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com.google.common.collect.Iterators Maven / Gradle / Ivy
Google Collections Library is a suite of new collections and collection-related goodness for Java 5.0
/*
* Copyright (C) 2007 Google Inc.
*
* 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 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.Preconditions;
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 com.google.common.base.Predicate;
import com.google.common.base.Predicates;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
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.
*
* @author Kevin Bourrillion
* @author Jared Levy
*/
@GwtCompatible
public final class Iterators {
private Iterators() {}
static final UnmodifiableIterator EMPTY_ITERATOR
= new UnmodifiableIterator() {
public boolean hasNext() {
return false;
}
public Object next() {
throw new NoSuchElementException();
}
};
/**
* Returns the empty iterator.
*
* The {@link Iterable} equivalent of this method is {@link
* Collections#emptySet}.
*/
// Casting to any type is safe since there are no actual elements.
@SuppressWarnings("unchecked")
public static UnmodifiableIterator emptyIterator() {
return (UnmodifiableIterator) EMPTY_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() {
throw new IllegalStateException();
}
};
/**
* 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);
return new UnmodifiableIterator() {
public boolean hasNext() {
return iterator.hasNext();
}
public T next() {
return iterator.next();
}
};
}
/**
* 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)
{
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 elements are removed from {@code iterator}
*/
public static boolean removeAll(
Iterator removeFrom, Collection elementsToRemove) {
checkNotNull(elementsToRemove);
boolean modified = false;
while (removeFrom.hasNext()) {
if (elementsToRemove.contains(removeFrom.next())) {
removeFrom.remove();
modified = true;
}
}
return modified;
}
/**
* 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
*/
// TODO: make public post-1.0
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 elements are removed from {@code iterator}
*/
public static boolean retainAll(
Iterator removeFrom, Collection elementsToRetain) {
checkNotNull(elementsToRetain);
boolean modified = false;
while (removeFrom.hasNext()) {
if (!elementsToRetain.contains(removeFrom.next())) {
removeFrom.remove();
modified = true;
}
}
return modified;
}
/**
* 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) {
if (!iterator.hasNext()) {
return "[]";
}
StringBuilder builder = new StringBuilder();
builder.append('[').append(iterator.next());
while (iterator.hasNext()) {
builder.append(", ").append(iterator.next());
}
return builder.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.
*/
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.newArray")
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);
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 result = 0;
if (element == null) {
while (iterator.hasNext()) {
if (iterator.next() == null) {
result++;
}
}
} else {
while (iterator.hasNext()) {
if (element.equals(iterator.next())) {
result++;
}
}
}
return result;
}
/**
* 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;
public boolean hasNext() {
if (!iterator.hasNext()) {
iterator = iterable.iterator();
}
return iterator.hasNext();
}
public T next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
removeFrom = iterator;
return iterator.next();
}
public void remove() {
checkState(removeFrom != null,
"no calls to next() since last call to remove()");
removeFrom.remove();
removeFrom = null;
}
};
}
/**
* Returns an iterator that cycles indefinitely over the provided elements.
*
* The returned iterator supports {@code remove()} if the provided iterator
* does. 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.
*/
@SuppressWarnings("unchecked")
public static Iterator concat(Iterator a,
Iterator b) {
checkNotNull(a);
checkNotNull(b);
return concat(Arrays.asList(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.
*/
@SuppressWarnings("unchecked")
public static Iterator concat(Iterator a,
Iterator b, Iterator c) {
checkNotNull(a);
checkNotNull(b);
checkNotNull(c);
return concat(Arrays.asList(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.
*/
@SuppressWarnings("unchecked")
public static Iterator concat(Iterator a,
Iterator b, Iterator c,
Iterator d) {
checkNotNull(a);
checkNotNull(b);
checkNotNull(c);
checkNotNull(d);
return concat(Arrays.asList(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.
*
* @throws NullPointerException if any of the provided iterators is null
*/
public static Iterator concat(Iterator... inputs) {
return concat(ImmutableList.of(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 are null.
*/
public static Iterator concat(
final Iterator> inputs) {
checkNotNull(inputs);
return new Iterator() {
Iterator current = emptyIterator();
Iterator removeFrom;
public boolean hasNext() {
// http://code.google.com/p/google-collections/issues/detail?id=151
// current.hasNext() might be relatively expensive, worth minimizing.
boolean currentHasNext;
while (!(currentHasNext = current.hasNext()) && inputs.hasNext()) {
current = inputs.next();
}
return currentHasNext;
}
public T next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
removeFrom = current;
return current.next();
}
public void remove() {
checkState(removeFrom != null,
"no calls to next() since last call to remove()");
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>() {
public boolean hasNext() {
return iterator.hasNext();
}
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();
}
@SuppressWarnings("unchecked") // we only put Ts in it
List list = Collections.unmodifiableList(
(List) Arrays.asList(array));
return (pad || count == size) ? list : Platform.subList(list, 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, Predicates.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) {
checkNotNull(predicate);
while (iterator.hasNext()) {
T element = iterator.next();
if (predicate.apply(element)) {
return true;
}
}
return false;
}
/**
* 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. If a matching element is found, the iterator will be left in a
* state such that calling {@code iterator.remove()} will remove the found
* item. If no such element is found, the iterator will be left exhausted: its
* {@code hasNext()} method will return {@code false}.
*
* @return the first matching element in {@code iterator}
* @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 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(fromIterator);
checkNotNull(function);
return new Iterator() {
public boolean hasNext() {
return fromIterator.hasNext();
}
public T next() {
F from = fromIterator.next();
return function.apply(from);
}
public void remove() {
fromIterator.remove();
}
};
}
/**
* 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) {
if (position < 0) {
throw new IndexOutOfBoundsException("position (" + position
+ ") must not be negative");
}
int skipped = 0;
while (iterator.hasNext()) {
T t = iterator.next();
if (skipped++ == position) {
return t;
}
}
throw new IndexOutOfBoundsException("position (" + position
+ ") must be less than the number of elements that remained ("
+ skipped + ")");
}
/**
* Advances {@code iterator} to the end, returning the last element.
*
* @return the last element of {@code iterator}
* @throws NoSuchElementException if the iterator has no remaining elements
*/
public static T getLast(Iterator iterator) {
while (true) {
T current = iterator.next();
if (!iterator.hasNext()) {
return current;
}
}
}
// Methods only in Iterators, not in Iterables
/**
* 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[])} or {@link ImmutableList#of(Object[])}}.
*/
public static UnmodifiableIterator forArray(final T... array) {
// optimized. benchmarks at nearly 2x of the straightforward impl
return new UnmodifiableIterator() {
final int length = array.length;
int i = 0;
public boolean hasNext() {
return i < length;
}
public T next() {
try {
// 'return array[i++];' almost works
T t = array[i];
i++;
return t;
} catch (ArrayIndexOutOfBoundsException e) {
throw new NoSuchElementException();
}
}
};
}
/**
* Returns an iterator containing the elements in the specified range 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.
*
* The {@code Iterable} equivalent of this method is {@code
* Arrays.asList(array).subList(offset, offset + length)}.
*
* @param array array to read elements out of
* @param offset index of first array element to retrieve
* @param length number of elements in iteration
*
* @throws IndexOutOfBoundsException if {@code offset} is negative,
* {@code length} is negative, or {@code offset + length > array.length}
*/
static UnmodifiableIterator forArray(
final T[] array, final int offset, int length) {
checkArgument(length >= 0);
final int end = offset + length;
// Technically we should give a slightly more descriptive error on overflow
Preconditions.checkPositionIndexes(offset, end, array.length);
// If length == 0 is a common enough case, we could return emptyIterator().
return new UnmodifiableIterator() {
int i = offset;
public boolean hasNext() {
return i < end;
}
public T next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
return array[i++];
}
};
}
/**
* 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;
public boolean hasNext() {
return !done;
}
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() {
public boolean hasNext() {
return enumeration.hasMoreElements();
}
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() {
public boolean hasMoreElements() {
return iterator.hasNext();
}
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);
}
public boolean hasNext() {
return hasPeeked || iterator.hasNext();
}
public E next() {
if (!hasPeeked) {
return iterator.next();
}
E result = peekedElement;
hasPeeked = false;
peekedElement = null;
return result;
}
public void remove() {
checkState(!hasPeeked, "Can't remove after you've peeked at next");
iterator.remove();
}
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);
}
}
