Package com.google.common.collect

Class FluentIterable<E extends @Nullable Object>

  • All Implemented Interfaces:
    Iterable<E>
    @GwtCompatible(emulated=true)
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:

    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 Detail

      • FluentIterable

        protected FluentIterable()
        Constructor for use by subclasses.

    • Method Detail

      • 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

        @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(java.lang.Iterable<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​(@CheckForNull
                              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​(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

        @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());

      • 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().

      • 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().

      • 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()).

      • 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(java.util.Comparator<? super E>) 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()).

      • 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(Collectors.groupingBy(keyFunction)) behaves similarly, but returns a mutable Map<K, List<E>> instead, and may not preserve the order of entries.

        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