedOperationException e) {
slowRemoveIfForRemainingElements(list, predicate, to, from);
return true;
}
}
to++;
}
}
// Clear the tail of any remaining items
list.subList(to, list.size()).clear();
return from != to;
}
private static void slowRemoveIfForRemainingElements(List list,
Predicate predicate, int to, int from) {
// Here we know that:
// * (to < from) and that both are valid indices.
// * Everything with (index < to) should be kept.
// * Everything with (to <= index < from) should be removed.
// * The element with (index == from) should be kept.
// * Everything with (index > from) has not been checked yet.
// Check from the end of the list backwards (minimize expected cost of
// moving elements when remove() is called). Stop before 'from' because
// we already know that should be kept.
for (int n = list.size() - 1; n > from; n--) {
if (predicate.apply(list.get(n))) {
list.remove(n);
}
}
// And now remove everything in the range [to, from) (going backwards).
for (int n = from - 1; n >= to; n--) {
list.remove(n);
}
}
/**
* Removes and returns the first matching element, or returns {@code null} if there is none.
*/
@Nullable
static T removeFirstMatching(Iterable removeFrom, Predicate predicate) {
checkNotNull(predicate);
Iterator iterator = removeFrom.iterator();
while (iterator.hasNext()) {
T next = iterator.next();
if (predicate.apply(next)) {
iterator.remove();
return next;
}
}
return null;
}
/**
* Determines whether two iterables contain equal elements in the same order.
* More specifically, this method returns {@code true} if {@code iterable1}
* and {@code iterable2} contain the same number of elements and every element
* of {@code iterable1} is equal to the corresponding element of
* {@code iterable2}.
*/
public static boolean elementsEqual(
Iterable iterable1, Iterable iterable2) {
if (iterable1 instanceof Collection && iterable2 instanceof Collection) {
Collection collection1 = (Collection) iterable1;
Collection collection2 = (Collection) iterable2;
if (collection1.size() != collection2.size()) {
return false;
}
}
return Iterators.elementsEqual(iterable1.iterator(), iterable2.iterator());
}
/**
* Returns a string representation of {@code iterable}, with the format {@code
* [e1, e2, ..., en]} (that is, identical to {@link java.util.Arrays
* Arrays}{@code .toString(Iterables.toArray(iterable))}). Note that for
* most implementations of {@link Collection}, {@code
* collection.toString()} also gives the same result, but that behavior is not
* generally guaranteed.
*/
public static String toString(Iterable iterable) {
return Iterators.toString(iterable.iterator());
}
/**
* Returns the single element contained in {@code iterable}.
*
* @throws NoSuchElementException if the iterable is empty
* @throws IllegalArgumentException if the iterable contains multiple
* elements
*/
public static T getOnlyElement(Iterable iterable) {
return Iterators.getOnlyElement(iterable.iterator());
}
/**
* Returns the single element contained in {@code iterable}, or {@code
* defaultValue} if the iterable is empty.
*
* @throws IllegalArgumentException if the iterator contains multiple
* elements
*/
@Nullable
public static T getOnlyElement(
Iterable iterable, @Nullable T defaultValue) {
return Iterators.getOnlyElement(iterable.iterator(), defaultValue);
}
/**
* Copies an iterable's elements into an array.
*
* @param iterable the iterable to copy
* @param type the type of the elements
* @return a newly-allocated array into which all the elements of the iterable
* have been copied
*/
@GwtIncompatible("Array.newInstance(Class, int)")
public static T[] toArray(Iterable iterable, Class type) {
Collection collection = toCollection(iterable);
T[] array = ObjectArrays.newArray(type, collection.size());
return collection.toArray(array);
}
/**
* Copies an iterable's elements into an array.
*
* @param iterable the iterable to copy
* @return a newly-allocated array into which all the elements of the iterable
* have been copied
*/
static Object[] toArray(Iterable iterable) {
return toCollection(iterable).toArray();
}
/**
* Converts an iterable into a collection. If the iterable is already a
* collection, it is returned. Otherwise, an {@link java.util.ArrayList} is
* created with the contents of the iterable in the same iteration order.
*/
private static Collection toCollection(Iterable iterable) {
return (iterable instanceof Collection)
? (Collection) iterable
: Lists.newArrayList(iterable.iterator());
}
/**
* Adds all elements in {@code iterable} to {@code collection}.
*
* @return {@code true} if {@code collection} was modified as a result of this
* operation.
*/
public static boolean addAll(
Collection addTo, Iterable elementsToAdd) {
if (elementsToAdd instanceof Collection) {
Collection c = Collections2.cast(elementsToAdd);
return addTo.addAll(c);
}
return Iterators.addAll(addTo, checkNotNull(elementsToAdd).iterator());
}
/**
* Returns the number of elements in the specified iterable that equal the
* specified object. This implementation avoids a full iteration when the
* iterable is a {@link Multiset} or {@link Set}.
*
* @see Collections#frequency
*/
public static int frequency(Iterable iterable, @Nullable Object element) {
if ((iterable instanceof Multiset)) {
return ((Multiset) iterable).count(element);
} else if ((iterable instanceof Set)) {
return ((Set) iterable).contains(element) ? 1 : 0;
}
return Iterators.frequency(iterable.iterator(), element);
}
/**
* Returns an iterable whose iterators cycle indefinitely over the elements of
* {@code iterable}.
*
* That iterator supports {@code remove()} if {@code iterable.iterator()}
* does. After {@code remove()} is called, subsequent cycles omit the removed
* element, which is no longer in {@code iterable}. The iterator's
* {@code hasNext()} method returns {@code true} until {@code iterable} is
* empty.
*
*
Warning: Typical uses of the resulting iterator may produce an
* infinite loop. You should use an explicit {@code break} or be certain that
* you will eventually remove all the elements.
*
*
To cycle over the iterable {@code n} times, use the following:
* {@code Iterables.concat(Collections.nCopies(n, iterable))}
*/
public static Iterable cycle(final Iterable iterable) {
checkNotNull(iterable);
return new FluentIterable() {
@Override
public Iterator iterator() {
return Iterators.cycle(iterable);
}
@Override public String toString() {
return iterable.toString() + " (cycled)";
}
};
}
/**
* Returns an iterable whose iterators cycle indefinitely over the provided
* elements.
*
* After {@code remove} is invoked on a generated iterator, the removed
* element will no longer appear in either that iterator or any other iterator
* created from the same source iterable. That is, this method behaves exactly
* as {@code Iterables.cycle(Lists.newArrayList(elements))}. The iterator's
* {@code hasNext} method returns {@code true} until all of the original
* elements have been removed.
*
*
Warning: Typical uses of the resulting iterator may produce an
* infinite loop. You should use an explicit {@code break} or be certain that
* you will eventually remove all the elements.
*
*
To cycle over the elements {@code n} times, use the following:
* {@code Iterables.concat(Collections.nCopies(n, Arrays.asList(elements)))}
*/
public static Iterable cycle(T... elements) {
return cycle(Lists.newArrayList(elements));
}
/**
* Combines two iterables into a single iterable. The returned iterable has an
* iterator that traverses the elements in {@code a}, followed by the elements
* in {@code b}. The source iterators are not polled until necessary.
*
* The returned iterable's iterator supports {@code remove()} when the
* corresponding input iterator supports it.
*/
public static Iterable concat(
Iterable a, Iterable b) {
return concat(ImmutableList.of(a, b));
}
/**
* Combines three iterables into a single iterable. The returned iterable has
* an iterator that traverses the elements in {@code a}, followed by the
* elements in {@code b}, followed by the elements in {@code c}. The source
* iterators are not polled until necessary.
*
* The returned iterable's iterator supports {@code remove()} when the
* corresponding input iterator supports it.
*/
public static Iterable concat(Iterable a,
Iterable b, Iterable c) {
return concat(ImmutableList.of(a, b, c));
}
/**
* Combines four iterables into a single iterable. The returned iterable has
* an iterator that traverses the elements in {@code a}, followed by the
* elements in {@code b}, followed by the elements in {@code c}, followed by
* the elements in {@code d}. The source iterators are not polled until
* necessary.
*
* The returned iterable's iterator supports {@code remove()} when the
* corresponding input iterator supports it.
*/
public static Iterable concat(Iterable a,
Iterable b, Iterable c,
Iterable d) {
return concat(ImmutableList.of(a, b, c, d));
}
/**
* Combines multiple iterables into a single iterable. The returned iterable
* has an iterator that traverses the elements of each iterable in
* {@code inputs}. The input iterators are not polled until necessary.
*
* The returned iterable's iterator supports {@code remove()} when the
* corresponding input iterator supports it.
*
* @throws NullPointerException if any of the provided iterables is null
*/
public static Iterable concat(Iterable... inputs) {
return concat(ImmutableList.copyOf(inputs));
}
/**
* Combines multiple iterables into a single iterable. The returned iterable
* has an iterator that traverses the elements of each iterable in
* {@code inputs}. The input iterators are not polled until necessary.
*
* The returned iterable's iterator supports {@code remove()} when the
* corresponding input iterator supports it. The methods of the returned
* iterable may throw {@code NullPointerException} if any of the input
* iterators is null.
*/
public static Iterable concat(
final Iterable> inputs) {
checkNotNull(inputs);
return new FluentIterable() {
@Override
public Iterator iterator() {
return Iterators.concat(iterators(inputs));
}
};
}
/**
* Returns an iterator over the iterators of the given iterables.
*/
private static Iterator> iterators(
Iterable> iterables) {
return new TransformedIterator, Iterator>(
iterables.iterator()) {
@Override
Iterator transform(Iterable from) {
return from.iterator();
}
};
}
/**
* Divides an iterable into unmodifiable sublists of the given size (the final
* iterable may be smaller). For example, partitioning an iterable containing
* {@code [a, b, c, d, e]} with a partition size of 3 yields {@code
* [[a, b, c], [d, e]]} -- an outer iterable containing two inner lists of
* three and two elements, all in the original order.
*
* Iterators returned by the returned iterable do not support the {@link
* Iterator#remove()} method. The returned lists implement {@link
* RandomAccess}, whether or not the input list does.
*
*
Note: if {@code iterable} is a {@link List}, use {@link
* Lists#partition(List, int)} instead.
*
* @param iterable the iterable to return a partitioned view of
* @param size the desired size of each partition (the last may be smaller)
* @return an iterable of unmodifiable lists containing the elements of {@code
* iterable} divided into partitions
* @throws IllegalArgumentException if {@code size} is nonpositive
*/
public static Iterable> partition(
final Iterable iterable, final int size) {
checkNotNull(iterable);
checkArgument(size > 0);
return new FluentIterable>() {
@Override
public Iterator> iterator() {
return Iterators.partition(iterable.iterator(), size);
}
};
}
/**
* Divides an iterable into unmodifiable sublists of the given size, padding
* the final iterable with null values if necessary. For example, partitioning
* an iterable containing {@code [a, b, c, d, e]} with a partition size of 3
* yields {@code [[a, b, c], [d, e, null]]} -- an outer iterable containing
* two inner lists of three elements each, all in the original order.
*
* Iterators returned by the returned iterable do not support the {@link
* Iterator#remove()} method.
*
* @param iterable the iterable to return a partitioned view of
* @param size the desired size of each partition
* @return an iterable of unmodifiable lists containing the elements of {@code
* iterable} divided into partitions (the final iterable may have
* trailing null elements)
* @throws IllegalArgumentException if {@code size} is nonpositive
*/
public static Iterable> paddedPartition(
final Iterable iterable, final int size) {
checkNotNull(iterable);
checkArgument(size > 0);
return new FluentIterable>() {
@Override
public Iterator> iterator() {
return Iterators.paddedPartition(iterable.iterator(), size);
}
};
}
/**
* Returns the elements of {@code unfiltered} that satisfy a predicate. The
* resulting iterable's iterator does not support {@code remove()}.
*/
public static Iterable filter(
final Iterable unfiltered, final Predicate predicate) {
checkNotNull(unfiltered);
checkNotNull(predicate);
return new FluentIterable() {
@Override
public Iterator iterator() {
return Iterators.filter(unfiltered.iterator(), predicate);
}
};
}
/**
* Returns all instances of class {@code type} in {@code unfiltered}. The
* returned iterable has elements whose class is {@code type} or a subclass of
* {@code type}. The returned iterable's iterator does not support
* {@code remove()}.
*
* @param unfiltered an iterable containing objects of any type
* @param type the type of elements desired
* @return an unmodifiable iterable containing all elements of the original
* iterable that were of the requested type
*/
@GwtIncompatible("Class.isInstance")
public static Iterable filter(
final Iterable unfiltered, final Class type) {
checkNotNull(unfiltered);
checkNotNull(type);
return new FluentIterable() {
@Override
public Iterator iterator() {
return Iterators.filter(unfiltered.iterator(), type);
}
};
}
/**
* Returns {@code true} if any element in {@code iterable} satisfies the predicate.
*/
public static boolean any(
Iterable iterable, Predicate predicate) {
return Iterators.any(iterable.iterator(), predicate);
}
/**
* Returns {@code true} if every element in {@code iterable} satisfies the
* predicate. If {@code iterable} is empty, {@code true} is returned.
*/
public static boolean all(
Iterable iterable, Predicate predicate) {
return Iterators.all(iterable.iterator(), predicate);
}
/**
* Returns the first element in {@code iterable} that satisfies the given
* predicate; use this method only when such an element is known to exist. If
* it is possible that no element will match, use {@link #tryFind} or
* {@link #find(Iterable, Predicate, Object)} instead.
*
* @throws NoSuchElementException if no element in {@code iterable} matches
* the given predicate
*/
public static T find(Iterable iterable,
Predicate predicate) {
return Iterators.find(iterable.iterator(), predicate);
}
/**
* Returns the first element in {@code iterable} that satisfies the given
* predicate, or {@code defaultValue} if none found. Note that this can
* usually be handled more naturally using {@code
* tryFind(iterable, predicate).or(defaultValue)}.
*
* @since 7.0
*/
@Nullable
public static T find(Iterable iterable,
Predicate predicate, @Nullable T defaultValue) {
return Iterators.find(iterable.iterator(), predicate, defaultValue);
}
/**
* Returns an {@link Optional} containing the first element in {@code
* iterable} that satisfies the given predicate, if such an element exists.
*
* Warning: avoid using a {@code predicate} that matches {@code
* null}. If {@code null} is matched in {@code iterable}, a
* NullPointerException will be thrown.
*
* @since 11.0
*/
public static Optional tryFind(Iterable iterable,
Predicate predicate) {
return Iterators.tryFind(iterable.iterator(), predicate);
}
/**
* Returns the index in {@code iterable} of the first element that satisfies
* the provided {@code predicate}, or {@code -1} if the Iterable has no such
* elements.
*
* More formally, returns the lowest index {@code i} such that
* {@code predicate.apply(Iterables.get(iterable, i))} returns {@code true},
* or {@code -1} if there is no such index.
*
* @since 2.0
*/
public static int indexOf(
Iterable iterable, Predicate predicate) {
return Iterators.indexOf(iterable.iterator(), predicate);
}
/**
* Returns an iterable that applies {@code function} to each element of {@code
* fromIterable}.
*
* The returned iterable's iterator supports {@code remove()} if the
* provided iterator does. After a successful {@code remove()} call,
* {@code fromIterable} no longer contains the corresponding element.
*
*
If the input {@code Iterable} is known to be a {@code List} or other
* {@code Collection}, consider {@link Lists#transform} and {@link
* Collections2#transform}.
*/
public static Iterable transform(final Iterable fromIterable,
final Function function) {
checkNotNull(fromIterable);
checkNotNull(function);
return new FluentIterable() {
@Override
public Iterator iterator() {
return Iterators.transform(fromIterable.iterator(), function);
}
};
}
/**
* Returns the element at the specified position in an iterable.
*
* @param position position of the element to return
* @return the element at the specified position in {@code iterable}
* @throws IndexOutOfBoundsException if {@code position} is negative or
* greater than or equal to the size of {@code iterable}
*/
public static T get(Iterable iterable, int position) {
checkNotNull(iterable);
return (iterable instanceof List)
? ((List) iterable).get(position)
: Iterators.get(iterable.iterator(), position);
}
/**
* Returns the element at the specified position in an iterable or a default
* value otherwise.
*
* @param position position of the element to return
* @param defaultValue the default value to return if {@code position} is
* greater than or equal to the size of the iterable
* @return the element at the specified position in {@code iterable} or
* {@code defaultValue} if {@code iterable} contains fewer than
* {@code position + 1} elements.
* @throws IndexOutOfBoundsException if {@code position} is negative
* @since 4.0
*/
@Nullable
public static T get(Iterable iterable, int position, @Nullable T defaultValue) {
checkNotNull(iterable);
Iterators.checkNonnegative(position);
if (iterable instanceof List) {
List list = Lists.cast(iterable);
return (position < list.size()) ? list.get(position) : defaultValue;
} else {
Iterator iterator = iterable.iterator();
Iterators.advance(iterator, position);
return Iterators.getNext(iterator, defaultValue);
}
}
/**
* Returns the first element in {@code iterable} or {@code defaultValue} if
* the iterable is empty. The {@link Iterators} analog to this method is
* {@link Iterators#getNext}.
*
* If no default value is desired (and the caller instead wants a
* {@link NoSuchElementException} to be thrown), it is recommended that
* {@code iterable.iterator().next()} is used instead.
*
* @param defaultValue the default value to return if the iterable is empty
* @return the first element of {@code iterable} or the default value
* @since 7.0
*/
@Nullable
public static T getFirst(Iterable iterable, @Nullable T defaultValue) {
return Iterators.getNext(iterable.iterator(), defaultValue);
}
/**
* Returns the last element of {@code iterable}.
*
* @return the last element of {@code iterable}
* @throws NoSuchElementException if the iterable is empty
*/
public static T getLast(Iterable iterable) {
// TODO(kevinb): Support a concurrently modified collection?
if (iterable instanceof List) {
List list = (List) iterable;
if (list.isEmpty()) {
throw new NoSuchElementException();
}
return getLastInNonemptyList(list);
}
return Iterators.getLast(iterable.iterator());
}
/**
* Returns the last element of {@code iterable} or {@code defaultValue} if
* the iterable is empty.
*
* @param defaultValue the value to return if {@code iterable} is empty
* @return the last element of {@code iterable} or the default value
* @since 3.0
*/
@Nullable
public static T getLast(Iterable iterable, @Nullable T defaultValue) {
if (iterable instanceof Collection) {
Collection c = Collections2.cast(iterable);
if (c.isEmpty()) {
return defaultValue;
} else if (iterable instanceof List) {
return getLastInNonemptyList(Lists.cast(iterable));
}
}
return Iterators.getLast(iterable.iterator(), defaultValue);
}
private static T getLastInNonemptyList(List list) {
return list.get(list.size() - 1);
}
/**
* Returns a view of {@code iterable} that skips its first
* {@code numberToSkip} elements. If {@code iterable} contains fewer than
* {@code numberToSkip} elements, the returned iterable skips all of its
* elements.
*
* Modifications to the underlying {@link Iterable} before a call to
* {@code iterator()} are reflected in the returned iterator. That is, the
* iterator skips the first {@code numberToSkip} elements that exist when the
* {@code Iterator} is created, not when {@code skip()} is called.
*
*
The returned iterable's iterator supports {@code remove()} if the
* iterator of the underlying iterable supports it. Note that it is
* not possible to delete the last skipped element by immediately
* calling {@code remove()} on that iterator, as the {@code Iterator}
* contract states that a call to {@code remove()} before a call to
* {@code next()} will throw an {@link IllegalStateException}.
*
* @since 3.0
*/
public static Iterable skip(final Iterable iterable,
final int numberToSkip) {
checkNotNull(iterable);
checkArgument(numberToSkip >= 0, "number to skip cannot be negative");
if (iterable instanceof List) {
final List list = (List) iterable;
return new FluentIterable() {
@Override
public Iterator iterator() {
// TODO(kevinb): Support a concurrently modified collection?
int toSkip = Math.min(list.size(), numberToSkip);
return list.subList(toSkip, list.size()).iterator();
}
};
}
return new FluentIterable() {
@Override
public Iterator iterator() {
final Iterator iterator = iterable.iterator();
Iterators.advance(iterator, numberToSkip);
/*
* We can't just return the iterator because an immediate call to its
* remove() method would remove one of the skipped elements instead of
* throwing an IllegalStateException.
*/
return new Iterator() {
boolean atStart = true;
@Override
public boolean hasNext() {
return iterator.hasNext();
}
@Override
public T next() {
T result = iterator.next();
atStart = false; // not called if next() fails
return result;
}
@Override
public void remove() {
checkRemove(!atStart);
iterator.remove();
}
};
}
};
}
/**
* Creates an iterable with the first {@code limitSize} elements of the given
* iterable. If the original iterable does not contain that many elements, the
* returned iterable will have the same behavior as the original iterable. The
* returned iterable's iterator supports {@code remove()} if the original
* iterator does.
*
* @param iterable the iterable to limit
* @param limitSize the maximum number of elements in the returned iterable
* @throws IllegalArgumentException if {@code limitSize} is negative
* @since 3.0
*/
public static Iterable limit(
final Iterable iterable, final int limitSize) {
checkNotNull(iterable);
checkArgument(limitSize >= 0, "limit is negative");
return new FluentIterable() {
@Override
public Iterator iterator() {
return Iterators.limit(iterable.iterator(), limitSize);
}
};
}
/**
* Returns a view of the supplied iterable that wraps each generated
* {@link Iterator} through {@link Iterators#consumingIterator(Iterator)}.
*
* Note: If {@code iterable} is a {@link Queue}, the returned iterable will
* get entries from {@link Queue#remove()} since {@link Queue}'s iteration
* order is undefined. Calling {@link Iterator#hasNext()} on a generated
* iterator from the returned iterable may cause an item to be immediately
* dequeued for return on a subsequent call to {@link Iterator#next()}.
*
* @param iterable the iterable to wrap
* @return a view of the supplied iterable that wraps each generated iterator
* through {@link Iterators#consumingIterator(Iterator)}; for queues,
* an iterable that generates iterators that return and consume the
* queue's elements in queue order
*
* @see Iterators#consumingIterator(Iterator)
* @since 2.0
*/
public static Iterable consumingIterable(final Iterable iterable) {
if (iterable instanceof Queue) {
return new FluentIterable() {
@Override
public Iterator iterator() {
return new ConsumingQueueIterator((Queue) iterable);
}
@Override
public String toString() {
return "Iterables.consumingIterable(...)";
}
};
}
checkNotNull(iterable);
return new FluentIterable() {
@Override
public Iterator iterator() {
return Iterators.consumingIterator(iterable.iterator());
}
@Override
public String toString() {
return "Iterables.consumingIterable(...)";
}
};
}
private static class ConsumingQueueIterator extends AbstractIterator {
private final Queue queue;
private ConsumingQueueIterator(Queue queue) {
this.queue = queue;
}
@Override public T computeNext() {
try {
return queue.remove();
} catch (NoSuchElementException e) {
return endOfData();
}
}
}
// Methods only in Iterables, not in Iterators
/**
* Determines if the given iterable contains no elements.
*
* There is no precise {@link Iterator} equivalent to this method, since
* one can only ask an iterator whether it has any elements remaining
* (which one does using {@link Iterator#hasNext}).
*
* @return {@code true} if the iterable contains no elements
*/
public static boolean isEmpty(Iterable iterable) {
if (iterable instanceof Collection) {
return ((Collection) iterable).isEmpty();
}
return !iterable.iterator().hasNext();
}
/**
* Returns an iterable over the merged contents of all given
* {@code iterables}. Equivalent entries will not be de-duplicated.
*
*
Callers must ensure that the source {@code iterables} are in
* non-descending order as this method does not sort its input.
*
*
For any equivalent elements across all {@code iterables}, it is
* undefined which element is returned first.
*
* @since 11.0
*/
@Beta
public static Iterable mergeSorted(
final Iterable> iterables,
final Comparator comparator) {
checkNotNull(iterables, "iterables");
checkNotNull(comparator, "comparator");
Iterable iterable = new FluentIterable() {
@Override
public Iterator iterator() {
return Iterators.mergeSorted(
Iterables.transform(iterables, Iterables.toIterator()),
comparator);
}
};
return new UnmodifiableIterable(iterable);
}
// TODO(user): Is this the best place for this? Move to fluent functions?
// Useful as a public method?
private static Function, Iterator>
toIterator() {
return new Function, Iterator>() {
@Override
public Iterator apply(Iterable iterable) {
return iterable.iterator();
}
};
}
}