Package com.google.common.collect

Class Iterators

  • @GwtCompatible(emulated=true)
public final class Iterators
extends Object
    This class contains static utility methods that operate on or return objects of type Iterator. Except as noted, each method has a corresponding Iterable-based method in the 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 Iterators.

    Since:
    2.0
    Author:
    Kevin Bourrillion, Jared Levy

    • Method Summary

      All Methods Static Methods Concrete Methods Deprecated Methods 
      Modifier and Type Method Description
      static <T> boolean addAll​(Collection<T> addTo, Iterator<? extends T> iterator)
      Adds all elements in iterator to collection.
      static int advance​(Iterator<?> iterator, int numberToAdvance)
      Calls next() on iterator, either numberToAdvance times or until hasNext() returns false, whichever comes first.
      static <T> boolean all​(Iterator<T> iterator, Predicate<? super T> predicate)
      Returns true if every element returned by iterator satisfies the given predicate.
      static <T> boolean any​(Iterator<T> iterator, Predicate<? super T> predicate)
      Returns true if one or more elements returned by iterator satisfy the given predicate.
      static <T> Enumeration<T> asEnumeration​(Iterator<T> iterator)
      Adapts an Iterator to the Enumeration interface.
      static <T> Iterator<T> concat​(Iterator<? extends Iterator<? extends T>> inputs)
      Combines multiple iterators into a single iterator.
      static <T> Iterator<T> concat​(Iterator<? extends T>... inputs)
      Combines multiple iterators into a single iterator.
      static <T> Iterator<T> concat​(Iterator<? extends T> a, Iterator<? extends T> b)
      Combines two iterators into a single iterator.
      static <T> Iterator<T> concat​(Iterator<? extends T> a, Iterator<? extends T> b, Iterator<? extends T> c)
      Combines three iterators into a single iterator.
      static <T> Iterator<T> concat​(Iterator<? extends T> a, Iterator<? extends T> b, Iterator<? extends T> c, Iterator<? extends T> d)
      Combines four iterators into a single iterator.
      static <T> Iterator<T> consumingIterator​(Iterator<T> iterator)
      Returns a view of the supplied iterator that removes each element from the supplied iterator as it is returned.
      static boolean contains​(Iterator<?> iterator, Object element)
      Returns true if iterator contains element.
      static <T> Iterator<T> cycle​(Iterable<T> iterable)
      Returns an iterator that cycles indefinitely over the elements of iterable.
      static <T> Iterator<T> cycle​(T... elements)
      Returns an iterator that cycles indefinitely over the provided elements.
      static boolean elementsEqual​(Iterator<?> iterator1, Iterator<?> iterator2)
      Determines whether two iterators contain equal elements in the same order.
      static <T> UnmodifiableIterator<T> filter​(Iterator<?> unfiltered, Class<T> desiredType)
      Returns a view of unfiltered containing all elements that are of the type desiredType.
      static <T> UnmodifiableIterator<T> filter​(Iterator<T> unfiltered, Predicate<? super T> retainIfTrue)
      Returns a view of unfiltered containing all elements that satisfy the input predicate retainIfTrue.
      static <T> T find​(Iterator<? extends T> iterator, Predicate<? super T> predicate, T defaultValue)
      Returns the first element in iterator that satisfies the given predicate.
      static <T> T find​(Iterator<T> iterator, Predicate<? super T> predicate)
      Returns the first element in iterator that satisfies the given predicate; use this method only when such an element is known to exist.
      static <T> UnmodifiableIterator<T> forArray​(T... array)
      Returns an iterator containing the elements of array in order.
      static <T> UnmodifiableIterator<T> forEnumeration​(Enumeration<T> enumeration)
      Adapts an Enumeration to the Iterator interface.
      static int frequency​(Iterator<?> iterator, Object element)
      Returns the number of elements in the specified iterator that equal the specified object.
      static <T> T get​(Iterator<? extends T> iterator, int position, T defaultValue)
      Advances iterator position + 1 times, returning the element at the positionth position or defaultValue otherwise.
      static <T> T get​(Iterator<T> iterator, int position)
      Advances iterator position + 1 times, returning the element at the positionth position.
      static <T> T getLast​(Iterator<? extends T> iterator, T defaultValue)
      Advances iterator to the end, returning the last element or defaultValue if the iterator is empty.
      static <T> T getLast​(Iterator<T> iterator)
      Advances iterator to the end, returning the last element.
      static <T> T getNext​(Iterator<? extends T> iterator, T defaultValue)
      Returns the next element in iterator or defaultValue if the iterator is empty.
      static <T> T getOnlyElement​(Iterator<? extends T> iterator, T defaultValue)
      Returns the single element contained in iterator, or defaultValue if the iterator is empty.
      static <T> T getOnlyElement​(Iterator<T> iterator)
      Returns the single element contained in iterator.
      static <T> int indexOf​(Iterator<T> iterator, Predicate<? super T> predicate)
      Returns the index in iterator of the first element that satisfies the provided predicate, or -1 if the Iterator has no such elements.
      static <T> Iterator<T> limit​(Iterator<T> iterator, int limitSize)
      Returns a view containing the first limitSize elements of iterator.
      static <T> UnmodifiableIterator<T> mergeSorted​(Iterable<? extends Iterator<? extends T>> iterators, Comparator<? super T> comparator)
      Returns an iterator over the merged contents of all given iterators, traversing every element of the input iterators.
      static <T> UnmodifiableIterator<List<T>> paddedPartition​(Iterator<T> iterator, int size)
      Divides an iterator into unmodifiable sublists of the given size, padding the final iterator with null values if necessary.
      static <T> UnmodifiableIterator<List<T>> partition​(Iterator<T> iterator, int size)
      Divides an iterator into unmodifiable sublists of the given size (the final list may be smaller).
      static <T> PeekingIterator<T> peekingIterator​(PeekingIterator<T> iterator)
      Deprecated.
      no need to use this
      static <T> PeekingIterator<T> peekingIterator​(Iterator<? extends T> iterator)
      Returns a PeekingIterator backed by the given iterator.
      static boolean removeAll​(Iterator<?> removeFrom, Collection<?> elementsToRemove)
      Traverses an iterator and removes every element that belongs to the provided collection.
      static <T> boolean removeIf​(Iterator<T> removeFrom, Predicate<? super T> predicate)
      Removes every element that satisfies the provided predicate from the iterator.
      static boolean retainAll​(Iterator<?> removeFrom, Collection<?> elementsToRetain)
      Traverses an iterator and removes every element that does not belong to the provided collection.
      static <T> UnmodifiableIterator<T> singletonIterator​(T value)
      Returns an iterator containing only value.
      static int size​(Iterator<?> iterator)
      Returns the number of elements remaining in iterator.
      static <T> T[] toArray​(Iterator<? extends T> iterator, Class<T> type)
      Copies an iterator's elements into an array.
      static String toString​(Iterator<?> iterator)
      Returns a string representation of iterator, with the format [e1, e2, ..., en].
      static <F,​T>
      Iterator<T>      		 transform​(Iterator<F> fromIterator, Function<? super F,​? extends T> function)
      Returns a view containing the result of applying function to each element of fromIterator.
      static <T> Optional<T> tryFind​(Iterator<T> iterator, Predicate<? super T> predicate)
      Returns an Optional containing the first element in iterator that satisfies the given predicate, if such an element exists.
      static <T> UnmodifiableIterator<T> unmodifiableIterator​(UnmodifiableIterator<T> iterator)
      Deprecated.
      no need to use this
      static <T> UnmodifiableIterator<T> unmodifiableIterator​(Iterator<? extends T> iterator)
      Returns an unmodifiable view of iterator.

    • Method Detail

      • size

        public static int size​(Iterator<?> iterator)
        Returns the number of elements remaining in iterator. The iterator will be left exhausted: its hasNext() method will return false.

      • removeAll

        @CanIgnoreReturnValue
public static boolean removeAll​(Iterator<?> removeFrom,
                                Collection<?> elementsToRemove)
        Traverses an iterator and removes every element that belongs to the provided collection. The iterator will be left exhausted: its hasNext() method will return false.
        Parameters:
        removeFrom - the iterator to (potentially) remove elements from
        elementsToRemove - the elements to remove
        Returns:
        true if any element was removed from iterator

      • removeIf

        @CanIgnoreReturnValue
public static <T> boolean removeIf​(Iterator<T> removeFrom,
                                   Predicate<? super T> predicate)
        Removes every element that satisfies the provided predicate from the iterator. The iterator will be left exhausted: its hasNext() method will return false.
        Parameters:
        removeFrom - the iterator to (potentially) remove elements from
        predicate - a predicate that determines whether an element should be removed
        Returns:
        true if any elements were removed from the iterator
        Since:
        2.0

      • retainAll

        @CanIgnoreReturnValue
public static boolean retainAll​(Iterator<?> removeFrom,
                                Collection<?> elementsToRetain)
        Traverses an iterator and removes every element that does not belong to the provided collection. The iterator will be left exhausted: its hasNext() method will return false.
        Parameters:
        removeFrom - the iterator to (potentially) remove elements from
        elementsToRetain - the elements to retain
        Returns:
        true if any element was removed from iterator

      • elementsEqual

        public static boolean elementsEqual​(Iterator<?> iterator1,
                                    Iterator<?> iterator2)
        Determines whether two iterators contain equal elements in the same order. More specifically, this method returns true if iterator1 and iterator2 contain the same number of elements and every element of iterator1 is equal to the corresponding element of iterator2.

        Note that this will modify the supplied iterators, since they will have been advanced some number of elements forward.

      • toString

        public static String toString​(Iterator<?> iterator)
        Returns a string representation of iterator, with the format [e1, e2, ..., en]. The iterator will be left exhausted: its hasNext() method will return false.

      • toArray

        @GwtIncompatible
public static <T> T[] toArray​(Iterator<? extends T> iterator,
                              Class<T> type)
        Copies an iterator's elements into an array. The iterator will be left exhausted: its hasNext() method will return false.
        Parameters:
        iterator - the iterator to copy
        type - the type of the elements
        Returns:
        a newly-allocated array into which all the elements of the iterator have been copied

      • addAll

        @CanIgnoreReturnValue
public static <T> boolean addAll​(Collection<T> addTo,
                                 Iterator<? extends T> iterator)
        Adds all elements in iterator to collection. The iterator will be left exhausted: its hasNext() method will return false.
        Returns:
        true if collection was modified as a result of this operation

      • cycle

        public static <T> Iterator<T> cycle​(Iterable<T> iterable)
        Returns an iterator that cycles indefinitely over the elements of iterable.

        The returned iterator supports remove() if the provided iterator does. After remove() is called, subsequent cycles omit the removed element, which is no longer in iterable. The iterator's hasNext() method returns true until iterable is empty.

        Warning: Typical uses of the resulting iterator may produce an infinite loop. You should use an explicit break or be certain that you will eventually remove all the elements.

      • cycle

        @SafeVarargs
public static <T> Iterator<T> cycle​(T... elements)
        Returns an iterator that cycles indefinitely over the provided elements.

        The returned iterator supports remove(). After remove() is called, subsequent cycles omit the removed element, but elements does not change. The iterator's hasNext() method returns 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 break or be certain that you will eventually remove all the elements.

      • concat

        public static <T> Iterator<T> concat​(Iterator<? extends T> a,
                                     Iterator<? extends T> b)
        Combines two iterators into a single iterator. The returned iterator iterates across the elements in a, followed by the elements in b. The source iterators are not polled until necessary.

        The returned iterator supports remove() when the corresponding input iterator supports it.

      • concat

        public static <T> Iterator<T> concat​(Iterator<? extends T> a,
                                     Iterator<? extends T> b,
                                     Iterator<? extends T> c)
        Combines three iterators into a single iterator. The returned iterator iterates across the elements in a, followed by the elements in b, followed by the elements in c. The source iterators are not polled until necessary.

        The returned iterator supports remove() when the corresponding input iterator supports it.

      • concat

        public static <T> Iterator<T> concat​(Iterator<? extends T> a,
                                     Iterator<? extends T> b,
                                     Iterator<? extends T> c,
                                     Iterator<? extends T> d)
        Combines four iterators into a single iterator. The returned iterator iterates across the elements in a, followed by the elements in b, followed by the elements in c, followed by the elements in d. The source iterators are not polled until necessary.

        The returned iterator supports remove() when the corresponding input iterator supports it.

      • concat

        public static <T> Iterator<T> concat​(Iterator<? extends T>... inputs)
        Combines multiple iterators into a single iterator. The returned iterator iterates across the elements of each iterator in inputs. The input iterators are not polled until necessary.

        The returned iterator supports remove() when the corresponding input iterator supports it.

        Throws:
        NullPointerException - if any of the provided iterators is null

      • concat

        public static <T> Iterator<T> concat​(Iterator<? extends Iterator<? extends T>> inputs)
        Combines multiple iterators into a single iterator. The returned iterator iterates across the elements of each iterator in inputs. The input iterators are not polled until necessary.

        The returned iterator supports remove() when the corresponding input iterator supports it. The methods of the returned iterator may throw NullPointerException if any of the input iterators is null.

      • partition

        public static <T> UnmodifiableIterator<List<T>> partition​(Iterator<T> iterator,
                                                          int size)
        Divides an iterator into unmodifiable sublists of the given size (the final list may be smaller). For example, partitioning an iterator containing [a, b, c, d, e] with a partition size of 3 yields [[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 RandomAccess.

        Parameters:
        iterator - the iterator to return a partitioned view of
        size - the desired size of each partition (the last may be smaller)
        Returns:
        an iterator of immutable lists containing the elements of iterator divided into partitions
        Throws:
        IllegalArgumentException - if size is nonpositive

      • paddedPartition

        public static <T> UnmodifiableIterator<List<T>> paddedPartition​(Iterator<T> iterator,
                                                                int size)
        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 [a, b, c, d, e] with a partition size of 3 yields [[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 RandomAccess.

        Parameters:
        iterator - the iterator to return a partitioned view of
        size - the desired size of each partition
        Returns:
        an iterator of immutable lists containing the elements of iterator divided into partitions (the final iterable may have trailing null elements)
        Throws:
        IllegalArgumentException - if size is nonpositive

      • filter

        public static <T> UnmodifiableIterator<T> filter​(Iterator<T> unfiltered,
                                                 Predicate<? super T> retainIfTrue)
        Returns a view of unfiltered containing all elements that satisfy the input predicate retainIfTrue.

      • any

        public static <T> boolean any​(Iterator<T> iterator,
                              Predicate<? super T> predicate)
        Returns true if one or more elements returned by iterator satisfy the given predicate.

      • all

        public static <T> boolean all​(Iterator<T> iterator,
                              Predicate<? super T> predicate)
        Returns true if every element returned by iterator satisfies the given predicate. If iterator is empty, true is returned.

      • find

        @NullableDecl
public static <T> T find​(Iterator<? extends T> iterator,
                         Predicate<? super T> predicate,
                         @NullableDecl
                         T defaultValue)
        Returns the first element in iterator that satisfies the given predicate. If no such element is found, defaultValue will be returned from this method and the iterator will be left exhausted: its hasNext() method will return false. Note that this can usually be handled more naturally using tryFind(iterator, predicate).or(defaultValue).
        Since:
        7.0

      • tryFind

        public static <T> Optional<T> tryFind​(Iterator<T> iterator,
                                      Predicate<? super T> predicate)
        Returns an Optional containing the first element in iterator that satisfies the given predicate, if such an element exists. If no such element is found, an empty Optional will be returned from this method and the iterator will be left exhausted: its hasNext() method will return false.

        Warning: avoid using a predicate that matches null. If null is matched in iterator, a NullPointerException will be thrown.

        Since:
        11.0

      • indexOf

        public static <T> int indexOf​(Iterator<T> iterator,
                              Predicate<? super T> predicate)
        Returns the index in iterator of the first element that satisfies the provided predicate, or -1 if the Iterator has no such elements.

        More formally, returns the lowest index i such that predicate.apply(Iterators.get(iterator, i)) returns true, or -1 if there is no such index.

        If -1 is returned, the iterator will be left exhausted: its hasNext() method will return false. Otherwise, the iterator will be set to the element which satisfies the predicate.

        Since:
        2.0

      • transform

        public static <F,​T> Iterator<T> transform​(Iterator<F> fromIterator,
                                                Function<? super F,​? extends T> function)
        Returns a view containing the result of applying function to each element of fromIterator.

        The returned iterator supports remove() if fromIterator does. After a successful remove() call, fromIterator no longer contains the corresponding element.

      • get

        public static <T> T get​(Iterator<T> iterator,
                        int position)
        Advances iterator position + 1 times, returning the element at the positionth position.
        Parameters:
        position - position of the element to return
        Returns:
        the element at the specified position in iterator
        Throws:
        IndexOutOfBoundsException - if position is negative or greater than or equal to the number of elements remaining in iterator

      • get

        @NullableDecl
public static <T> T get​(Iterator<? extends T> iterator,
                        int position,
                        @NullableDecl
                        T defaultValue)
        Advances iterator position + 1 times, returning the element at the positionth position or defaultValue otherwise.
        Parameters:
        position - position of the element to return
        defaultValue - the default value to return if the iterator is empty or if position is greater than the number of elements remaining in iterator
        Returns:
        the element at the specified position in iterator or defaultValue if iterator produces fewer than position + 1 elements.
        Throws:
        IndexOutOfBoundsException - if position is negative
        Since:
        4.0

      • getLast

        public static <T> T getLast​(Iterator<T> iterator)
        Advances iterator to the end, returning the last element.
        Returns:
        the last element of iterator
        Throws:
        NoSuchElementException - if the iterator is empty

      • getLast

        @NullableDecl
public static <T> T getLast​(Iterator<? extends T> iterator,
                            @NullableDecl
                            T defaultValue)
        Advances iterator to the end, returning the last element or defaultValue if the iterator is empty.
        Parameters:
        defaultValue - the default value to return if the iterator is empty
        Returns:
        the last element of iterator
        Since:
        3.0

      • advance

        @CanIgnoreReturnValue
public static int advance​(Iterator<?> iterator,
                          int numberToAdvance)
        Calls next() on iterator, either numberToAdvance times or until hasNext() returns false, whichever comes first.
        Returns:
        the number of elements the iterator was advanced
        Since:
        13.0 (since 3.0 as Iterators.skip)

      • limit

        public static <T> Iterator<T> limit​(Iterator<T> iterator,
                                    int limitSize)
        Returns a view containing the first limitSize elements of iterator. If iterator contains fewer than limitSize elements, the returned view contains all of its elements. The returned iterator supports remove() if iterator does.
        Parameters:
        iterator - the iterator to limit
        limitSize - the maximum number of elements in the returned iterator
        Throws:
        IllegalArgumentException - if limitSize is negative
        Since:
        3.0

      • consumingIterator

        public static <T> Iterator<T> consumingIterator​(Iterator<T> iterator)
        Returns a view of the supplied iterator that removes each element from the supplied iterator as it is returned.

        The provided iterator must support Iterator.remove() or else the returned iterator will fail on the first call to next.

        Parameters:
        iterator - the iterator to remove and return elements from
        Returns:
        an iterator that removes and returns elements from the supplied iterator
        Since:
        2.0

      • forEnumeration

        public static <T> UnmodifiableIterator<T> forEnumeration​(Enumeration<T> enumeration)
        Adapts an Enumeration to the Iterator interface.

        This method has no equivalent in Iterables because viewing an Enumeration as an Iterable is impossible. However, the contents can be copied into a collection using Collections.list(java.util.Enumeration<T>).

        Java 9 users: use enumeration.asIterator() instead, unless it is important to return an UnmodifiableIterator instead of a plain Iterator.

      • peekingIterator

        public static <T> PeekingIterator<T> peekingIterator​(Iterator<? extends T> iterator)
        Returns a PeekingIterator backed by the given iterator.

        Calls to the peek method with no intervening calls to next do not affect the iteration, and hence return the same object each time. A subsequent call to next is guaranteed to return the same object again. For example: 

        
 PeekingIterator<String> peekingIterator =
     Iterators.peekingIterator(Iterators.forArray("a", "b"));
 String a1 = peekingIterator.peek(); // returns "a"
 String a2 = peekingIterator.peek(); // also returns "a"
 String a3 = peekingIterator.next(); // also returns "a"

        Any structural changes to the underlying iteration (aside from those performed by the iterator's own PeekingIterator.remove() method) will leave the iterator in an undefined state.

        The returned iterator does not support removal after peeking, as explained by PeekingIterator.remove().

        Note: If the given iterator is already a 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 PeekingIterator when the behavior of the implementation is known to meet the contract guaranteed by this method.

        There is no Iterable equivalent to this method, so use this method to wrap each individual iterator as it is generated.

        Parameters:
        iterator - the backing iterator. The PeekingIterator assumes ownership of this iterator, so users should cease making direct calls to it after calling this method.
        Returns:
        a peeking iterator backed by that iterator. Apart from the additional PeekingIterator.peek() method, this iterator behaves exactly the same as iterator.

      • mergeSorted

        @Beta
public static <T> UnmodifiableIterator<T> mergeSorted​(Iterable<? extends Iterator<? extends T>> iterators,
                                                      Comparator<? super T> comparator)
        Returns an iterator over the merged contents of all given iterators, traversing every element of the input iterators. Equivalent entries will not be de-duplicated.

        Callers must ensure that the source iterators are in non-descending order as this method does not sort its input.

        For any equivalent elements across all iterators, it is undefined which element is returned first.

        Since:
        11.0