/
Iterators.java
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/
Iterators.java
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/*
* 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.equalTo;
import static com.google.common.base.Predicates.in;
import static com.google.common.base.Predicates.not;
import static com.google.common.collect.CollectPreconditions.checkRemove;
import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
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 java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
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 javax.annotation.CheckReturnValue;
import javax.annotation.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() {}
static final UnmodifiableListIterator<Object> EMPTY_LIST_ITERATOR =
new UnmodifiableListIterator<Object>() {
@Override
public boolean hasNext() {
return false;
}
@Override
public Object next() {
throw new NoSuchElementException();
}
@Override
public boolean hasPrevious() {
return false;
}
@Override
public Object previous() {
throw new NoSuchElementException();
}
@Override
public int nextIndex() {
return 0;
}
@Override
public int previousIndex() {
return -1;
}
};
/**
* Returns the empty iterator.
*
* <p>The {@link Iterable} equivalent of this method is {@link
* ImmutableSet#of()}.
*
* @deprecated Use {@code ImmutableSet.<T>of().iterator()} instead; or for
* Java 7 or later, {@link Collections#emptyIterator}. This method is
* scheduled for removal in May 2016.
*/
@Deprecated
public 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>) EMPTY_LIST_ITERATOR;
}
private static final Iterator<Object> EMPTY_MODIFIABLE_ITERATOR =
new Iterator<Object>() {
@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>) EMPTY_MODIFIABLE_ITERATOR;
}
/** Returns an unmodifiable view of {@code iterator}. */
public static <T> UnmodifiableIterator<T> unmodifiableIterator(final Iterator<T> iterator) {
checkNotNull(iterator);
if (iterator instanceof UnmodifiableIterator) {
return (UnmodifiableIterator<T>) iterator;
}
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) {
int count = 0;
while (iterator.hasNext()) {
iterator.next();
count++;
}
return count;
}
/**
* Returns {@code true} if {@code iterator} contains {@code element}.
*/
public static boolean contains(Iterator<?> iterator, @Nullable Object element) {
return any(iterator, equalTo(element));
}
/**
* 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}
*/
public static boolean removeAll(Iterator<?> removeFrom, Collection<?> elementsToRemove) {
return removeIf(removeFrom, in(elementsToRemove));
}
/**
* 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
*/
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}
*/
public static boolean retainAll(Iterator<?> removeFrom, Collection<?> elementsToRetain) {
return removeIf(removeFrom, not(in(elementsToRetain)));
}
/**
* 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) {
return Collections2.STANDARD_JOINER
.appendTo(new StringBuilder().append('['), iterator)
.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();
sb.append("expected one element but was: <" + first);
for (int i = 0; i < 4 && iterator.hasNext(); i++) {
sb.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.
*/
@Nullable
public static <T> T getOnlyElement(Iterator<? extends T> iterator, @Nullable T defaultValue) {
return iterator.hasNext() ? getOnlyElement(iterator) : defaultValue;
}
/**
* 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
*/
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) {
return size(filter(iterator, equalTo(element)));
}
/**
* 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.
*/
public static <T> Iterator<T> cycle(T... elements) {
return cycle(Lists.newArrayList(elements));
}
/**
* 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.
*
* <p><b>Note:</b> the current implementation is not suitable for nested
* concatenated iterators, i.e. the following should be avoided when in a loop:
* {@code iterator = Iterators.concat(iterator, suffix);}, since iteration over the
* resulting iterator has a cubic complexity to the depth of the nesting.
*/
public static <T> Iterator<T> concat(Iterator<? extends T> a, Iterator<? extends T> b) {
checkNotNull(a);
checkNotNull(b);
return concat(new ConsumingQueueIterator<Iterator<? extends T>>(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.
*
* <p><b>Note:</b> the current implementation is not suitable for nested
* concatenated iterators, i.e. the following should be avoided when in a loop:
* {@code iterator = Iterators.concat(iterator, suffix);}, since iteration over the
* resulting iterator has a cubic complexity to the depth of the nesting.
*/
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(new ConsumingQueueIterator<Iterator<? extends T>>(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.
*
* <p><b>Note:</b> the current implementation is not suitable for nested
* concatenated iterators, i.e. the following should be avoided when in a loop:
* {@code iterator = Iterators.concat(iterator, suffix);}, since iteration over the
* resulting iterator has a cubic complexity to the depth of the nesting.
*/
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(new ConsumingQueueIterator<Iterator<? extends T>>(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.
*
* <p><b>Note:</b> the current implementation is not suitable for nested
* concatenated iterators, i.e. the following should be avoided when in a loop:
* {@code iterator = Iterators.concat(iterator, suffix);}, since iteration over the
* resulting iterator has a cubic complexity to the depth of the nesting.
*
* @throws NullPointerException if any of the provided iterators is null
*/
public static <T> Iterator<T> concat(Iterator<? extends T>... inputs) {
for (Iterator<? extends T> input : checkNotNull(inputs)) {
checkNotNull(input);
}
return concat(new ConsumingQueueIterator<Iterator<? extends T>>(inputs));
}
/**
* 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.
*
* <p><b>Note:</b> the current implementation is not suitable for nested
* concatenated iterators, i.e. the following should be avoided when in a loop:
* {@code iterator = Iterators.concat(iterator, suffix);}, since iteration over the
* resulting iterator has a cubic complexity to the depth of the nesting.
*/
public static <T> Iterator<T> concat(final Iterator<? extends Iterator<? extends T>> inputs) {
checkNotNull(inputs);
return new Iterator<T>() {
Iterator<? extends T> current = emptyIterator();
Iterator<? extends T> removeFrom;
@Override
public boolean hasNext() {
// http://code.google.com/p/google-collections/issues/detail?id=151
// current.hasNext() might be relatively expensive, worth minimizing.
boolean currentHasNext;
// checkNotNull eager for GWT
// note: it must be here & not where 'current' is assigned,
// because otherwise we'll have called inputs.next() before throwing
// the first NPE, and the next time around we'll call inputs.next()
// again, incorrectly moving beyond the error.
while (!(currentHasNext = checkNotNull(current).hasNext()) && inputs.hasNext()) {
current = inputs.next();
}
return currentHasNext;
}
@Override
public T next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
removeFrom = current;
return current.next();
}
@Override
public void remove() {
checkRemove(removeFrom != null);
removeFrom.remove();
removeFrom = null;
}
};
}
/**
* 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 the elements of {@code unfiltered} that satisfy a predicate.
*/
@CheckReturnValue
public static <T> UnmodifiableIterator<T> filter(
final Iterator<T> unfiltered, final Predicate<? super T> predicate) {
checkNotNull(unfiltered);
checkNotNull(predicate);
return new AbstractIterator<T>() {
@Override
protected T computeNext() {
while (unfiltered.hasNext()) {
T element = unfiltered.next();
if (predicate.apply(element)) {
return element;
}
}
return endOfData();
}
};
}
/**
* 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) {
return filter(iterator, predicate).next();
}
/**
* 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
*/
@Nullable
public static <T> T find(
Iterator<? extends T> iterator, Predicate<? super T> predicate, @Nullable T defaultValue) {
return getNext(filter(iterator, predicate), 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) {
UnmodifiableIterator<T> filteredIterator = filter(iterator, predicate);
return filteredIterator.hasNext()
? Optional.of(filteredIterator.next())
: Optional.<T>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 an iterator that applies {@code function} to each element of {@code
* fromIterator}.
*
* <p>The returned iterator supports {@code remove()} if the provided iterator
* 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();
}
static void checkNonnegative(int position) {
if (position < 0) {
throw new IndexOutOfBoundsException("position (" + position + ") must not be negative");
}
}
/**
* 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
*/
@Nullable
public static <T> T get(Iterator<? extends T> iterator, int position, @Nullable T defaultValue) {
checkNonnegative(position);
advance(iterator, position);
return getNext(iterator, defaultValue);
}
/**
* 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
*/
@Nullable
public static <T> 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
*/
@Nullable
public static <T> 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})
*/
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;
}
/**
* Creates an iterator returning the first {@code limitSize} elements of the
* given iterator. If the original iterator does not contain that many
* elements, the returned iterator will have the same behavior as the original
* iterator. The returned iterator supports {@code remove()} if the original
* 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.
*/
@Nullable
static <T> 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();
}
}