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com.google.common.collect.Iterators Maven / Gradle / Ivy
/*
* 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.base.Function;
import com.google.common.base.Join;
import com.google.common.base.Nullable;
import com.google.common.base.Objects;
import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkContentsNotNull;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import com.google.common.base.Predicate;
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.ListIterator;
import java.util.NoSuchElementException;
/**
* This class contains static utility methods that operate on or return objects
* of type {@code Iterator}. Also see the parallel implementations in {@link
* Iterables}.
*
* @author Kevin Bourrillion
* @author Scott Bonneau
*/
public final class Iterators {
private Iterators() {}
static final Iterator EMPTY_ITERATOR = new Iterator() {
public boolean hasNext() {
return false;
}
public Object next() {
throw new NoSuchElementException();
}
public void remove() {
throw new UnsupportedOperationException();
}
};
/** Returns the empty {@code Iterator}. */
// Casting to any type is safe since there are no actual elements.
@SuppressWarnings("unchecked")
public static Iterator emptyIterator() {
return (Iterator) EMPTY_ITERATOR;
}
private static final ListIterator EMPTY_LIST_ITERATOR =
new ListIterator() {
public boolean hasNext() {
return false;
}
public boolean hasPrevious() {
return false;
}
public int nextIndex() {
return 0;
}
public int previousIndex() {
return -1;
}
public Object next() {
throw new NoSuchElementException();
}
public Object previous() {
throw new NoSuchElementException();
}
public void set(Object o) {
throw new UnsupportedOperationException();
}
public void add(Object o) {
throw new UnsupportedOperationException();
}
public void remove() {
throw new UnsupportedOperationException();
}
};
/** Returns the empty {@code ListIterator}. */
// Casting to any type is safe since there are no actual elements.
@SuppressWarnings("unchecked")
public static ListIterator emptyListIterator() {
return (ListIterator) EMPTY_LIST_ITERATOR;
}
/** Returns an unmodifiable view of {@code iterator}. */
public static Iterator unmodifiableIterator(
final Iterator iterator) {
checkNotNull(iterator);
return new Iterator() {
public boolean hasNext() {
return iterator.hasNext();
}
public T next() {
return iterator.next();
}
public void remove() {
throw new UnsupportedOperationException();
}
};
}
/**
* 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;
}
/**
* Determines whether two iterators contain equal elements. 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 the elements in {@code iterator}, with
* the format "{@code [e1, e2, ..., en]}". The iterator will be left
* exhausted: its {@code hasNext()} method will return {@code false}.
*/
public static String toString(Iterator iterator) {
StringBuilder builder = new StringBuilder().append('[');
Join.join(builder, ", ", iterator);
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
*/
public static T getOnlyElement(Iterator iterator) {
if (!iterator.hasNext()) {
throw new NoSuchElementException();
}
T element = iterator.next();
if (iterator.hasNext()) {
throw new IllegalArgumentException(
"extra elements found: " + toString(iterator));
}
return element;
}
/**
* Returns the single element contained in {@code iterator}, or {@code
* defaultValue} if the iterator is empty.
*
* @throws IllegalArgumentException if the iterator contains multiple
* elements
*/
public static T getOnlyElement(
Iterator iterator, @Nullable T defaultValue) {
return iterator.hasNext() ? getOnlyElement(iterator) : defaultValue;
}
/**
* Converts an iterator into an array. The iterator will be left exhausted:
* its {@code hasNext()} method will return {@code false}.
*
* @param iterator the iterator to convert
* @param type the type of the elements
* @return a newly-allocated array into which all the elements of the iterator
* have been copied. It may be empty, but it never will be null.
*/
public static T[] newArray(Iterator iterator, Class type) {
List list = Lists.newArrayList(iterator);
return Iterables.newArray(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 collection, Iterator iterator) {
checkNotNull(collection);
boolean wasModified = false;
while (iterator.hasNext()) {
wasModified |= collection.add(iterator.next());
}
return wasModified;
}
/**
* Returns the number of elements in the specified iterator equal to 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 calling {@code remove()}, 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 appropriate
* source 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 multiple iterators into a single iterator. The returned iterator
* iterates across the elements in each source iterator. The source iterators
* are not polled until necessary.
*
* The returned iterator supports {@code remove()} when the appropriate
* source iterator supports it.
*
* @throws NullPointerException if any of the provided iterators is null
*/
public static Iterator concat(Iterator... iterators) {
return concat(checkContentsNotNull(Arrays.asList(iterators)).iterator());
}
/**
* Combines multiple iterators into a single iterator. The returned iterator
* iterates across the elements in each source iterator. The source iterators
* are not polled until necessary.
*
* The returned iterator supports {@link Iterator#remove()} when the
* appropriate source iterator supports it. The methods of the returned
* iterator may throw {@code NullPointerException} if any of the source
* iterators are null.
*/
public static Iterator concat(
final Iterator> iterators) {
checkNotNull(iterators);
return new Iterator() {
Iterator current = emptyIterator();
Iterator removeFrom;
public boolean hasNext() {
while (!current.hasNext() && iterators.hasNext()) {
current = iterators.next();
}
return current.hasNext();
}
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;
}
};
}
/**
* Partition an iterator into sub-iterators of the given size like so: {A, B,
* C, D, E, F} with partition size 3 => {A, B, C} and {D, E, F}. The returned
* iterator does not support {@code remove()}.
*
* After you call {@code next()} on the returned iterator, the iterators
* from prior {@code next()} calls become invalid.
*
* @param iterator the iterator to partition
* @param partitionSize the size of each partition
* @param padToSize whether to pad the last partition to the partition size
* with {@code null}
* @return an iterator across partitioned iterators
*/
public static Iterator> partition(
final Iterator iterator,
final int partitionSize, final boolean padToSize) {
checkNotNull(iterator);
return new AbstractIterator>() {
Iterator currentRow;
@Override protected Iterator computeNext() {
if (currentRow != null) {
while (currentRow.hasNext()) {
currentRow.next();
}
}
if (!iterator.hasNext()) {
return endOfData();
}
currentRow = new AbstractIterator() {
int count = partitionSize;
@Override protected T computeNext() {
if (count == 0) {
return endOfData();
}
count--;
if (iterator.hasNext()) {
return iterator.next();
} else {
if (!padToSize) {
endOfData();
}
return null;
}
}
};
return currentRow;
}
};
}
/**
* Returns the elements of {@code unfiltered} that satisfy a predicate. It may
* return an empty iterator, but never {@code null}. The resulting iterator
* does not support {@code remove()}.
*/
public static Iterator 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}. The returned iterator does not support {@code remove()}.
*
* @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")
public static Iterator filter(
Iterator unfiltered, final Class type) {
checkNotNull(type);
Predicate predicate = new Predicate() {
public boolean apply(Object object) {
return type.isInstance(object);
}
};
return (Iterator) filter(unfiltered, predicate);
}
/**
* 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 E 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} 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;
}
}
}
/**
* Calls {@code next()} on {@code iterator}, either {@code numberToSkip} times
* or until {@code hasNext()} returns {@code false}, whichever comes first.
*
* @return the number of elements skipped
*/
public static int skip(Iterator iterator, int numberToSkip) {
checkNotNull(iterator);
checkArgument(numberToSkip >= 0, "number to skip cannot be negative");
int i;
for (i = 0; i < numberToSkip && iterator.hasNext(); i++) {
iterator.next();
}
return i;
}
// Methods only in Iterators, not in Iterables
/**
* Returns an iterator containing the elements of {@code array} in order. Note
* that you can also use the iterator of {@link Arrays#asList}.
*/
static Iterator forArray(final T[] array) {
checkNotNull(array);
if (array.length == 0) {
return emptyIterator();
}
return new Iterator() {
int i = 0;
public boolean hasNext() {
return i < array.length;
}
public T next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
return array[i++];
}
public void remove() {
throw new UnsupportedOperationException();
}
};
}
/**
* Adapts an {@code Enumeration} to the {@code Iterator} interface. The
* returned iterator does not support {@code remove()}.
*/
public static Iterator forEnumeration(final Enumeration enumeration)
{
checkNotNull(enumeration);
return new Iterator() {
public boolean hasNext() {
return enumeration.hasMoreElements();
}
public T next() {
return enumeration.nextElement();
}
public void remove() {
throw new UnsupportedOperationException();
}
};
}
/**
* Adapts an {@code Iterator} to the {@code Enumeration} interface.
*
* @see Collections#enumeration(Collection)
*/
public static Enumeration asEnumeration(final Iterator iterator) {
checkNotNull(iterator);
return new Enumeration() {
public boolean hasMoreElements() {
return iterator.hasNext();
}
public T nextElement() {
return iterator.next();
}
};
}
/**
* Creates an iterator returning the first {@code limit} 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
* supports it.
*
* @param iterator the iterator to limit
* @param limit the maximum number of elements in the returned iterator
* @throws IllegalArgumentException if limit is negative
*/
public static Iterator limit(
final Iterator iterator, final int limit) {
checkNotNull(iterator);
checkArgument(limit >= 0, "limit is negative");
return new Iterator() {
private int count;
public boolean hasNext() {
return count < limit && iterator.hasNext();
}
public T next() {
if (!hasNext()) {
throw new NoSuchElementException();
}
count++;
return iterator.next();
}
public void remove() {
iterator.remove();
}
};
}
/**
* 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;
}
}
/**
* Wraps the supplied iterator in a {@code PeekingIterator}. The
* {@link PeekingIterator} assumes ownership of the supplied iterator, so
* users should cease making direct calls to it after calling this method.
*
* If the {@link PeekingIterator#peek()} method of the constructed
* {@code PeekingIterator} is never called, the returned iterator will
* behave exactly the same as the supplied iterator.
*
*
Subsequent calls to {@code peek()} with no intervening calls to
* {@code next()} do not affect the iteration, and hence return the same
* object each time. After a call to {@code peek()}, the next call to
* {@code next()} is guaranteed to return the same object that the
* {@code peek()} call returned. For example:
*
*
* PeekingIterator<E> peekingIterator = ...;
* // Either the next three calls will each throw
* // NoSuchElementExceptions, or...
* E e1 = peekingIterator.peek();
* E e2 = peekingIterator.peek(); // e2 is the same as e1
* E e3 = peekingIterator.next(); // e3 is the same as e1/e2
*
*
* Calling {@link Iterator#remove()} after {@link PeekingIterator#peek()}
* is unsupported by the returned iterator and will throw an
* {@link IllegalStateException}.
*/
public static PeekingIterator peekingIterator(Iterator iterator) {
return new PeekingImpl(iterator);
}
}
