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com.google.common.collect.Iterators 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.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.instanceOf;
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.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 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.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.
*
* Performance notes: Unless otherwise noted, all of the iterators
* produced in this class are lazy , which means that they only advance
* the backing iteration when absolutely necessary.
*
*
See the Guava User Guide section on
* {@code Iterators} .
*
* @author Kevin Bourrillion
* @author Jared Levy
* @since 2.0 (imported from Google Collections Library)
*/
@GwtCompatible(emulated = true)
public final class Iterators {
private Iterators() {}
static final UnmodifiableListIterator EMPTY_LIST_ITERATOR
= new UnmodifiableListIterator() {
@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.
*
* The {@link Iterable} equivalent of this method is {@link
* ImmutableSet#of()}.
*
* @deprecated Use {@code ImmutableSet.of().iterator()} instead; or for
* Java 7 or later, {@link Collections#emptyIterator}. This method is
* scheduled for removal in May 2016.
*/
@Deprecated
public static UnmodifiableIterator emptyIterator() {
return emptyListIterator();
}
/**
* Returns the empty iterator.
*
* 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 UnmodifiableListIterator emptyListIterator() {
return (UnmodifiableListIterator) EMPTY_LIST_ITERATOR;
}
private static final Iterator EMPTY_MODIFIABLE_ITERATOR =
new Iterator() {
@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 Iterator emptyModifiableIterator() {
return (Iterator) EMPTY_MODIFIABLE_ITERATOR;
}
/** Returns an unmodifiable view of {@code iterator}. */
public static UnmodifiableIterator unmodifiableIterator(
final Iterator iterator) {
checkNotNull(iterator);
if (iterator instanceof UnmodifiableIterator) {
return (UnmodifiableIterator) iterator;
}
return new UnmodifiableIterator() {
@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 UnmodifiableIterator unmodifiableIterator(
UnmodifiableIterator 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 boolean removeIf(
Iterator removeFrom, Predicate 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}.
*
* 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 getOnlyElement(Iterator 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 getOnlyElement(Iterator 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[] toArray(
Iterator iterator, Class type) {
List 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
*/
public static boolean addAll(
Collection addTo, Iterator 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}.
*
* 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.
*
*
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.
*/
public static Iterator cycle(final Iterable iterable) {
checkNotNull(iterable);
return new Iterator() {
Iterator iterator = emptyIterator();
Iterator removeFrom;
@Override
public boolean hasNext() {
if (!iterator.hasNext()) {
iterator = iterable.iterator();
}
return iterator.hasNext();
}
@Override
public T next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
removeFrom = iterator;
return iterator.next();
}
@Override
public void remove() {
checkRemove(removeFrom != null);
removeFrom.remove();
removeFrom = null;
}
};
}
/**
* Returns an iterator that cycles indefinitely over the provided elements.
*
* 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.
*
*
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.
*/
public static Iterator 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.
*
* The returned iterator supports {@code remove()} when the corresponding
* input iterator supports it.
*
*
Note: 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 Iterator concat(Iterator a,
Iterator b) {
return concat(ImmutableList.of(a, b).iterator());
}
/**
* 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.
*
* The returned iterator supports {@code remove()} when the corresponding
* input iterator supports it.
*
*
Note: 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 Iterator concat(Iterator a,
Iterator b, Iterator c) {
return concat(ImmutableList.of(a, b, c).iterator());
}
/**
* 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.
*
* The returned iterator supports {@code remove()} when the corresponding
* input iterator supports it.
*
*
Note: 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 Iterator concat(Iterator a,
Iterator b, Iterator c,
Iterator d) {
return concat(ImmutableList.of(a, b, c, d).iterator());
}
/**
* 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.
*
* The returned iterator supports {@code remove()} when the corresponding
* input iterator supports it.
*
*
Note: 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 Iterator concat(Iterator... inputs) {
return concat(ImmutableList.copyOf(inputs).iterator());
}
/**
* 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.
*
* 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.
*
*
Note: 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 Iterator concat(
final Iterator> inputs) {
checkNotNull(inputs);
return new Iterator() {
Iterator current = emptyIterator();
Iterator 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.
*
* 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 UnmodifiableIterator> partition(
Iterator 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.
*
* 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 UnmodifiableIterator> paddedPartition(
Iterator iterator, int size) {
return partitionImpl(iterator, size, true);
}
private static UnmodifiableIterator> partitionImpl(
final Iterator iterator, final int size, final boolean pad) {
checkNotNull(iterator);
checkArgument(size > 0);
return new UnmodifiableIterator>() {
@Override
public boolean hasNext() {
return iterator.hasNext();
}
@Override
public List 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 list = Collections.unmodifiableList(
(List) Arrays.asList(array));
return (pad || count == size) ? list : list.subList(0, count);
}
};
}
/**
* Returns the elements of {@code unfiltered} that satisfy a predicate.
*/
public static UnmodifiableIterator filter(
final Iterator unfiltered, final Predicate predicate) {
checkNotNull(unfiltered);
checkNotNull(predicate);
return new AbstractIterator() {
@Override protected T computeNext() {
while (unfiltered.hasNext()) {
T element = unfiltered.next();
if (predicate.apply(element)) {
return element;
}
}
return endOfData();
}
};
}
/**
* Returns all instances of class {@code type} in {@code unfiltered}. The
* returned iterator has elements whose class is {@code type} or a subclass of
* {@code type}.
*
* @param unfiltered an iterator containing objects of any type
* @param type the type of elements desired
* @return an unmodifiable iterator containing all elements of the original
* iterator that were of the requested type
*/
@SuppressWarnings("unchecked") // can cast to because non-Ts are removed
@GwtIncompatible("Class.isInstance")
public static UnmodifiableIterator filter(
Iterator unfiltered, Class type) {
return (UnmodifiableIterator) filter(unfiltered, instanceOf(type));
}
/**
* Returns {@code true} if one or more elements returned by {@code iterator}
* satisfy the given predicate.
*/
public static boolean any(
Iterator iterator, Predicate 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 boolean all(
Iterator iterator, Predicate 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
* no 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 find(
Iterator iterator, Predicate 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 find(Iterator iterator, Predicate 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}.
*
* Warning: 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 Optional tryFind(
Iterator iterator, Predicate predicate) {
UnmodifiableIterator filteredIterator = filter(iterator, predicate);
return filteredIterator.hasNext()
? Optional.of(filteredIterator.next())
: 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.
*
* 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.
*
*
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 int indexOf(
Iterator iterator, Predicate 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}.
*
* 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 Iterator transform(final Iterator fromIterator,
final Function function) {
checkNotNull(function);
return new TransformedIterator(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 get(Iterator 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 get(Iterator 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 getNext(Iterator 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 getLast(Iterator 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 getLast(Iterator 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 Iterator limit(
final Iterator iterator, final int limitSize) {
checkNotNull(iterator);
checkArgument(limitSize >= 0, "limit is negative");
return new Iterator() {
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.
*
* 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 Iterator consumingIterator(final Iterator iterator) {
checkNotNull(iterator);
return new UnmodifiableIterator() {
@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 pollNext(Iterator 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.
*
* Note: It is often preferable to represent your data using a
* collection type, for example using {@link Arrays#asList(Object[])}, making
* this method unnecessary.
*
*
The {@code Iterable} equivalent of this method is either {@link
* Arrays#asList(Object[])}, {@link ImmutableList#copyOf(Object[])}},
* or {@link ImmutableList#of}.
*/
public static UnmodifiableIterator 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.
*
* The {@code Iterable} equivalent of this method is {@code
* Arrays.asList(array).subList(offset, offset + length).listIterator(index)}.
*/
static UnmodifiableListIterator 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();
}
/*
* We can't use call the two-arg constructor with arguments (offset, end)
* because the returned Iterator is a ListIterator that may be moved back
* past the beginning of the iteration.
*/
return new AbstractIndexedListIterator(length, index) {
@Override protected T get(int index) {
return array[offset + index];
}
};
}
/**
* Returns an iterator containing only {@code value}.
*
* The {@link Iterable} equivalent of this method is {@link
* Collections#singleton}.
*/
public static UnmodifiableIterator singletonIterator(
@Nullable final T value) {
return new UnmodifiableIterator() {
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.
*
* This method has no equivalent in {@link Iterables} because viewing an
* {@code Enumeration} as an {@code Iterable} is impossible. However, the
* contents can be copied into a collection using {@link
* Collections#list}.
*/
public static UnmodifiableIterator forEnumeration(
final Enumeration enumeration) {
checkNotNull(enumeration);
return new UnmodifiableIterator() {
@Override
public boolean hasNext() {
return enumeration.hasMoreElements();
}
@Override
public T next() {
return enumeration.nextElement();
}
};
}
/**
* Adapts an {@code Iterator} to the {@code Enumeration} interface.
*
* 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 Enumeration asEnumeration(final Iterator iterator) {
checkNotNull(iterator);
return new Enumeration() {
@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 implements PeekingIterator {
private final Iterator iterator;
private boolean hasPeeked;
private E peekedElement;
public PeekingImpl(Iterator 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.
*
* 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:
{@code
*
* PeekingIterator 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"}
*
* 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.
*
*
The returned iterator does not support removal after peeking, as
* explained by {@link PeekingIterator#remove()}.
*
*
Note: If the given iterator is already a {@code PeekingIterator},
* it might be returned to the caller, although this is neither
* guaranteed to occur nor required to be consistent. For example, this
* method might 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.
*
*
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 PeekingIterator peekingIterator(
Iterator iterator) {
if (iterator instanceof PeekingImpl) {
// Safe to cast to because PeekingImpl only uses T
// covariantly (and cannot be subclassed to add non-covariant uses).
@SuppressWarnings("unchecked")
PeekingImpl peeking = (PeekingImpl) iterator;
return peeking;
}
return new PeekingImpl(iterator);
}
/**
* Simply returns its argument.
*
* @deprecated no need to use this
* @since 10.0
*/
@Deprecated public static PeekingIterator peekingIterator(
PeekingIterator 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.
*
* Callers must ensure that the source {@code iterators} are in
* non-descending order as this method does not sort its input.
*
*
For any equivalent elements across all {@code iterators}, it is
* undefined which element is returned first.
*
* @since 11.0
*/
@Beta
public static UnmodifiableIterator mergeSorted(
Iterable> iterators,
Comparator comparator) {
checkNotNull(iterators, "iterators");
checkNotNull(comparator, "comparator");
return new MergingIterator(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.
*
* 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 extends UnmodifiableIterator {
final Queue> queue;
public MergingIterator(Iterable> iterators,
final Comparator itemComparator) {
// A comparator that's used by the heap, allowing the heap
// to be sorted based on the top of each iterator.
Comparator> heapComparator =
new Comparator>() {
@Override
public int compare(PeekingIterator o1, PeekingIterator o2) {
return itemComparator.compare(o1.peek(), o2.peek());
}
};
queue = new PriorityQueue>(2, heapComparator);
for (Iterator iterator : iterators) {
if (iterator.hasNext()) {
queue.add(Iterators.peekingIterator(iterator));
}
}
}
@Override
public boolean hasNext() {
return !queue.isEmpty();
}
@Override
public T next() {
PeekingIterator nextIter = queue.remove();
T next = nextIter.next();
if (nextIter.hasNext()) {
queue.add(nextIter);
}
return next;
}
}
/**
* Used to avoid http://bugs.sun.com/view_bug.do?bug_id=6558557
*/
static ListIterator cast(Iterator iterator) {
return (ListIterator) iterator;
}
}
