Iterators.java
/*
* 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.base.Preconditions.checkState;
import static com.google.common.base.Predicates.instanceOf;
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.Objects;
import com.google.common.base.Optional;
import com.google.common.base.Preconditions;
import com.google.common.base.Predicate;
import com.google.common.primitives.Ints;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.util.ArrayDeque;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.Deque;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
import java.util.NoSuchElementException;
import java.util.PriorityQueue;
import java.util.Queue;
import org.checkerframework.checker.nullness.qual.Nullable;
/**
* This class contains static utility methods that operate on or return objects of type {@link
* Iterator}. Except as noted, each method has a corresponding {@link Iterable}-based method in the
* {@link Iterables} class.
*
* <p><i>Performance notes:</i> Unless otherwise noted, all of the iterators produced in this class
* are <i>lazy</i>, which means that they only advance the backing iteration when absolutely
* necessary.
*
* <p>See the Guava User Guide section on <a href=
* "https://github.com/google/guava/wiki/CollectionUtilitiesExplained#iterables"> {@code
* Iterators}</a>.
*
* @author Kevin Bourrillion
* @author Jared Levy
* @since 2.0
*/
@GwtCompatible(emulated = true)
public final class Iterators {
private Iterators() {}
/**
* Returns the empty iterator.
*
* <p>The {@link Iterable} equivalent of this method is {@link ImmutableSet#of()}.
*/
static <T> UnmodifiableIterator<T> emptyIterator() {
return emptyListIterator();
}
/**
* Returns the empty iterator.
*
* <p>The {@link Iterable} equivalent of this method is {@link ImmutableSet#of()}.
*/
// Casting to any type is safe since there are no actual elements.
@SuppressWarnings("unchecked")
static <T> UnmodifiableListIterator<T> emptyListIterator() {
return (UnmodifiableListIterator<T>) ArrayItr.EMPTY;
}
/**
* This is an enum singleton rather than an anonymous class so ProGuard can figure out it's only
* referenced by emptyModifiableIterator().
*/
private enum EmptyModifiableIterator implements Iterator<Object> {
INSTANCE;
@Override
public boolean hasNext() {
return false;
}
@Override
public Object next() {
throw new NoSuchElementException();
}
@Override
public void remove() {
checkRemove(false);
}
}
/**
* Returns the empty {@code Iterator} that throws {@link IllegalStateException} instead of {@link
* UnsupportedOperationException} on a call to {@link Iterator#remove()}.
*/
// Casting to any type is safe since there are no actual elements.
@SuppressWarnings("unchecked")
static <T> Iterator<T> emptyModifiableIterator() {
return (Iterator<T>) EmptyModifiableIterator.INSTANCE;
}
/** Returns an unmodifiable view of {@code iterator}. */
public static <T> UnmodifiableIterator<T> unmodifiableIterator(
final Iterator<? extends T> iterator) {
checkNotNull(iterator);
if (iterator instanceof UnmodifiableIterator) {
@SuppressWarnings("unchecked") // Since it's unmodifiable, the covariant cast is safe
UnmodifiableIterator<T> result = (UnmodifiableIterator<T>) iterator;
return result;
}
return new UnmodifiableIterator<T>() {
@Override
public boolean hasNext() {
return iterator.hasNext();
}
@Override
public T next() {
return iterator.next();
}
};
}
/**
* Simply returns its argument.
*
* @deprecated no need to use this
* @since 10.0
*/
@Deprecated
public static <T> UnmodifiableIterator<T> unmodifiableIterator(UnmodifiableIterator<T> iterator) {
return checkNotNull(iterator);
}
/**
* Returns the number of elements remaining in {@code iterator}. The iterator will be left
* exhausted: its {@code hasNext()} method will return {@code false}.
*/
public static int size(Iterator<?> iterator) {
long count = 0L;
while (iterator.hasNext()) {
iterator.next();
count++;
}
return Ints.saturatedCast(count);
}
/** Returns {@code true} if {@code iterator} contains {@code element}. */
public static boolean contains(Iterator<?> iterator, @Nullable Object element) {
if (element == null) {
while (iterator.hasNext()) {
if (iterator.next() == null) {
return true;
}
}
} else {
while (iterator.hasNext()) {
if (element.equals(iterator.next())) {
return true;
}
}
}
return false;
}
/**
* Traverses an iterator and removes every element that belongs to the provided collection. The
* iterator will be left exhausted: its {@code hasNext()} method will return {@code false}.
*
* @param removeFrom the iterator to (potentially) remove elements from
* @param elementsToRemove the elements to remove
* @return {@code true} if any element was removed from {@code iterator}
*/
@CanIgnoreReturnValue
public static boolean removeAll(Iterator<?> removeFrom, Collection<?> elementsToRemove) {
checkNotNull(elementsToRemove);
boolean result = false;
while (removeFrom.hasNext()) {
if (elementsToRemove.contains(removeFrom.next())) {
removeFrom.remove();
result = true;
}
}
return result;
}
/**
* Removes every element that satisfies the provided predicate from the iterator. The iterator
* will be left exhausted: its {@code hasNext()} method will return {@code false}.
*
* @param removeFrom the iterator 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 iterator
* @since 2.0
*/
@CanIgnoreReturnValue
public static <T> boolean removeIf(Iterator<T> removeFrom, Predicate<? super T> predicate) {
checkNotNull(predicate);
boolean modified = false;
while (removeFrom.hasNext()) {
if (predicate.apply(removeFrom.next())) {
removeFrom.remove();
modified = true;
}
}
return modified;
}
/**
* Traverses an iterator and removes every element that does not belong to the provided
* collection. The iterator will be left exhausted: its {@code hasNext()} method will return
* {@code false}.
*
* @param removeFrom the iterator to (potentially) remove elements from
* @param elementsToRetain the elements to retain
* @return {@code true} if any element was removed from {@code iterator}
*/
@CanIgnoreReturnValue
public static boolean retainAll(Iterator<?> removeFrom, Collection<?> elementsToRetain) {
checkNotNull(elementsToRetain);
boolean result = false;
while (removeFrom.hasNext()) {
if (!elementsToRetain.contains(removeFrom.next())) {
removeFrom.remove();
result = true;
}
}
return result;
}
/**
* Determines whether two iterators contain equal elements in the same order. More specifically,
* this method returns {@code true} if {@code iterator1} and {@code iterator2} contain the same
* number of elements and every element of {@code iterator1} is equal to the corresponding element
* of {@code iterator2}.
*
* <p>Note that this will modify the supplied iterators, since they will have been advanced some
* number of elements forward.
*/
public static boolean elementsEqual(Iterator<?> iterator1, Iterator<?> iterator2) {
while (iterator1.hasNext()) {
if (!iterator2.hasNext()) {
return false;
}
Object o1 = iterator1.next();
Object o2 = iterator2.next();
if (!Objects.equal(o1, o2)) {
return false;
}
}
return !iterator2.hasNext();
}
/**
* Returns a string representation of {@code iterator}, with the format {@code [e1, e2, ..., en]}.
* The iterator will be left exhausted: its {@code hasNext()} method will return {@code false}.
*/
public static String toString(Iterator<?> iterator) {
StringBuilder sb = new StringBuilder().append('[');
boolean first = true;
while (iterator.hasNext()) {
if (!first) {
sb.append(", ");
}
first = false;
sb.append(iterator.next());
}
return sb.append(']').toString();
}
/**
* Returns the single element contained in {@code iterator}.
*
* @throws NoSuchElementException if the iterator is empty
* @throws IllegalArgumentException if the iterator contains multiple elements. The state of the
* iterator is unspecified.
*/
public static <T> T getOnlyElement(Iterator<T> iterator) {
T first = iterator.next();
if (!iterator.hasNext()) {
return first;
}
StringBuilder sb = new StringBuilder().append("expected one element but was: <").append(first);
for (int i = 0; i < 4 && iterator.hasNext(); i++) {
sb.append(", ").append(iterator.next());
}
if (iterator.hasNext()) {
sb.append(", ...");
}
sb.append('>');
throw new IllegalArgumentException(sb.toString());
}
/**
* Returns the single element contained in {@code iterator}, or {@code defaultValue} if the
* iterator is empty.
*
* @throws IllegalArgumentException if the iterator contains multiple elements. The state of the
* iterator is unspecified.
*/
public static <T> @Nullable T getOnlyElement(
Iterator<? extends T> iterator, @Nullable T defaultValue) {
return iterator.hasNext() ? getOnlyElement(iterator) : defaultValue;
}
/**
* Copies an iterator's elements into an array. The iterator will be left exhausted: its {@code
* hasNext()} method will return {@code false}.
*
* @param iterator the iterator to copy
* @param type the type of the elements
* @return a newly-allocated array into which all the elements of the iterator have been copied
*/
@GwtIncompatible // Array.newInstance(Class, int)
public static <T> T[] toArray(Iterator<? extends T> iterator, Class<T> type) {
List<T> list = Lists.newArrayList(iterator);
return Iterables.toArray(list, type);
}
/**
* Adds all elements in {@code iterator} to {@code collection}. The iterator will be left
* exhausted: its {@code hasNext()} method will return {@code false}.
*
* @return {@code true} if {@code collection} was modified as a result of this operation
*/
@CanIgnoreReturnValue
public static <T> boolean addAll(Collection<T> addTo, Iterator<? extends T> iterator) {
checkNotNull(addTo);
checkNotNull(iterator);
boolean wasModified = false;
while (iterator.hasNext()) {
wasModified |= addTo.add(iterator.next());
}
return wasModified;
}
/**
* Returns the number of elements in the specified iterator that equal the specified object. The
* iterator will be left exhausted: its {@code hasNext()} method will return {@code false}.
*
* @see Collections#frequency
*/
public static int frequency(Iterator<?> iterator, @Nullable Object element) {
int count = 0;
while (contains(iterator, element)) {
// Since it lives in the same class, we know contains gets to the element and then stops,
// though that isn't currently publicly documented.
count++;
}
return count;
}
/**
* Returns an iterator that cycles indefinitely over the elements of {@code iterable}.
*
* <p>The returned iterator supports {@code remove()} if the provided 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.
*
* <p><b>Warning:</b> 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.
*/
public static <T> Iterator<T> cycle(final Iterable<T> iterable) {
checkNotNull(iterable);
return new Iterator<T>() {
Iterator<T> iterator = emptyModifiableIterator();
@Override
public boolean hasNext() {
/*
* Don't store a new Iterator until we know the user can't remove() the last returned
* element anymore. Otherwise, when we remove from the old iterator, we may be invalidating
* the new one. The result is a ConcurrentModificationException or other bad behavior.
*
* (If we decide that we really, really hate allocating two Iterators per cycle instead of
* one, we can optimistically store the new Iterator and then be willing to throw it out if
* the user calls remove().)
*/
return iterator.hasNext() || iterable.iterator().hasNext();
}
@Override
public T next() {
if (!iterator.hasNext()) {
iterator = iterable.iterator();
if (!iterator.hasNext()) {
throw new NoSuchElementException();
}
}
return iterator.next();
}
@Override
public void remove() {
iterator.remove();
}
};
}
/**
* Returns an iterator that cycles indefinitely over the provided elements.
*
* <p>The returned iterator supports {@code remove()}. After {@code remove()} is called,
* subsequent cycles omit the removed element, but {@code elements} does not change. The
* iterator's {@code hasNext()} method returns {@code true} until all of the original elements
* have been removed.
*
* <p><b>Warning:</b> 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.
*/
@SafeVarargs
public static <T> Iterator<T> cycle(T... elements) {
return cycle(Lists.newArrayList(elements));
}
/**
* Returns an Iterator that walks the specified array, nulling out elements behind it. This can
* avoid memory leaks when an element is no longer necessary.
*
* <p>This is mainly just to avoid the intermediate ArrayDeque in ConsumingQueueIterator.
*/
private static <T> Iterator<T> consumingForArray(final T... elements) {
return new UnmodifiableIterator<T>() {
int index = 0;
@Override
public boolean hasNext() {
return index < elements.length;
}
@Override
public T next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
T result = elements[index];
elements[index] = null;
index++;
return result;
}
};
}
/**
* Combines two iterators into a single iterator. The returned iterator iterates across the
* elements in {@code a}, followed by the elements in {@code b}. The source iterators are not
* polled until necessary.
*
* <p>The returned iterator supports {@code remove()} when the corresponding input iterator
* supports it.
*/
public static <T> Iterator<T> concat(Iterator<? extends T> a, Iterator<? extends T> b) {
checkNotNull(a);
checkNotNull(b);
return concat(consumingForArray(a, b));
}
/**
* Combines three iterators into a single iterator. The returned iterator iterates across 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.
*
* <p>The returned iterator supports {@code remove()} when the corresponding input iterator
* supports it.
*/
public static <T> Iterator<T> concat(
Iterator<? extends T> a, Iterator<? extends T> b, Iterator<? extends T> c) {
checkNotNull(a);
checkNotNull(b);
checkNotNull(c);
return concat(consumingForArray(a, b, c));
}
/**
* Combines four iterators into a single iterator. The returned iterator iterates across 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.
*
* <p>The returned iterator supports {@code remove()} when the corresponding input iterator
* supports it.
*/
public static <T> Iterator<T> concat(
Iterator<? extends T> a,
Iterator<? extends T> b,
Iterator<? extends T> c,
Iterator<? extends T> d) {
checkNotNull(a);
checkNotNull(b);
checkNotNull(c);
checkNotNull(d);
return concat(consumingForArray(a, b, c, d));
}
/**
* Combines multiple iterators into a single iterator. The returned iterator iterates across the
* elements of each iterator in {@code inputs}. The input iterators are not polled until
* necessary.
*
* <p>The returned iterator supports {@code remove()} when the corresponding input iterator
* supports it.
*
* @throws NullPointerException if any of the provided iterators is null
*/
public static <T> Iterator<T> concat(Iterator<? extends T>... inputs) {
return concatNoDefensiveCopy(Arrays.copyOf(inputs, inputs.length));
}
/**
* Combines multiple iterators into a single iterator. The returned iterator iterates across the
* elements of each iterator in {@code inputs}. The input iterators are not polled until
* necessary.
*
* <p>The returned iterator supports {@code remove()} when the corresponding input iterator
* supports it. The methods of the returned iterator may throw {@code NullPointerException} if any
* of the input iterators is null.
*/
public static <T> Iterator<T> concat(Iterator<? extends Iterator<? extends T>> inputs) {
return new ConcatenatedIterator<T>(inputs);
}
/** Concats a varargs array of iterators without making a defensive copy of the array. */
static <T> Iterator<T> concatNoDefensiveCopy(Iterator<? extends T>... inputs) {
for (Iterator<? extends T> input : checkNotNull(inputs)) {
checkNotNull(input);
}
return concat(consumingForArray(inputs));
}
/**
* Divides an iterator into unmodifiable sublists of the given size (the final list may be
* smaller). For example, partitioning an iterator containing {@code [a, b, c, d, e]} with a
* partition size of 3 yields {@code [[a, b, c], [d, e]]} -- an outer iterator containing two
* inner lists of three and two elements, all in the original order.
*
* <p>The returned lists implement {@link java.util.RandomAccess}.
*
* @param iterator the iterator to return a partitioned view of
* @param size the desired size of each partition (the last may be smaller)
* @return an iterator of immutable lists containing the elements of {@code iterator} divided into
* partitions
* @throws IllegalArgumentException if {@code size} is nonpositive
*/
public static <T> UnmodifiableIterator<List<T>> partition(Iterator<T> iterator, int size) {
return partitionImpl(iterator, size, false);
}
/**
* Divides an iterator into unmodifiable sublists of the given size, padding the final iterator
* with null values if necessary. For example, partitioning an iterator containing {@code [a, b,
* c, d, e]} with a partition size of 3 yields {@code [[a, b, c], [d, e, null]]} -- an outer
* iterator containing two inner lists of three elements each, all in the original order.
*
* <p>The returned lists implement {@link java.util.RandomAccess}.
*
* @param iterator the iterator to return a partitioned view of
* @param size the desired size of each partition
* @return an iterator of immutable lists containing the elements of {@code iterator} divided into
* partitions (the final iterable may have trailing null elements)
* @throws IllegalArgumentException if {@code size} is nonpositive
*/
public static <T> UnmodifiableIterator<List<T>> paddedPartition(Iterator<T> iterator, int size) {
return partitionImpl(iterator, size, true);
}
private static <T> UnmodifiableIterator<List<T>> partitionImpl(
final Iterator<T> iterator, final int size, final boolean pad) {
checkNotNull(iterator);
checkArgument(size > 0);
return new UnmodifiableIterator<List<T>>() {
@Override
public boolean hasNext() {
return iterator.hasNext();
}
@Override
public List<T> next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
Object[] array = new Object[size];
int count = 0;
for (; count < size && iterator.hasNext(); count++) {
array[count] = iterator.next();
}
for (int i = count; i < size; i++) {
array[i] = null; // for GWT
}
@SuppressWarnings("unchecked") // we only put Ts in it
List<T> list = Collections.unmodifiableList((List<T>) Arrays.asList(array));
return (pad || count == size) ? list : list.subList(0, count);
}
};
}
/**
* Returns a view of {@code unfiltered} containing all elements that satisfy the input predicate
* {@code retainIfTrue}.
*/
public static <T> UnmodifiableIterator<T> filter(
final Iterator<T> unfiltered, final Predicate<? super T> retainIfTrue) {
checkNotNull(unfiltered);
checkNotNull(retainIfTrue);
return new AbstractIterator<T>() {
@Override
protected T computeNext() {
while (unfiltered.hasNext()) {
T element = unfiltered.next();
if (retainIfTrue.apply(element)) {
return element;
}
}
return endOfData();
}
};
}
/**
* Returns a view of {@code unfiltered} containing all elements that are of the type {@code
* desiredType}.
*/
@SuppressWarnings("unchecked") // can cast to <T> because non-Ts are removed
@GwtIncompatible // Class.isInstance
public static <T> UnmodifiableIterator<T> filter(Iterator<?> unfiltered, Class<T> desiredType) {
return (UnmodifiableIterator<T>) filter(unfiltered, instanceOf(desiredType));
}
/**
* Returns {@code true} if one or more elements returned by {@code iterator} satisfy the given
* predicate.
*/
public static <T> boolean any(Iterator<T> iterator, Predicate<? super T> predicate) {
return indexOf(iterator, predicate) != -1;
}
/**
* Returns {@code true} if every element returned by {@code iterator} satisfies the given
* predicate. If {@code iterator} is empty, {@code true} is returned.
*/
public static <T> boolean all(Iterator<T> iterator, Predicate<? super T> predicate) {
checkNotNull(predicate);
while (iterator.hasNext()) {
T element = iterator.next();
if (!predicate.apply(element)) {
return false;
}
}
return true;
}
/**
* Returns the first element in {@code iterator} that satisfies the given predicate; use this
* method only when such an element is known to exist. If no such element is found, the iterator
* will be left exhausted: its {@code hasNext()} method will return {@code false}. If it is
* possible that <i>no</i> element will match, use {@link #tryFind} or {@link #find(Iterator,
* Predicate, Object)} instead.
*
* @throws NoSuchElementException if no element in {@code iterator} matches the given predicate
*/
public static <T> T find(Iterator<T> iterator, Predicate<? super T> predicate) {
checkNotNull(iterator);
checkNotNull(predicate);
while (iterator.hasNext()) {
T t = iterator.next();
if (predicate.apply(t)) {
return t;
}
}
throw new NoSuchElementException();
}
/**
* Returns the first element in {@code iterator} that satisfies the given predicate. If no such
* element is found, {@code defaultValue} will be returned from this method and the iterator will
* be left exhausted: its {@code hasNext()} method will return {@code false}. Note that this can
* usually be handled more naturally using {@code tryFind(iterator, predicate).or(defaultValue)}.
*
* @since 7.0
*/
public static <T> @Nullable T find(
Iterator<? extends T> iterator, Predicate<? super T> predicate, @Nullable T defaultValue) {
checkNotNull(iterator);
checkNotNull(predicate);
while (iterator.hasNext()) {
T t = iterator.next();
if (predicate.apply(t)) {
return t;
}
}
return defaultValue;
}
/**
* Returns an {@link Optional} containing the first element in {@code iterator} that satisfies the
* given predicate, if such an element exists. If no such element is found, an empty {@link
* Optional} will be returned from this method and the iterator will be left exhausted: its {@code
* hasNext()} method will return {@code false}.
*
* <p><b>Warning:</b> avoid using a {@code predicate} that matches {@code null}. If {@code null}
* is matched in {@code iterator}, a NullPointerException will be thrown.
*
* @since 11.0
*/
public static <T> Optional<T> tryFind(Iterator<T> iterator, Predicate<? super T> predicate) {
checkNotNull(iterator);
checkNotNull(predicate);
while (iterator.hasNext()) {
T t = iterator.next();
if (predicate.apply(t)) {
return Optional.of(t);
}
}
return Optional.absent();
}
/**
* Returns the index in {@code iterator} of the first element that satisfies the provided {@code
* predicate}, or {@code -1} if the Iterator has no such elements.
*
* <p>More formally, returns the lowest index {@code i} such that {@code
* predicate.apply(Iterators.get(iterator, i))} returns {@code true}, or {@code -1} if there is no
* such index.
*
* <p>If -1 is returned, the iterator will be left exhausted: its {@code hasNext()} method will
* return {@code false}. Otherwise, the iterator will be set to the element which satisfies the
* {@code predicate}.
*
* @since 2.0
*/
public static <T> int indexOf(Iterator<T> iterator, Predicate<? super T> predicate) {
checkNotNull(predicate, "predicate");
for (int i = 0; iterator.hasNext(); i++) {
T current = iterator.next();
if (predicate.apply(current)) {
return i;
}
}
return -1;
}
/**
* Returns a view containing the result of applying {@code function} to each element of {@code
* fromIterator}.
*
* <p>The returned iterator supports {@code remove()} if {@code fromIterator} does. After a
* successful {@code remove()} call, {@code fromIterator} no longer contains the corresponding
* element.
*/
public static <F, T> Iterator<T> transform(
final Iterator<F> fromIterator, final Function<? super F, ? extends T> function) {
checkNotNull(function);
return new TransformedIterator<F, T>(fromIterator) {
@Override
T transform(F from) {
return function.apply(from);
}
};
}
/**
* Advances {@code iterator} {@code position + 1} times, returning the element at the {@code
* position}th position.
*
* @param position position of the element to return
* @return the element at the specified position in {@code iterator}
* @throws IndexOutOfBoundsException if {@code position} is negative or greater than or equal to
* the number of elements remaining in {@code iterator}
*/
public static <T> T get(Iterator<T> iterator, int position) {
checkNonnegative(position);
int skipped = advance(iterator, position);
if (!iterator.hasNext()) {
throw new IndexOutOfBoundsException(
"position ("
+ position
+ ") must be less than the number of elements that remained ("
+ skipped
+ ")");
}
return iterator.next();
}
/**
* Advances {@code iterator} {@code position + 1} times, returning the element at the {@code
* position}th position or {@code defaultValue} otherwise.
*
* @param position position of the element to return
* @param defaultValue the default value to return if the iterator is empty or if {@code position}
* is greater than the number of elements remaining in {@code iterator}
* @return the element at the specified position in {@code iterator} or {@code defaultValue} if
* {@code iterator} produces fewer than {@code position + 1} elements.
* @throws IndexOutOfBoundsException if {@code position} is negative
* @since 4.0
*/
public static <T> @Nullable T get(
Iterator<? extends T> iterator, int position, @Nullable T defaultValue) {
checkNonnegative(position);
advance(iterator, position);
return getNext(iterator, defaultValue);
}
static void checkNonnegative(int position) {
if (position < 0) {
throw new IndexOutOfBoundsException("position (" + position + ") must not be negative");
}
}
/**
* Returns the next element in {@code iterator} or {@code defaultValue} if the iterator is empty.
* The {@link Iterables} analog to this method is {@link Iterables#getFirst}.
*
* @param defaultValue the default value to return if the iterator is empty
* @return the next element of {@code iterator} or the default value
* @since 7.0
*/
public static <T> @Nullable T getNext(Iterator<? extends T> iterator, @Nullable T defaultValue) {
return iterator.hasNext() ? iterator.next() : defaultValue;
}
/**
* Advances {@code iterator} to the end, returning the last element.
*
* @return the last element of {@code iterator}
* @throws NoSuchElementException if the iterator is empty
*/
public static <T> T getLast(Iterator<T> iterator) {
while (true) {
T current = iterator.next();
if (!iterator.hasNext()) {
return current;
}
}
}
/**
* Advances {@code iterator} to the end, returning the last element or {@code defaultValue} if the
* iterator is empty.
*
* @param defaultValue the default value to return if the iterator is empty
* @return the last element of {@code iterator}
* @since 3.0
*/
public static <T> @Nullable T getLast(Iterator<? extends T> iterator, @Nullable T defaultValue) {
return iterator.hasNext() ? getLast(iterator) : defaultValue;
}
/**
* Calls {@code next()} on {@code iterator}, either {@code numberToAdvance} times or until {@code
* hasNext()} returns {@code false}, whichever comes first.
*
* @return the number of elements the iterator was advanced
* @since 13.0 (since 3.0 as {@code Iterators.skip})
*/
@CanIgnoreReturnValue
public static int advance(Iterator<?> iterator, int numberToAdvance) {
checkNotNull(iterator);
checkArgument(numberToAdvance >= 0, "numberToAdvance must be nonnegative");
int i;
for (i = 0; i < numberToAdvance && iterator.hasNext(); i++) {
iterator.next();
}
return i;
}
/**
* Returns a view containing the first {@code limitSize} elements of {@code iterator}. If {@code
* iterator} contains fewer than {@code limitSize} elements, the returned view contains all of its
* elements. The returned iterator supports {@code remove()} if {@code iterator} does.
*
* @param iterator the iterator to limit
* @param limitSize the maximum number of elements in the returned iterator
* @throws IllegalArgumentException if {@code limitSize} is negative
* @since 3.0
*/
public static <T> Iterator<T> limit(final Iterator<T> iterator, final int limitSize) {
checkNotNull(iterator);
checkArgument(limitSize >= 0, "limit is negative");
return new Iterator<T>() {
private int count;
@Override
public boolean hasNext() {
return count < limitSize && iterator.hasNext();
}
@Override
public T next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
count++;
return iterator.next();
}
@Override
public void remove() {
iterator.remove();
}
};
}
/**
* Returns a view of the supplied {@code iterator} that removes each element from the supplied
* {@code iterator} as it is returned.
*
* <p>The provided iterator must support {@link Iterator#remove()} or else the returned iterator
* will fail on the first call to {@code next}.
*
* @param iterator the iterator to remove and return elements from
* @return an iterator that removes and returns elements from the supplied iterator
* @since 2.0
*/
public static <T> Iterator<T> consumingIterator(final Iterator<T> iterator) {
checkNotNull(iterator);
return new UnmodifiableIterator<T>() {
@Override
public boolean hasNext() {
return iterator.hasNext();
}
@Override
public T next() {
T next = iterator.next();
iterator.remove();
return next;
}
@Override
public String toString() {
return "Iterators.consumingIterator(...)";
}
};
}
/**
* Deletes and returns the next value from the iterator, or returns {@code null} if there is no
* such value.
*/
static <T> @Nullable T pollNext(Iterator<T> iterator) {
if (iterator.hasNext()) {
T result = iterator.next();
iterator.remove();
return result;
} else {
return null;
}
}
// Methods only in Iterators, not in Iterables
/** Clears the iterator using its remove method. */
static void clear(Iterator<?> iterator) {
checkNotNull(iterator);
while (iterator.hasNext()) {
iterator.next();
iterator.remove();
}
}
/**
* Returns an iterator containing the elements of {@code array} in order. The returned iterator is
* a view of the array; subsequent changes to the array will be reflected in the iterator.
*
* <p><b>Note:</b> It is often preferable to represent your data using a collection type, for
* example using {@link Arrays#asList(Object[])}, making this method unnecessary.
*
* <p>The {@code Iterable} equivalent of this method is either {@link Arrays#asList(Object[])},
* {@link ImmutableList#copyOf(Object[])}}, or {@link ImmutableList#of}.
*/
@SafeVarargs
public static <T> UnmodifiableIterator<T> forArray(final T... array) {
return forArray(array, 0, array.length, 0);
}
/**
* Returns a list iterator containing the elements in the specified range of {@code array} in
* order, starting at the specified index.
*
* <p>The {@code Iterable} equivalent of this method is {@code
* Arrays.asList(array).subList(offset, offset + length).listIterator(index)}.
*/
static <T> UnmodifiableListIterator<T> forArray(
final T[] array, final int offset, int length, int index) {
checkArgument(length >= 0);
int end = offset + length;
// Technically we should give a slightly more descriptive error on overflow
Preconditions.checkPositionIndexes(offset, end, array.length);
Preconditions.checkPositionIndex(index, length);
if (length == 0) {
return emptyListIterator();
}
return new ArrayItr<T>(array, offset, length, index);
}
private static final class ArrayItr<T> extends AbstractIndexedListIterator<T> {
static final UnmodifiableListIterator<Object> EMPTY = new ArrayItr<>(new Object[0], 0, 0, 0);
private final T[] array;
private final int offset;
ArrayItr(T[] array, int offset, int length, int index) {
super(length, index);
this.array = array;
this.offset = offset;
}
@Override
protected T get(int index) {
return array[offset + index];
}
}
/**
* Returns an iterator containing only {@code value}.
*
* <p>The {@link Iterable} equivalent of this method is {@link Collections#singleton}.
*/
public static <T> UnmodifiableIterator<T> singletonIterator(final @Nullable T value) {
return new UnmodifiableIterator<T>() {
boolean done;
@Override
public boolean hasNext() {
return !done;
}
@Override
public T next() {
if (done) {
throw new NoSuchElementException();
}
done = true;
return value;
}
};
}
/**
* Adapts an {@code Enumeration} to the {@code Iterator} interface.
*
* <p>This method has no equivalent in {@link Iterables} because viewing an {@code Enumeration} as
* an {@code Iterable} is impossible. However, the contents can be <i>copied</i> into a collection
* using {@link Collections#list}.
*
* <p><b>Java 9 users:</b> use {@code enumeration.asIterator()} instead, unless it is important to
* return an {@code UnmodifiableIterator} instead of a plain {@code Iterator}.
*/
public static <T> UnmodifiableIterator<T> forEnumeration(final Enumeration<T> enumeration) {
checkNotNull(enumeration);
return new UnmodifiableIterator<T>() {
@Override
public boolean hasNext() {
return enumeration.hasMoreElements();
}
@Override
public T next() {
return enumeration.nextElement();
}
};
}
/**
* Adapts an {@code Iterator} to the {@code Enumeration} interface.
*
* <p>The {@code Iterable} equivalent of this method is either {@link Collections#enumeration} (if
* you have a {@link Collection}), or {@code Iterators.asEnumeration(collection.iterator())}.
*/
public static <T> Enumeration<T> asEnumeration(final Iterator<T> iterator) {
checkNotNull(iterator);
return new Enumeration<T>() {
@Override
public boolean hasMoreElements() {
return iterator.hasNext();
}
@Override
public T nextElement() {
return iterator.next();
}
};
}
/** Implementation of PeekingIterator that avoids peeking unless necessary. */
private static class PeekingImpl<E> implements PeekingIterator<E> {
private final Iterator<? extends E> iterator;
private boolean hasPeeked;
private @Nullable E peekedElement;
public PeekingImpl(Iterator<? extends E> iterator) {
this.iterator = checkNotNull(iterator);
}
@Override
public boolean hasNext() {
return hasPeeked || iterator.hasNext();
}
@Override
public E next() {
if (!hasPeeked) {
return iterator.next();
}
E result = peekedElement;
hasPeeked = false;
peekedElement = null;
return result;
}
@Override
public void remove() {
checkState(!hasPeeked, "Can't remove after you've peeked at next");
iterator.remove();
}
@Override
public E peek() {
if (!hasPeeked) {
peekedElement = iterator.next();
hasPeeked = true;
}
return peekedElement;
}
}
/**
* Returns a {@code PeekingIterator} backed by the given iterator.
*
* <p>Calls to the {@code peek} method with no intervening calls to {@code next} do not affect the
* iteration, and hence return the same object each time. A subsequent call to {@code next} is
* guaranteed to return the same object again. For example:
*
* <pre>{@code
* PeekingIterator<String> peekingIterator =
* Iterators.peekingIterator(Iterators.forArray("a", "b"));
* String a1 = peekingIterator.peek(); // returns "a"
* String a2 = peekingIterator.peek(); // also returns "a"
* String a3 = peekingIterator.next(); // also returns "a"
* }</pre>
*
* <p>Any structural changes to the underlying iteration (aside from those performed by the
* iterator's own {@link PeekingIterator#remove()} method) will leave the iterator in an undefined
* state.
*
* <p>The returned iterator does not support removal after peeking, as explained by {@link
* PeekingIterator#remove()}.
*
* <p>Note: If the given iterator is already a {@code PeekingIterator}, it <i>might</i> be
* returned to the caller, although this is neither guaranteed to occur nor required to be
* consistent. For example, this method <i>might</i> choose to pass through recognized
* implementations of {@code PeekingIterator} when the behavior of the implementation is known to
* meet the contract guaranteed by this method.
*
* <p>There is no {@link Iterable} equivalent to this method, so use this method to wrap each
* individual iterator as it is generated.
*
* @param iterator the backing iterator. The {@link PeekingIterator} assumes ownership of this
* iterator, so users should cease making direct calls to it after calling this method.
* @return a peeking iterator backed by that iterator. Apart from the additional {@link
* PeekingIterator#peek()} method, this iterator behaves exactly the same as {@code iterator}.
*/
public static <T> PeekingIterator<T> peekingIterator(Iterator<? extends T> iterator) {
if (iterator instanceof PeekingImpl) {
// Safe to cast <? extends T> to <T> because PeekingImpl only uses T
// covariantly (and cannot be subclassed to add non-covariant uses).
@SuppressWarnings("unchecked")
PeekingImpl<T> peeking = (PeekingImpl<T>) iterator;
return peeking;
}
return new PeekingImpl<T>(iterator);
}
/**
* Simply returns its argument.
*
* @deprecated no need to use this
* @since 10.0
*/
@Deprecated
public static <T> PeekingIterator<T> peekingIterator(PeekingIterator<T> iterator) {
return checkNotNull(iterator);
}
/**
* Returns an iterator over the merged contents of all given {@code iterators}, traversing every
* element of the input iterators. Equivalent entries will not be de-duplicated.
*
* <p>Callers must ensure that the source {@code iterators} are in non-descending order as this
* method does not sort its input.
*
* <p>For any equivalent elements across all {@code iterators}, it is undefined which element is
* returned first.
*
* @since 11.0
*/
@Beta
public static <T> UnmodifiableIterator<T> mergeSorted(
Iterable<? extends Iterator<? extends T>> iterators, Comparator<? super T> comparator) {
checkNotNull(iterators, "iterators");
checkNotNull(comparator, "comparator");
return new MergingIterator<T>(iterators, comparator);
}
/**
* An iterator that performs a lazy N-way merge, calculating the next value each time the iterator
* is polled. This amortizes the sorting cost over the iteration and requires less memory than
* sorting all elements at once.
*
* <p>Retrieving a single element takes approximately O(log(M)) time, where M is the number of
* iterators. (Retrieving all elements takes approximately O(N*log(M)) time, where N is the total
* number of elements.)
*/
private static class MergingIterator<T> extends UnmodifiableIterator<T> {
final Queue<PeekingIterator<T>> queue;
public MergingIterator(
Iterable<? extends Iterator<? extends T>> iterators,
final Comparator<? super T> itemComparator) {
// A comparator that's used by the heap, allowing the heap
// to be sorted based on the top of each iterator.
Comparator<PeekingIterator<T>> heapComparator =
new Comparator<PeekingIterator<T>>() {
@Override
public int compare(PeekingIterator<T> o1, PeekingIterator<T> o2) {
return itemComparator.compare(o1.peek(), o2.peek());
}
};
queue = new PriorityQueue<>(2, heapComparator);
for (Iterator<? extends T> iterator : iterators) {
if (iterator.hasNext()) {
queue.add(Iterators.peekingIterator(iterator));
}
}
}
@Override
public boolean hasNext() {
return !queue.isEmpty();
}
@Override
public T next() {
PeekingIterator<T> nextIter = queue.remove();
T next = nextIter.next();
if (nextIter.hasNext()) {
queue.add(nextIter);
}
return next;
}
}
private static class ConcatenatedIterator<T> implements Iterator<T> {
/* The last iterator to return an element. Calls to remove() go to this iterator. */
private @Nullable Iterator<? extends T> toRemove;
/* The iterator currently returning elements. */
private Iterator<? extends T> iterator;
/*
* We track the "meta iterators," the iterators-of-iterators, below. Usually, topMetaIterator
* is the only one in use, but if we encounter nested concatenations, we start a deque of
* meta-iterators rather than letting the nesting get arbitrarily deep. This keeps each
* operation O(1).
*/
private Iterator<? extends Iterator<? extends T>> topMetaIterator;
// Only becomes nonnull if we encounter nested concatenations.
private @Nullable Deque<Iterator<? extends Iterator<? extends T>>> metaIterators;
ConcatenatedIterator(Iterator<? extends Iterator<? extends T>> metaIterator) {
iterator = emptyIterator();
topMetaIterator = checkNotNull(metaIterator);
}
// Returns a nonempty meta-iterator or, if all meta-iterators are empty, null.
private @Nullable Iterator<? extends Iterator<? extends T>> getTopMetaIterator() {
while (topMetaIterator == null || !topMetaIterator.hasNext()) {
if (metaIterators != null && !metaIterators.isEmpty()) {
topMetaIterator = metaIterators.removeFirst();
} else {
return null;
}
}
return topMetaIterator;
}
@Override
public boolean hasNext() {
while (!checkNotNull(iterator).hasNext()) {
// this weird checkNotNull positioning appears required by our tests, which expect
// both hasNext and next to throw NPE if an input iterator is null.
topMetaIterator = getTopMetaIterator();
if (topMetaIterator == null) {
return false;
}
iterator = topMetaIterator.next();
if (iterator instanceof ConcatenatedIterator) {
// Instead of taking linear time in the number of nested concatenations, unpack
// them into the queue
@SuppressWarnings("unchecked")
ConcatenatedIterator<T> topConcat = (ConcatenatedIterator<T>) iterator;
iterator = topConcat.iterator;
// topConcat.topMetaIterator, then topConcat.metaIterators, then this.topMetaIterator,
// then this.metaIterators
if (this.metaIterators == null) {
this.metaIterators = new ArrayDeque<>();
}
this.metaIterators.addFirst(this.topMetaIterator);
if (topConcat.metaIterators != null) {
while (!topConcat.metaIterators.isEmpty()) {
this.metaIterators.addFirst(topConcat.metaIterators.removeLast());
}
}
this.topMetaIterator = topConcat.topMetaIterator;
}
}
return true;
}
@Override
public T next() {
if (hasNext()) {
toRemove = iterator;
return iterator.next();
} else {
throw new NoSuchElementException();
}
}
@Override
public void remove() {
CollectPreconditions.checkRemove(toRemove != null);
toRemove.remove();
toRemove = null;
}
}
/** Used to avoid http://bugs.sun.com/view_bug.do?bug_id=6558557 */
static <T> ListIterator<T> cast(Iterator<T> iterator) {
return (ListIterator<T>) iterator;
}
}