Skip to content

Most visited

Recently visited

navigation

Stream

public interface Stream
implements BaseStream<T, Stream<T>>

java.util.stream.Stream<T>


A sequence of elements supporting sequential and parallel aggregate operations. The following example illustrates an aggregate operation using Stream and IntStream:

int sum = widgets.stream()
                      .filter(w -> w.getColor() == RED)
                      .mapToInt(w -> w.getWeight())
                      .sum();
 
In this example, widgets is a Collection<Widget>. We create a stream of Widget objects via Collection.stream(), filter it to produce a stream containing only the red widgets, and then transform it into a stream of int values representing the weight of each red widget. Then this stream is summed to produce a total weight.

In addition to Stream, which is a stream of object references, there are primitive specializations for IntStream, LongStream, and DoubleStream, all of which are referred to as "streams" and conform to the characteristics and restrictions described here.

To perform a computation, stream operations are composed into a stream pipeline. A stream pipeline consists of a source (which might be an array, a collection, a generator function, an I/O channel, etc), zero or more intermediate operations (which transform a stream into another stream, such as filter(Predicate)), and a terminal operation (which produces a result or side-effect, such as count() or forEach(Consumer)). Streams are lazy; computation on the source data is only performed when the terminal operation is initiated, and source elements are consumed only as needed.

Collections and streams, while bearing some superficial similarities, have different goals. Collections are primarily concerned with the efficient management of, and access to, their elements. By contrast, streams do not provide a means to directly access or manipulate their elements, and are instead concerned with declaratively describing their source and the computational operations which will be performed in aggregate on that source. However, if the provided stream operations do not offer the desired functionality, the iterator() and spliterator() operations can be used to perform a controlled traversal.

A stream pipeline, like the "widgets" example above, can be viewed as a query on the stream source. Unless the source was explicitly designed for concurrent modification (such as a ConcurrentHashMap), unpredictable or erroneous behavior may result from modifying the stream source while it is being queried.

Most stream operations accept parameters that describe user-specified behavior, such as the lambda expression w -> w.getWeight() passed to mapToInt in the example above. To preserve correct behavior, these behavioral parameters:

  • must be non-interfering (they do not modify the stream source); and
  • in most cases must be stateless (their result should not depend on any state that might change during execution of the stream pipeline).

Such parameters are always instances of a functional interface such as Function, and are often lambda expressions or method references. Unless otherwise specified these parameters must be non-null.

A stream should be operated on (invoking an intermediate or terminal stream operation) only once. This rules out, for example, "forked" streams, where the same source feeds two or more pipelines, or multiple traversals of the same stream. A stream implementation may throw IllegalStateException if it detects that the stream is being reused. However, since some stream operations may return their receiver rather than a new stream object, it may not be possible to detect reuse in all cases.

Streams have a close() method and implement AutoCloseable, but nearly all stream instances do not actually need to be closed after use. Generally, only streams whose source is an IO channel will require closing. Most streams are backed by collections, arrays, or generating functions, which require no special resource management. (If a stream does require closing, it can be declared as a resource in a try-with-resources statement.)

Stream pipelines may execute either sequentially or in parallel. This execution mode is a property of the stream. Streams are created with an initial choice of sequential or parallel execution. (For example, Collection.stream() creates a sequential stream, and Collection.parallelStream() creates a parallel one.) This choice of execution mode may be modified by the sequential() or parallel() methods, and may be queried with the isParallel() method.

See also:

Summary

Nested classes

interface Stream.Builder<T>

A mutable builder for a Stream

Public methods

abstract boolean allMatch(Predicate<? super T> predicate)

Returns whether all elements of this stream match the provided predicate.

abstract boolean anyMatch(Predicate<? super T> predicate)

Returns whether any elements of this stream match the provided predicate.

static <T> Builder<T> builder()

Returns a builder for a Stream.

abstract <R, A> R collect(Collector<? super T, A, R> collector)

Performs a mutable reduction operation on the elements of this stream using a Collector.

abstract <R> R collect(Supplier<R> supplier, BiConsumer<R, ? super T> accumulator, BiConsumer<R, R> combiner)

Performs a mutable reduction operation on the elements of this stream.

static <T> Stream<T> concat(Stream<? extends T> a, Stream<? extends T> b)

Creates a lazily concatenated stream whose elements are all the elements of the first stream followed by all the elements of the second stream.

abstract long count()

Returns the count of elements in this stream.

abstract Stream<T> distinct()

Returns a stream consisting of the distinct elements (according to equals(Object)) of this stream.

static <T> Stream<T> empty()

Returns an empty sequential Stream.

abstract Stream<T> filter(Predicate<? super T> predicate)

Returns a stream consisting of the elements of this stream that match the given predicate.

abstract Optional<T> findAny()

Returns an Optional describing some element of the stream, or an empty Optional if the stream is empty.

abstract Optional<T> findFirst()

Returns an Optional describing the first element of this stream, or an empty Optional if the stream is empty.

abstract <R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper)

Returns a stream consisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element.

abstract DoubleStream flatMapToDouble(Function<? super T, ? extends DoubleStream> mapper)

Returns an DoubleStream consisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element.

abstract IntStream flatMapToInt(Function<? super T, ? extends IntStream> mapper)

Returns an IntStream consisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element.

abstract LongStream flatMapToLong(Function<? super T, ? extends LongStream> mapper)

Returns an LongStream consisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element.

abstract void forEach(Consumer<? super T> action)

Performs an action for each element of this stream.

abstract void forEachOrdered(Consumer<? super T> action)

Performs an action for each element of this stream, in the encounter order of the stream if the stream has a defined encounter order.

static <T> Stream<T> generate(Supplier<T> s)

Returns an infinite sequential unordered stream where each element is generated by the provided Supplier.

static <T> Stream<T> iterate(T seed, UnaryOperator<T> f)

Returns an infinite sequential ordered Stream produced by iterative application of a function f to an initial element seed, producing a Stream consisting of seed, f(seed), f(f(seed)), etc.

abstract Stream<T> limit(long maxSize)

Returns a stream consisting of the elements of this stream, truncated to be no longer than maxSize in length.

abstract <R> Stream<R> map(Function<? super T, ? extends R> mapper)

Returns a stream consisting of the results of applying the given function to the elements of this stream.

abstract DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper)

Returns a DoubleStream consisting of the results of applying the given function to the elements of this stream.

abstract IntStream mapToInt(ToIntFunction<? super T> mapper)

Returns an IntStream consisting of the results of applying the given function to the elements of this stream.

abstract LongStream mapToLong(ToLongFunction<? super T> mapper)

Returns a LongStream consisting of the results of applying the given function to the elements of this stream.

abstract Optional<T> max(Comparator<? super T> comparator)

Returns the maximum element of this stream according to the provided Comparator.

abstract Optional<T> min(Comparator<? super T> comparator)

Returns the minimum element of this stream according to the provided Comparator.

abstract boolean noneMatch(Predicate<? super T> predicate)

Returns whether no elements of this stream match the provided predicate.

static <T> Stream<T> of(T t)

Returns a sequential Stream containing a single element.

static <T> Stream<T> of(T... values)

Returns a sequential ordered stream whose elements are the specified values.

abstract Stream<T> peek(Consumer<? super T> action)

Returns a stream consisting of the elements of this stream, additionally performing the provided action on each element as elements are consumed from the resulting stream.

abstract T reduce(T identity, BinaryOperator<T> accumulator)

Performs a reduction on the elements of this stream, using the provided identity value and an associative accumulation function, and returns the reduced value.

abstract Optional<T> reduce(BinaryOperator<T> accumulator)

Performs a reduction on the elements of this stream, using an associative accumulation function, and returns an Optional describing the reduced value, if any.

abstract <U> U reduce(U identity, BiFunction<U, ? super T, U> accumulator, BinaryOperator<U> combiner)

Performs a reduction on the elements of this stream, using the provided identity, accumulation and combining functions.

abstract Stream<T> skip(long n)

Returns a stream consisting of the remaining elements of this stream after discarding the first n elements of the stream.

abstract Stream<T> sorted(Comparator<? super T> comparator)

Returns a stream consisting of the elements of this stream, sorted according to the provided Comparator.

abstract Stream<T> sorted()

Returns a stream consisting of the elements of this stream, sorted according to natural order.

abstract Object[] toArray()

Returns an array containing the elements of this stream.

abstract <A> A[] toArray(IntFunction<A[]> generator)

Returns an array containing the elements of this stream, using the provided generator function to allocate the returned array, as well as any additional arrays that might be required for a partitioned execution or for resizing.

Inherited methods

From interface java.util.stream.BaseStream
From interface java.lang.AutoCloseable

Public methods

allMatch

added in API level 24
boolean allMatch (Predicate<? super T> predicate)

Returns whether all elements of this stream match the provided predicate. May not evaluate the predicate on all elements if not necessary for determining the result. If the stream is empty then true is returned and the predicate is not evaluated.

This is a short-circuiting terminal operation.

API Note:
  • This method evaluates the universal quantification of the predicate over the elements of the stream (for all x P(x)). If the stream is empty, the quantification is said to be vacuously satisfied and is always true (regardless of P(x)).
Parameters
predicate Predicate: a non-interfering, stateless predicate to apply to elements of this stream

Returns
boolean true if either all elements of the stream match the provided predicate or the stream is empty, otherwise false

anyMatch

added in API level 24
boolean anyMatch (Predicate<? super T> predicate)

Returns whether any elements of this stream match the provided predicate. May not evaluate the predicate on all elements if not necessary for determining the result. If the stream is empty then false is returned and the predicate is not evaluated.

This is a short-circuiting terminal operation.

API Note:
  • This method evaluates the existential quantification of the predicate over the elements of the stream (for some x P(x)).
Parameters
predicate Predicate: a non-interfering, stateless predicate to apply to elements of this stream

Returns
boolean true if any elements of the stream match the provided predicate, otherwise false

builder

added in API level 24
Builder<T> builder ()

Returns a builder for a Stream.

Returns
Builder<T> a stream builder

collect

added in API level 24
R collect (Collector<? super T, A, R> collector)

Performs a mutable reduction operation on the elements of this stream using a Collector. A Collector encapsulates the functions used as arguments to collect(Supplier, BiConsumer, BiConsumer), allowing for reuse of collection strategies and composition of collect operations such as multiple-level grouping or partitioning.

If the stream is parallel, and the Collector is concurrent, and either the stream is unordered or the collector is unordered, then a concurrent reduction will be performed (see Collector for details on concurrent reduction.)

This is a terminal operation.

When executed in parallel, multiple intermediate results may be instantiated, populated, and merged so as to maintain isolation of mutable data structures. Therefore, even when executed in parallel with non-thread-safe data structures (such as ArrayList), no additional synchronization is needed for a parallel reduction.

API Note:
  • The following will accumulate strings into an ArrayList:
    List<String> asList = stringStream.collect(Collectors.toList());
     

    The following will classify Person objects by city:

    Map<String, List<Person>> peopleByCity
             = personStream.collect(Collectors.groupingBy(Person::getCity));
     

    The following will classify Person objects by state and city, cascading two Collectors together:

    Map<String, Map<String, List<Person>>> peopleByStateAndCity
             = personStream.collect(Collectors.groupingBy(Person::getState,
                                                          Collectors.groupingBy(Person::getCity)));
     
Parameters
collector Collector: the Collector describing the reduction

Returns
R the result of the reduction

See also:

collect

added in API level 24
R collect (Supplier<R> supplier, 
                BiConsumer<R, ? super T> accumulator, 
                BiConsumer<R, R> combiner)

Performs a mutable reduction operation on the elements of this stream. A mutable reduction is one in which the reduced value is a mutable result container, such as an ArrayList, and elements are incorporated by updating the state of the result rather than by replacing the result. This produces a result equivalent to:

R result = supplier.get();
     for (T element : this stream)
         accumulator.accept(result, element);
     return result;
 

Like reduce(Object, BinaryOperator), collect operations can be parallelized without requiring additional synchronization.

This is a terminal operation.

API Note:
  • There are many existing classes in the JDK whose signatures are well-suited for use with method references as arguments to collect(). For example, the following will accumulate strings into an ArrayList:
    List<String> asList = stringStream.collect(ArrayList::new, ArrayList::add,
                                                    ArrayList::addAll);
     

    The following will take a stream of strings and concatenates them into a single string:

    String concat = stringStream.collect(StringBuilder::new, StringBuilder::append,
                                              StringBuilder::append)
                                     .toString();
     
Parameters
supplier Supplier: a function that creates a new result container. For a parallel execution, this function may be called multiple times and must return a fresh value each time.

accumulator BiConsumer: an associative, non-interfering, stateless function for incorporating an additional element into a result

combiner BiConsumer: an associative, non-interfering, stateless function for combining two values, which must be compatible with the accumulator function

Returns
R the result of the reduction

concat

added in API level 24
Stream<T> concat (Stream<? extends T> a, 
                Stream<? extends T> b)

Creates a lazily concatenated stream whose elements are all the elements of the first stream followed by all the elements of the second stream. The resulting stream is ordered if both of the input streams are ordered, and parallel if either of the input streams is parallel. When the resulting stream is closed, the close handlers for both input streams are invoked.

Implementation Note:
  • Use caution when constructing streams from repeated concatenation. Accessing an element of a deeply concatenated stream can result in deep call chains, or even StackOverflowException.
Parameters
a Stream: the first stream

b Stream: the second stream

Returns
Stream<T> the concatenation of the two input streams

count

added in API level 24
long count ()

Returns the count of elements in this stream. This is a special case of a reduction and is equivalent to:

return mapToLong(e -> 1L).sum();
 

This is a terminal operation.

Returns
long the count of elements in this stream

distinct

added in API level 24
Stream<T> distinct ()

Returns a stream consisting of the distinct elements (according to equals(Object)) of this stream.

For ordered streams, the selection of distinct elements is stable (for duplicated elements, the element appearing first in the encounter order is preserved.) For unordered streams, no stability guarantees are made.

This is a stateful intermediate operation.

API Note:
  • Preserving stability for distinct() in parallel pipelines is relatively expensive (requires that the operation act as a full barrier, with substantial buffering overhead), and stability is often not needed. Using an unordered stream source (such as generate(Supplier)) or removing the ordering constraint with unordered() may result in significantly more efficient execution for distinct() in parallel pipelines, if the semantics of your situation permit. If consistency with encounter order is required, and you are experiencing poor performance or memory utilization with distinct() in parallel pipelines, switching to sequential execution with sequential() may improve performance.
Returns
Stream<T> the new stream

empty

added in API level 24
Stream<T> empty ()

Returns an empty sequential Stream.

Returns
Stream<T> an empty sequential stream

filter

added in API level 24
Stream<T> filter (Predicate<? super T> predicate)

Returns a stream consisting of the elements of this stream that match the given predicate.

This is an intermediate operation.

Parameters
predicate Predicate: a non-interfering, stateless predicate to apply to each element to determine if it should be included

Returns
Stream<T> the new stream

findAny

added in API level 24
Optional<T> findAny ()

Returns an Optional describing some element of the stream, or an empty Optional if the stream is empty.

This is a short-circuiting terminal operation.

The behavior of this operation is explicitly nondeterministic; it is free to select any element in the stream. This is to allow for maximal performance in parallel operations; the cost is that multiple invocations on the same source may not return the same result. (If a stable result is desired, use findFirst() instead.)

Returns
Optional<T> an Optional describing some element of this stream, or an empty Optional if the stream is empty

Throws
NullPointerException if the element selected is null

See also:

findFirst

added in API level 24
Optional<T> findFirst ()

Returns an Optional describing the first element of this stream, or an empty Optional if the stream is empty. If the stream has no encounter order, then any element may be returned.

This is a short-circuiting terminal operation.

Returns
Optional<T> an Optional describing the first element of this stream, or an empty Optional if the stream is empty

Throws
NullPointerException if the element selected is null

flatMap

added in API level 24
Stream<R> flatMap (Function<? super T, ? extends Stream<? extends R>> mapper)

Returns a stream consisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element. Each mapped stream is closed after its contents have been placed into this stream. (If a mapped stream is null an empty stream is used, instead.)

This is an intermediate operation.

API Note:
  • The flatMap() operation has the effect of applying a one-to-many transformation to the elements of the stream, and then flattening the resulting elements into a new stream.

    Examples.

    If orders is a stream of purchase orders, and each purchase order contains a collection of line items, then the following produces a stream containing all the line items in all the orders:

    orders.flatMap(order -> order.getLineItems().stream())...
     

    If path is the path to a file, then the following produces a stream of the words contained in that file:

    Stream<String> lines = Files.lines(path, StandardCharsets.UTF_8);
         Stream<String> words = lines.flatMap(line -> Stream.of(line.split(" +")));
     
    The mapper function passed to flatMap splits a line, using a simple regular expression, into an array of words, and then creates a stream of words from that array.
Parameters
mapper Function: a non-interfering, stateless function to apply to each element which produces a stream of new values

Returns
Stream<R> the new stream

flatMapToDouble

added in API level 24
DoubleStream flatMapToDouble (Function<? super T, ? extends DoubleStream> mapper)

Returns an DoubleStream consisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element. Each mapped stream is closed after its contents have placed been into this stream. (If a mapped stream is null an empty stream is used, instead.)

This is an intermediate operation.

Parameters
mapper Function: a non-interfering, stateless function to apply to each element which produces a stream of new values

Returns
DoubleStream the new stream

See also:

flatMapToInt

added in API level 24
IntStream flatMapToInt (Function<? super T, ? extends IntStream> mapper)

Returns an IntStream consisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element. Each mapped stream is closed after its contents have been placed into this stream. (If a mapped stream is null an empty stream is used, instead.)

This is an intermediate operation.

Parameters
mapper Function: a non-interfering, stateless function to apply to each element which produces a stream of new values

Returns
IntStream the new stream

See also:

flatMapToLong

added in API level 24
LongStream flatMapToLong (Function<? super T, ? extends LongStream> mapper)

Returns an LongStream consisting of the results of replacing each element of this stream with the contents of a mapped stream produced by applying the provided mapping function to each element. Each mapped stream is closed after its contents have been placed into this stream. (If a mapped stream is null an empty stream is used, instead.)

This is an intermediate operation.

Parameters
mapper Function: a non-interfering, stateless function to apply to each element which produces a stream of new values

Returns
LongStream the new stream

See also:

forEach

added in API level 24
void forEach (Consumer<? super T> action)

Performs an action for each element of this stream.

This is a terminal operation.

The behavior of this operation is explicitly nondeterministic. For parallel stream pipelines, this operation does not guarantee to respect the encounter order of the stream, as doing so would sacrifice the benefit of parallelism. For any given element, the action may be performed at whatever time and in whatever thread the library chooses. If the action accesses shared state, it is responsible for providing the required synchronization.

Parameters
action Consumer: a non-interfering action to perform on the elements

forEachOrdered

added in API level 24
void forEachOrdered (Consumer<? super T> action)

Performs an action for each element of this stream, in the encounter order of the stream if the stream has a defined encounter order.

This is a terminal operation.

This operation processes the elements one at a time, in encounter order if one exists. Performing the action for one element happens-before performing the action for subsequent elements, but for any given element, the action may be performed in whatever thread the library chooses.

Parameters
action Consumer: a non-interfering action to perform on the elements

See also:

generate

added in API level 24
Stream<T> generate (Supplier<T> s)

Returns an infinite sequential unordered stream where each element is generated by the provided Supplier. This is suitable for generating constant streams, streams of random elements, etc.

Parameters
s Supplier: the Supplier of generated elements

Returns
Stream<T> a new infinite sequential unordered Stream

iterate

added in API level 24
Stream<T> iterate (T seed, 
                UnaryOperator<T> f)

Returns an infinite sequential ordered Stream produced by iterative application of a function f to an initial element seed, producing a Stream consisting of seed, f(seed), f(f(seed)), etc.

The first element (position 0) in the Stream will be the provided seed. For n > 0, the element at position n, will be the result of applying the function f to the element at position n - 1.

Parameters
seed T: the initial element

f UnaryOperator: a function to be applied to to the previous element to produce a new element

Returns
Stream<T> a new sequential Stream

limit

added in API level 24
Stream<T> limit (long maxSize)

Returns a stream consisting of the elements of this stream, truncated to be no longer than maxSize in length.

This is a short-circuiting stateful intermediate operation.

API Note:
  • While limit() is generally a cheap operation on sequential stream pipelines, it can be quite expensive on ordered parallel pipelines, especially for large values of maxSize, since limit(n) is constrained to return not just any n elements, but the first n elements in the encounter order. Using an unordered stream source (such as generate(Supplier)) or removing the ordering constraint with unordered() may result in significant speedups of limit() in parallel pipelines, if the semantics of your situation permit. If consistency with encounter order is required, and you are experiencing poor performance or memory utilization with limit() in parallel pipelines, switching to sequential execution with sequential() may improve performance.
Parameters
maxSize long: the number of elements the stream should be limited to

Returns
Stream<T> the new stream

Throws
IllegalArgumentException if maxSize is negative

map

added in API level 24
Stream<R> map (Function<? super T, ? extends R> mapper)

Returns a stream consisting of the results of applying the given function to the elements of this stream.

This is an intermediate operation.

Parameters
mapper Function: a non-interfering, stateless function to apply to each element

Returns
Stream<R> the new stream

mapToDouble

added in API level 24
DoubleStream mapToDouble (ToDoubleFunction<? super T> mapper)

Returns a DoubleStream consisting of the results of applying the given function to the elements of this stream.

This is an intermediate operation.

Parameters
mapper ToDoubleFunction: a non-interfering, stateless function to apply to each element

Returns
DoubleStream the new stream

mapToInt

added in API level 24
IntStream mapToInt (ToIntFunction<? super T> mapper)

Returns an IntStream consisting of the results of applying the given function to the elements of this stream.

This is an intermediate operation.

Parameters
mapper ToIntFunction: a non-interfering, stateless function to apply to each element

Returns
IntStream the new stream

mapToLong

added in API level 24
LongStream mapToLong (ToLongFunction<? super T> mapper)

Returns a LongStream consisting of the results of applying the given function to the elements of this stream.

This is an intermediate operation.

Parameters
mapper ToLongFunction: a non-interfering, stateless function to apply to each element

Returns
LongStream the new stream

max

added in API level 24
Optional<T> max (Comparator<? super T> comparator)

Returns the maximum element of this stream according to the provided Comparator. This is a special case of a reduction.

This is a terminal operation.

Parameters
comparator Comparator: a non-interfering, stateless Comparator to compare elements of this stream

Returns
Optional<T> an Optional describing the maximum element of this stream, or an empty Optional if the stream is empty

Throws
NullPointerException if the maximum element is null

min

added in API level 24
Optional<T> min (Comparator<? super T> comparator)

Returns the minimum element of this stream according to the provided Comparator. This is a special case of a reduction.

This is a terminal operation.

Parameters
comparator Comparator: a non-interfering, stateless Comparator to compare elements of this stream

Returns
Optional<T> an Optional describing the minimum element of this stream, or an empty Optional if the stream is empty

Throws
NullPointerException if the minimum element is null

noneMatch

added in API level 24
boolean noneMatch (Predicate<? super T> predicate)

Returns whether no elements of this stream match the provided predicate. May not evaluate the predicate on all elements if not necessary for determining the result. If the stream is empty then true is returned and the predicate is not evaluated.

This is a short-circuiting terminal operation.

API Note:
  • This method evaluates the universal quantification of the negated predicate over the elements of the stream (for all x ~P(x)). If the stream is empty, the quantification is said to be vacuously satisfied and is always true, regardless of P(x).
Parameters
predicate Predicate: a non-interfering, stateless predicate to apply to elements of this stream

Returns
boolean true if either no elements of the stream match the provided predicate or the stream is empty, otherwise false

of

added in API level 24
Stream<T> of (T t)

Returns a sequential Stream containing a single element.

Parameters
t T: the single element

Returns
Stream<T> a singleton sequential stream

of

added in API level 24
Stream<T> of (T... values)

Returns a sequential ordered stream whose elements are the specified values.

Parameters
values T: the elements of the new stream

Returns
Stream<T> the new stream

peek

added in API level 24
Stream<T> peek (Consumer<? super T> action)

Returns a stream consisting of the elements of this stream, additionally performing the provided action on each element as elements are consumed from the resulting stream.

This is an intermediate operation.

For parallel stream pipelines, the action may be called at whatever time and in whatever thread the element is made available by the upstream operation. If the action modifies shared state, it is responsible for providing the required synchronization.

API Note:
  • This method exists mainly to support debugging, where you want to see the elements as they flow past a certain point in a pipeline:
    Stream.of("one", "two", "three", "four")
             .filter(e -> e.length() > 3)
             .peek(e -> System.out.println("Filtered value: " + e))
             .map(String::toUpperCase)
             .peek(e -> System.out.println("Mapped value: " + e))
             .collect(Collectors.toList());
     
Parameters
action Consumer: a non-interfering action to perform on the elements as they are consumed from the stream

Returns
Stream<T> the new stream

reduce

added in API level 24
T reduce (T identity, 
                BinaryOperator<T> accumulator)

Performs a reduction on the elements of this stream, using the provided identity value and an associative accumulation function, and returns the reduced value. This is equivalent to:

T result = identity;
     for (T element : this stream)
         result = accumulator.apply(result, element)
     return result;
 
but is not constrained to execute sequentially.

The identity value must be an identity for the accumulator function. This means that for all t, accumulator.apply(identity, t) is equal to t. The accumulator function must be an associative function.

This is a terminal operation.

API Note:
  • Sum, min, max, average, and string concatenation are all special cases of reduction. Summing a stream of numbers can be expressed as:
    Integer sum = integers.reduce(0, (a, b) -> a+b);
     
    or:
    Integer sum = integers.reduce(0, Integer::sum);
     

    While this may seem a more roundabout way to perform an aggregation compared to simply mutating a running total in a loop, reduction operations parallelize more gracefully, without needing additional synchronization and with greatly reduced risk of data races.

Parameters
identity T: the identity value for the accumulating function

accumulator BinaryOperator: an associative, non-interfering, stateless function for combining two values

Returns
T the result of the reduction

reduce

added in API level 24
Optional<T> reduce (BinaryOperator<T> accumulator)

Performs a reduction on the elements of this stream, using an associative accumulation function, and returns an Optional describing the reduced value, if any. This is equivalent to:

boolean foundAny = false;
     T result = null;
     for (T element : this stream) {
         if (!foundAny) {
             foundAny = true;
             result = element;
         }
         else
             result = accumulator.apply(result, element);
     }
     return foundAny ? Optional.of(result) : Optional.empty();
 
but is not constrained to execute sequentially.

The accumulator function must be an associative function.

This is a terminal operation.

Parameters
accumulator BinaryOperator: an associative, non-interfering, stateless function for combining two values

Returns
Optional<T> an Optional describing the result of the reduction

Throws
NullPointerException if the result of the reduction is null

See also:

reduce

added in API level 24
U reduce (U identity, 
                BiFunction<U, ? super T, U> accumulator, 
                BinaryOperator<U> combiner)

Performs a reduction on the elements of this stream, using the provided identity, accumulation and combining functions. This is equivalent to:

U result = identity;
     for (T element : this stream)
         result = accumulator.apply(result, element)
     return result;
 
but is not constrained to execute sequentially.

The identity value must be an identity for the combiner function. This means that for all u, combiner(identity, u) is equal to u. Additionally, the combiner function must be compatible with the accumulator function; for all u and t, the following must hold:

combiner.apply(u, accumulator.apply(identity, t)) == accumulator.apply(u, t)
 

This is a terminal operation.

API Note:
  • Many reductions using this form can be represented more simply by an explicit combination of map and reduce operations. The accumulator function acts as a fused mapper and accumulator, which can sometimes be more efficient than separate mapping and reduction, such as when knowing the previously reduced value allows you to avoid some computation.
Parameters
identity U: the identity value for the combiner function

accumulator BiFunction: an associative, non-interfering, stateless function for incorporating an additional element into a result

combiner BinaryOperator: an associative, non-interfering, stateless function for combining two values, which must be compatible with the accumulator function

Returns
U the result of the reduction

See also:

skip

added in API level 24
Stream<T> skip (long n)

Returns a stream consisting of the remaining elements of this stream after discarding the first n elements of the stream. If this stream contains fewer than n elements then an empty stream will be returned.

This is a stateful intermediate operation.

API Note:
  • While skip() is generally a cheap operation on sequential stream pipelines, it can be quite expensive on ordered parallel pipelines, especially for large values of n, since skip(n) is constrained to skip not just any n elements, but the first n elements in the encounter order. Using an unordered stream source (such as generate(Supplier)) or removing the ordering constraint with unordered() may result in significant speedups of skip() in parallel pipelines, if the semantics of your situation permit. If consistency with encounter order is required, and you are experiencing poor performance or memory utilization with skip() in parallel pipelines, switching to sequential execution with sequential() may improve performance.
Parameters
n long: the number of leading elements to skip

Returns
Stream<T> the new stream

Throws
IllegalArgumentException if n is negative

sorted

added in API level 24
Stream<T> sorted (Comparator<? super T> comparator)

Returns a stream consisting of the elements of this stream, sorted according to the provided Comparator.

For ordered streams, the sort is stable. For unordered streams, no stability guarantees are made.

This is a stateful intermediate operation.

Parameters
comparator Comparator: a non-interfering, stateless Comparator to be used to compare stream elements

Returns
Stream<T> the new stream

sorted

added in API level 24
Stream<T> sorted ()

Returns a stream consisting of the elements of this stream, sorted according to natural order. If the elements of this stream are not Comparable, a java.lang.ClassCastException may be thrown when the terminal operation is executed.

For ordered streams, the sort is stable. For unordered streams, no stability guarantees are made.

This is a stateful intermediate operation.

Returns
Stream<T> the new stream

toArray

added in API level 24
Object[] toArray ()

Returns an array containing the elements of this stream.

This is a terminal operation.

Returns
Object[] an array containing the elements of this stream

toArray

added in API level 24
A[] toArray (IntFunction<A[]> generator)

Returns an array containing the elements of this stream, using the provided generator function to allocate the returned array, as well as any additional arrays that might be required for a partitioned execution or for resizing.

This is a terminal operation.

API Note:
  • The generator function takes an integer, which is the size of the desired array, and produces an array of the desired size. This can be concisely expressed with an array constructor reference:
    Person[] men = people.stream()
                              .filter(p -> p.getGender() == MALE)
                              .toArray(Person[]::new);
     
Parameters
generator IntFunction: a function which produces a new array of the desired type and the provided length

Returns
A[] an array containing the elements in this stream

Throws
ArrayStoreException if the runtime type of the array returned from the array generator is not a supertype of the runtime type of every element in this stream
This site uses cookies to store your preferences for site-specific language and display options.

Get the latest Android developer news and tips that will help you find success on Google Play.

* Required Fields

Hooray!

Follow Google Developers on WeChat

Browse this site in ?

You requested a page in , but your language preference for this site is .

Would you like to change your language preference and browse this site in ? If you want to change your language preference later, use the language menu at the bottom of each page.

This class requires API level or higher

This doc is hidden because your selected API level for the documentation is . You can change the documentation API level with the selector above the left navigation.

For more information about specifying the API level your app requires, read Supporting Different Platform Versions.

Take a short survey?
Help us improve the Android developer experience.
(Sep 2017 survey)