Class FluentIterable<E extends @Nullable Object>

java.lang.Object
com.google.common.collect.FluentIterable<E>
All Implemented Interfaces:
Iterable<E>
@GwtCompatible public abstract class FluentIterable<E extends @Nullable Object> extends Object implements Iterable<E>
A discouraged (but not deprecated) precursor to Java's superior Stream library.

The following types of methods are provided:

  • chaining methods which return a new FluentIterable based in some way on the contents of the current one (for example transform(Function))
  • element extraction methods which facilitate the retrieval of certain elements (for example last())
  • query methods which answer questions about the FluentIterable's contents (for example anyMatch(Predicate))
  • conversion methods which copy the FluentIterable's contents into a new collection or array (for example toList())

Several lesser-used features are currently available only as static methods on the Iterables class.

Comparison to streams

Stream is similar to this class, but generally more powerful, and certainly more standard. Key differences include:

  • A stream is single-use; it becomes invalid as soon as any "terminal operation" such as findFirst() or iterator() is invoked. (Even though Stream contains all the right method signatures to implement Iterable, it does not actually do so, to avoid implying repeat-iterability.) FluentIterable, on the other hand, is multiple-use, and does implement Iterable.
  • Streams offer many features not found here, including min/max, distinct, reduce, sorted, the very powerful collect, and built-in support for parallelizing stream operations.
  • FluentIterable contains several features not available on Stream, which are noted in the method descriptions below.
  • Streams include primitive-specialized variants such as IntStream, the use of which is strongly recommended.
  • Streams are standard Java, not requiring a third-party dependency.

Example

Here is an example that accepts a list from a database call, filters it based on a predicate, transforms it by invoking toString() on each element, and returns the first 10 elements as a List: 

ImmutableList<String> results =
    FluentIterable.from(database.getClientList())
        .filter(Client::isActiveInLastMonth)
        .transform(Object::toString)
        .limit(10)
        .toList();
The approximate stream equivalent is: 
List<String> results =
    database.getClientList()
        .stream()
        .filter(Client::isActiveInLastMonth)
        .map(Object::toString)
        .limit(10)
        .collect(Collectors.toList());
Since:
12.0
Author:
Marcin Mikosik

  • Constructor Details

    • FluentIterable

      protected FluentIterable()
      Constructor for use by subclasses.

  • Method Details

    • from

      public static <E extends @Nullable Object> FluentIterable<E> from(Iterable<E> iterable)
      Returns a fluent iterable that wraps iterable, or iterable itself if it is already a FluentIterable.

      Stream equivalent: Collection.stream() if iterable is a Collection; Streams.stream(Iterable) otherwise.

    • from

      public static <E extends @Nullable Object> FluentIterable<E> from(E[] elements)
      Returns a fluent iterable containing elements in the specified order.

      The returned iterable is an unmodifiable view of the input array.

      Stream equivalent: Stream.of(T...).

      Since:
      20.0 (since 18.0 as an overload of of)

    • from

      @Deprecated @InlineMe(replacement="checkNotNull(iterable)", staticImports="com.google.common.base.Preconditions.checkNotNull") public static <E extends @Nullable Object> FluentIterable<E> from(FluentIterable<E> iterable)
      Deprecated.
      instances of FluentIterable don't need to be converted to FluentIterable
      Construct a fluent iterable from another fluent iterable. This is obviously never necessary, but is intended to help call out cases where one migration from Iterable to FluentIterable has obviated the need to explicitly convert to a FluentIterable.

    • concat

      public static <T extends @Nullable Object> FluentIterable<T> concat(Iterable<? extends T> a, Iterable<? extends T> b)
      Returns a fluent iterable that combines two iterables. The returned iterable has an iterator that traverses the elements in a, followed by the elements in b. The source iterators are not polled until necessary.

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

      Stream equivalent: Stream.concat(Stream, Stream).

      Since:
      20.0

    • concat

      public static <T extends @Nullable Object> FluentIterable<T> concat(Iterable<? extends T> a, Iterable<? extends T> b, Iterable<? extends T> c)
      Returns a fluent iterable that combines three iterables. The returned iterable has an iterator that traverses 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 iterable's iterator supports remove() when the corresponding input iterator supports it.

      Stream equivalent: use nested calls to Stream.concat(Stream, Stream), or see the advice in concat(Iterable...).

      Since:
      20.0

    • concat

      public static <T extends @Nullable Object> FluentIterable<T> concat(Iterable<? extends T> a, Iterable<? extends T> b, Iterable<? extends T> c, Iterable<? extends T> d)
      Returns a fluent iterable that combines four iterables. The returned iterable has an iterator that traverses 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 iterable's iterator supports remove() when the corresponding input iterator supports it.

      Stream equivalent: use nested calls to Stream.concat(Stream, Stream), or see the advice in concat(Iterable...).

      Since:
      20.0

    • concat

      @SafeVarargs public static <T extends @Nullable Object> FluentIterable<T> concat(Iterable<? extends T>... inputs)
      Returns a fluent iterable that combines several iterables. The returned iterable has an iterator that traverses the elements of each iterable in inputs. The input iterators are not polled until necessary.

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

      Stream equivalent: to concatenate an arbitrary number of streams, use Stream.of(stream1, stream2, ...).flatMap(s -> s). If the sources are iterables, use Stream.of(iter1, iter2, ...).flatMap(Streams::stream).

      Throws:
      NullPointerException - if any of the provided iterables is null
      Since:
      20.0

    • concat

      public static <T extends @Nullable Object> FluentIterable<T> concat(Iterable<? extends Iterable<? extends T>> inputs)
      Returns a fluent iterable that combines several iterables. The returned iterable has an iterator that traverses the elements of each iterable in inputs. The input iterators are not polled until necessary.

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

      Stream equivalent: streamOfStreams.flatMap(s -> s) or streamOfIterables.flatMap(Streams::stream). (See Streams.stream(Iterable).)

      Since:
      20.0

    • of

      public static <E extends @Nullable Object> FluentIterable<E> of()
      Returns a fluent iterable containing no elements.

      Stream equivalent: Stream.empty().

      Since:
      20.0

    • of

      public static <E extends @Nullable Object> FluentIterable<E> of(E element, E... elements)
      Returns a fluent iterable containing the specified elements in order.

      Stream equivalent: Stream.of(T...).

      Since:
      20.0

    • toString

      public String toString()
      Returns a string representation of this fluent iterable, with the format [e1, e2, ..., en].

      Stream equivalent: stream.collect(Collectors.joining(", ", "[", "]")) or (less efficiently) stream.collect(Collectors.toList()).toString().

      Overrides:
      toString in class Object

    • size

      public final int size()
      Returns the number of elements in this fluent iterable.

      Stream equivalent: Stream.count().

    • contains

      public final boolean contains(@Nullable Object target)
      Returns true if this fluent iterable contains any object for which equals(target) is true.

      Stream equivalent: stream.anyMatch(Predicate.isEqual(target)).

    • cycle

      public final FluentIterable<E> cycle()
      Returns a fluent iterable whose Iterator cycles indefinitely over the elements of this fluent iterable.

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

      Stream equivalent: if the source iterable has only a single element e, use Stream.generate(() -> e). Otherwise, collect your stream into a collection and use Stream.generate(() -> collection).flatMap(Collection::stream).

    • append

      public final FluentIterable<E> append(Iterable<? extends E> other)
      Returns a fluent iterable whose iterators traverse first the elements of this fluent iterable, followed by those of other. The iterators are not polled until necessary.

      The returned iterable's Iterator supports remove() when the corresponding Iterator supports it.

      Stream equivalent: Stream.concat(Stream, Stream).

      Since:
      18.0

    • append

      public final FluentIterable<E> append(E... elements)
      Returns a fluent iterable whose iterators traverse first the elements of this fluent iterable, followed by elements.

      Stream equivalent: Stream.concat(thisStream, Stream.of(elements)).

      Since:
      18.0

    • filter

      public final FluentIterable<E> filter(Predicate<? super E> predicate)
      Returns the elements from this fluent iterable that satisfy a predicate. The resulting fluent iterable's iterator does not support remove().

      Stream equivalent: Stream.filter(Predicate) (same).

    • filter

      @GwtIncompatible public final <T> FluentIterable<T> filter(Class<T> type)
      Returns the elements from this fluent iterable that are instances of class type.

      Stream equivalent: stream.filter(type::isInstance).map(type::cast). This does perform a little more work than necessary, so another option is to insert an unchecked cast at some later point: 

      @SuppressWarnings("unchecked") // safe because of ::isInstance check
ImmutableList<NewType> result =
    (ImmutableList) stream.filter(NewType.class::isInstance).collect(toImmutableList());

    • anyMatch

      public final boolean anyMatch(Predicate<? super E> predicate)
      Returns true if any element in this fluent iterable satisfies the predicate.

      Stream equivalent: Stream.anyMatch(Predicate) (same).

    • allMatch

      public final boolean allMatch(Predicate<? super E> predicate)
      Returns true if every element in this fluent iterable satisfies the predicate. If this fluent iterable is empty, true is returned.

      Stream equivalent: Stream.allMatch(Predicate) (same).

    • firstMatch

      public final Optional<@NonNull E> firstMatch(Predicate<? super E> predicate)
      Returns an Optional containing the first element in this fluent iterable that satisfies the given predicate, if such an element exists.

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

      Stream equivalent: stream.filter(predicate).findFirst().

    • transform

      public final <T extends @Nullable Object> FluentIterable<T> transform(Function<? super E, T> function)
      Returns a fluent iterable that applies function to each element of this fluent iterable.

      The returned fluent iterable's iterator supports remove() if this iterable's iterator does. After a successful remove() call, this fluent iterable no longer contains the corresponding element.

      Stream equivalent: Stream.map(Function).

    • transformAndConcat

      public <T extends @Nullable Object> FluentIterable<T> transformAndConcat(Function<? super E, ? extends Iterable<? extends T>> function)
      Applies function to each element of this fluent iterable and returns a fluent iterable with the concatenated combination of results. function returns an Iterable of results.

      The returned fluent iterable's iterator supports remove() if this function-returned iterables' iterator does. After a successful remove() call, the returned fluent iterable no longer contains the corresponding element.

      Stream equivalent: Stream.flatMap(Function) (using a function that produces streams, not iterables).

      Since:
      13.0 (required Function<E, Iterable<T>> until 14.0)

    • first

      public final Optional<@NonNull E> first()
      Returns an Optional containing the first element in this fluent iterable. If the iterable is empty, Optional.absent() is returned.

      Stream equivalent: if the goal is to obtain any element, Stream.findAny(); if it must specifically be the first element, Stream#findFirst.

      Throws:
      NullPointerException - if the first element is null; if this is a possibility, use iterator().next() or Iterables.getFirst(Iterable, T) instead.

    • last

      public final Optional<@NonNull E> last()
      Returns an Optional containing the last element in this fluent iterable. If the iterable is empty, Optional.absent() is returned. If the underlying iterable is a List with RandomAccess support, then this operation is guaranteed to be O(1).

      Stream equivalent: stream.reduce((a, b) -> b).

      Throws:
      NullPointerException - if the last element is null; if this is a possibility, use Iterables.getLast(Iterable) instead.

    • skip

      public final FluentIterable<E> skip(int numberToSkip)
      Returns a view of this fluent iterable that skips its first numberToSkip elements. If this fluent iterable contains fewer than numberToSkip elements, the returned fluent iterable skips all of its elements.

      Modifications to this fluent iterable before a call to iterator() are reflected in the returned fluent iterable. That is, the iterator skips the first numberToSkip elements that exist when the iterator is created, not when skip() is called.

      The returned fluent iterable's iterator supports remove() if the Iterator of this fluent iterable supports it. Note that it is not possible to delete the last skipped element by immediately calling remove() on the returned fluent iterable's iterator, as the Iterator contract states that a call to * remove() before a call to next() will throw an IllegalStateException.

      Stream equivalent: Stream.skip(long) (same).

    • limit

      public final FluentIterable<E> limit(int maxSize)
      Creates a fluent iterable with the first size elements of this fluent iterable. If this fluent iterable does not contain that many elements, the returned fluent iterable will have the same behavior as this fluent iterable. The returned fluent iterable's iterator supports remove() if this fluent iterable's iterator does.

      Stream equivalent: Stream.limit(long) (same).

      Parameters:
      maxSize - the maximum number of elements in the returned fluent iterable
      Throws:
      IllegalArgumentException - if size is negative

    • isEmpty

      public final boolean isEmpty()
      Determines whether this fluent iterable is empty.

      Stream equivalent: !stream.findAny().isPresent().

    • toList

      public final ImmutableList<@NonNull E> toList()
      Returns an ImmutableList containing all of the elements from this fluent iterable in proper sequence.

      Stream equivalent: pass ImmutableList.toImmutableList() to stream.collect().

      Throws:
      NullPointerException - if any element is null
      Since:
      14.0 (since 12.0 as toImmutableList()).

    • toSortedList

      public final ImmutableList<@NonNull E> toSortedList(Comparator<? super E> comparator)
      Returns an ImmutableList containing all of the elements from this FluentIterable in the order specified by comparator. To produce an ImmutableList sorted by its natural ordering, use toSortedList(Ordering.natural()).

      Stream equivalent: pass ImmutableList.toImmutableList() to stream.sorted(comparator).collect().

      Parameters:
      comparator - the function by which to sort list elements
      Throws:
      NullPointerException - if any element of this iterable is null
      Since:
      14.0 (since 13.0 as toSortedImmutableList()).

    • toSet

      public final ImmutableSet<@NonNull E> toSet()
      Returns an ImmutableSet containing all of the elements from this fluent iterable with duplicates removed.

      Stream equivalent: pass ImmutableSet.toImmutableSet() to stream.collect().

      Throws:
      NullPointerException - if any element is null
      Since:
      14.0 (since 12.0 as toImmutableSet()).

    • toSortedSet

      public final ImmutableSortedSet<@NonNull E> toSortedSet(Comparator<? super E> comparator)
      Returns an ImmutableSortedSet containing all of the elements from this FluentIterable in the order specified by comparator, with duplicates (determined by comparator.compare(x, y) == 0) removed. To produce an ImmutableSortedSet sorted by its natural ordering, use toSortedSet(Ordering.natural()).

      Stream equivalent: pass ImmutableSortedSet.toImmutableSortedSet(Comparator) to stream.collect().

      Parameters:
      comparator - the function by which to sort set elements
      Throws:
      NullPointerException - if any element of this iterable is null
      Since:
      14.0 (since 12.0 as toImmutableSortedSet()).

    • toMultiset

      public final ImmutableMultiset<@NonNull E> toMultiset()
      Returns an ImmutableMultiset containing all of the elements from this fluent iterable.

      Stream equivalent: pass ImmutableMultiset.toImmutableMultiset() to stream.collect().

      Throws:
      NullPointerException - if any element is null
      Since:
      19.0

    • toMap

      public final <V> ImmutableMap<@NonNull E, V> toMap(Function<? super E, V> valueFunction)
      Returns an immutable map whose keys are the distinct elements of this FluentIterable and whose value for each key was computed by valueFunction. The map's iteration order is the order of the first appearance of each key in this iterable.

      When there are multiple instances of a key in this iterable, it is unspecified whether valueFunction will be applied to more than one instance of that key and, if it is, which result will be mapped to that key in the returned map.

      Stream equivalent: stream.collect(ImmutableMap.toImmutableMap(k -> k, valueFunction)).

      Throws:
      NullPointerException - if any element of this iterable is null, or if valueFunction produces null for any key
      Since:
      14.0

    • index

      public final <K> ImmutableListMultimap<K, @NonNull E> index(Function<? super E, K> keyFunction)
      Creates an index ImmutableListMultimap that contains the results of applying a specified function to each item in this FluentIterable of values. Each element of this iterable will be stored as a value in the resulting multimap, yielding a multimap with the same size as this iterable. The key used to store that value in the multimap will be the result of calling the function on that value. The resulting multimap is created as an immutable snapshot. In the returned multimap, keys appear in the order they are first encountered, and the values corresponding to each key appear in the same order as they are encountered.

      Stream equivalent: stream.collect(ImmutableListMultimap.toImmutableListMultimap(keyFunction, v -> v)).

      Parameters:
      keyFunction - the function used to produce the key for each value
      Throws:
      NullPointerException - if any element of this iterable is null, or if keyFunction produces null for any key
      Since:
      14.0

    • uniqueIndex

      public final <K> ImmutableMap<K, @NonNull E> uniqueIndex(Function<? super E, K> keyFunction)
      Returns a map with the contents of this FluentIterable as its values, indexed by keys derived from those values. In other words, each input value produces an entry in the map whose key is the result of applying keyFunction to that value. These entries appear in the same order as they appeared in this fluent iterable. Example usage: 
      Color red = new Color("red", 255, 0, 0);
...
FluentIterable<Color> allColors = FluentIterable.from(ImmutableSet.of(red, green, blue));

Map<String, Color> colorForName = allColors.uniqueIndex(toStringFunction());
assertThat(colorForName).containsEntry("red", red);

      If your index may associate multiple values with each key, use index.

      Stream equivalent: stream.collect(ImmutableMap.toImmutableMap(keyFunction, v -> v)).

      Parameters:
      keyFunction - the function used to produce the key for each value
      Returns:
      a map mapping the result of evaluating the function keyFunction on each value in this fluent iterable to that value
      Throws:
      IllegalArgumentException - if keyFunction produces the same key for more than one value in this fluent iterable
      NullPointerException - if any element of this iterable is null, or if keyFunction produces null for any key
      Since:
      14.0

    • toArray

      @GwtIncompatible public final E[] toArray(Class<@NonNull E> type)
      Returns an array containing all of the elements from this fluent iterable in iteration order.

      Stream equivalent: if an object array is acceptable, use stream.toArray(); if type is a class literal such as MyType.class, use stream.toArray(MyType[]::new). Otherwise use stream.toArray( len -> (E[]) Array.newInstance(type, len)).

      Parameters:
      type - the type of the elements
      Returns:
      a newly-allocated array into which all the elements of this fluent iterable have been copied

    • copyInto

      @CanIgnoreReturnValue public final <C extends Collection<? super E>> C copyInto(C collection)
      Copies all the elements from this fluent iterable to collection. This is equivalent to calling Iterables.addAll(collection, this).

      Stream equivalent: stream.forEachOrdered(collection::add) or stream.forEach(collection::add).

      Parameters:
      collection - the collection to copy elements to
      Returns:
      collection, for convenience
      Since:
      14.0

    • join

      public final String join(Joiner joiner)
      Returns a String containing all of the elements of this fluent iterable joined with joiner.

      Stream equivalent: joiner.join(stream.iterator()), or, if you are not using any optional Joiner features, stream.collect(Collectors.joining(delimiter).

      Since:
      18.0

    • get

      public final E get(int position)
      Returns the element at the specified position in this fluent iterable.

      Stream equivalent: stream.skip(position).findFirst().get() (but note that this throws different exception types, and throws an exception if null would be returned).

      Parameters:
      position - position of the element to return
      Returns:
      the element at the specified position in this fluent iterable
      Throws:
      IndexOutOfBoundsException - if position is negative or greater than or equal to the size of this fluent iterable

    • stream

      public final Stream<E> stream()
      Returns a stream of this fluent iterable's contents (similar to calling Collection.stream() on a collection).

      Note: the earlier in the chain you can switch to Stream usage (ideally not going through FluentIterable at all), the more performant and idiomatic your code will be. This method is a transitional aid, to be used only when really necessary.

      Since:
      21.0