Module java.base
Package java.util.stream

Interface DoubleStream

  • All Superinterfaces:
    AutoCloseable,BaseStream<Double,​DoubleStream>
    public interfaceDoubleStreamextendsBaseStream<Double,​DoubleStream>
    A sequence of primitive double-valued elements supporting sequential and parallel aggregate operations. This is thedouble primitive specialization ofStream.

    The following example illustrates an aggregate operation usingStream andDoubleStream, computing the sum of the weights of the red widgets:

         double sum = widgets.stream()                         .filter(w -> w.getColor() == RED)                         .mapToDouble(w -> w.getWeight())                         .sum();
    See the class documentation forStream and the package documentation forjava.util.stream for additional specification of streams, stream operations, stream pipelines, and parallelism.

    Since:
    1.8
    See Also:
    Stream,java.util.stream

    • Method Detail

      • mapToObj

        <U> Stream<U> mapToObj​(DoubleFunction<? extends U> mapper)
        Returns an object-valuedStream consisting of the results of applying the given function to the elements of this stream.

        This is an intermediate operation.

        Type Parameters:
        U - the element type of the new stream
        Parameters:
        mapper - anon-interfering,stateless function to apply to each element
        Returns:
        the new stream

      • flatMap

        DoubleStream flatMap​(DoubleFunction<? extendsDoubleStream> 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 isclosed after its contents have been placed into this stream. (If a mapped stream isnull an empty stream is used, instead.)

        This is anintermediate operation.

        Parameters:
        mapper - anon-interfering,stateless function to apply to each element which produces aDoubleStream of new values
        Returns:
        the new stream
        See Also:
        Stream.flatMap(Function)

      • peek

        DoubleStream peek​(DoubleConsumer 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 anintermediate 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:
             DoubleStream.of(1, 2, 3, 4)         .filter(e -> e > 2)         .peek(e -> System.out.println("Filtered value: " + e))         .map(e -> e * e)         .peek(e -> System.out.println("Mapped value: " + e))         .sum();

        In cases where the stream implementation is able to optimize away the production of some or all the elements (such as with short-circuiting operations likefindFirst, or in the example described incount()), the action will not be invoked for those elements.

        Parameters:
        action - a non-interfering action to perform on the elements as they are consumed from the stream
        Returns:
        the new stream

      • limit

        DoubleStream limit​(long maxSize)
        Returns a stream consisting of the elements of this stream, truncated to be no longer thanmaxSize in length.

        This is ashort-circuiting stateful intermediate operation.

        API Note:
        Whilelimit() is generally a cheap operation on sequential stream pipelines, it can be quite expensive on ordered parallel pipelines, especially for large values ofmaxSize, sincelimit(n) is constrained to return not just anyn elements, but thefirst n elements in the encounter order. Using an unordered stream source (such asgenerate(DoubleSupplier)) or removing the ordering constraint withBaseStream.unordered() may result in significant speedups oflimit() 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 withlimit() in parallel pipelines, switching to sequential execution withBaseStream.sequential() may improve performance.
        Parameters:
        maxSize - the number of elements the stream should be limited to
        Returns:
        the new stream
        Throws:
        IllegalArgumentException - ifmaxSize is negative

      • skip

        DoubleStream skip​(long n)
        Returns a stream consisting of the remaining elements of this stream after discarding the firstn elements of the stream. If this stream contains fewer thann elements then an empty stream will be returned.

        This is astateful intermediate operation.

        API Note:
        Whileskip() is generally a cheap operation on sequential stream pipelines, it can be quite expensive on ordered parallel pipelines, especially for large values ofn, sinceskip(n) is constrained to skip not just anyn elements, but thefirst n elements in the encounter order. Using an unordered stream source (such asgenerate(DoubleSupplier)) or removing the ordering constraint withBaseStream.unordered() may result in significant speedups ofskip() 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 withskip() in parallel pipelines, switching to sequential execution withBaseStream.sequential() may improve performance.
        Parameters:
        n - the number of leading elements to skip
        Returns:
        the new stream
        Throws:
        IllegalArgumentException - ifn is negative

      • takeWhile

        default DoubleStream takeWhile​(DoublePredicate predicate)
        Returns, if this stream is ordered, a stream consisting of the longest prefix of elements taken from this stream that match the given predicate. Otherwise returns, if this stream is unordered, a stream consisting of a subset of elements taken from this stream that match the given predicate.

        If this stream is ordered then the longest prefix is a contiguous sequence of elements of this stream that match the given predicate. The first element of the sequence is the first element of this stream, and the element immediately following the last element of the sequence does not match the given predicate.

        If this stream is unordered, and some (but not all) elements of this stream match the given predicate, then the behavior of this operation is nondeterministic; it is free to take any subset of matching elements (which includes the empty set).

        Independent of whether this stream is ordered or unordered if all elements of this stream match the given predicate then this operation takes all elements (the result is the same as the input), or if no elements of the stream match the given predicate then no elements are taken (the result is an empty stream).

        This is ashort-circuiting stateful intermediate operation.

        API Note:
        WhiletakeWhile() is generally a cheap operation on sequential stream pipelines, it can be quite expensive on ordered parallel pipelines, since the operation is constrained to return not just any valid prefix, but the longest prefix of elements in the encounter order. Using an unordered stream source (such asgenerate(DoubleSupplier)) or removing the ordering constraint withBaseStream.unordered() may result in significant speedups oftakeWhile() 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 withtakeWhile() in parallel pipelines, switching to sequential execution withBaseStream.sequential() may improve performance.
        Implementation Requirements:
        The default implementation obtains thespliterator of this stream, wraps that spliterator so as to support the semantics of this operation on traversal, and returns a new stream associated with the wrapped spliterator. The returned stream preserves the execution characteristics of this stream (namely parallel or sequential execution as perBaseStream.isParallel()) but the wrapped spliterator may choose to not support splitting. When the returned stream is closed, the close handlers for both the returned and this stream are invoked.
        Parameters:
        predicate - anon-interfering,stateless predicate to apply to elements to determine the longest prefix of elements.
        Returns:
        the new stream
        Since:
        9

      • dropWhile

        default DoubleStream dropWhile​(DoublePredicate predicate)
        Returns, if this stream is ordered, a stream consisting of the remaining elements of this stream after dropping the longest prefix of elements that match the given predicate. Otherwise returns, if this stream is unordered, a stream consisting of the remaining elements of this stream after dropping a subset of elements that match the given predicate.

        If this stream is ordered then the longest prefix is a contiguous sequence of elements of this stream that match the given predicate. The first element of the sequence is the first element of this stream, and the element immediately following the last element of the sequence does not match the given predicate.

        If this stream is unordered, and some (but not all) elements of this stream match the given predicate, then the behavior of this operation is nondeterministic; it is free to drop any subset of matching elements (which includes the empty set).

        Independent of whether this stream is ordered or unordered if all elements of this stream match the given predicate then this operation drops all elements (the result is an empty stream), or if no elements of the stream match the given predicate then no elements are dropped (the result is the same as the input).

        This is astateful intermediate operation.

        API Note:
        WhiledropWhile() is generally a cheap operation on sequential stream pipelines, it can be quite expensive on ordered parallel pipelines, since the operation is constrained to return not just any valid prefix, but the longest prefix of elements in the encounter order. Using an unordered stream source (such asgenerate(DoubleSupplier)) or removing the ordering constraint withBaseStream.unordered() may result in significant speedups ofdropWhile() 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 withdropWhile() in parallel pipelines, switching to sequential execution withBaseStream.sequential() may improve performance.
        Implementation Requirements:
        The default implementation obtains thespliterator of this stream, wraps that spliterator so as to support the semantics of this operation on traversal, and returns a new stream associated with the wrapped spliterator. The returned stream preserves the execution characteristics of this stream (namely parallel or sequential execution as perBaseStream.isParallel()) but the wrapped spliterator may choose to not support splitting. When the returned stream is closed, the close handlers for both the returned and this stream are invoked.
        Parameters:
        predicate - anon-interfering,stateless predicate to apply to elements to determine the longest prefix of elements.
        Returns:
        the new stream
        Since:
        9

      • forEach

        void forEach​(DoubleConsumer action)
        Performs an action for each element of this stream.

        This is aterminal operation.

        For parallel stream pipelines, this operation doesnot 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 - a non-interfering action to perform on the elements

      • forEachOrdered

        void forEachOrdered​(DoubleConsumer action)
        Performs an action for each element of this stream, guaranteeing that each element is processed in encounter order for streams that have a defined encounter order.

        This is aterminal operation.

        Parameters:
        action - a non-interfering action to perform on the elements
        See Also:
        forEach(DoubleConsumer)

      • toArray

        double[] toArray()
        Returns an array containing the elements of this stream.

        This is aterminal operation.

        Returns:
        an array containing the elements of this stream

      • reduce

        double reduce​(double identity,DoubleBinaryOperator op)
        Performs areduction on the elements of this stream, using the provided identity value and anassociative accumulation function, and returns the reduced value. This is equivalent to:
             double result = identity;     for (double element : this stream)         result = accumulator.applyAsDouble(result, element)     return result;
        but is not constrained to execute sequentially.

        Theidentity value must be an identity for the accumulator function. This means that for allx,accumulator.apply(identity, x) is equal tox. Theaccumulator function must be anassociative function.

        This is aterminal operation.

        API Note:
        Sum, min, max, and average are all special cases of reduction. Summing a stream of numbers can be expressed as:
             double sum = numbers.reduce(0, (a, b) -> a+b);
        or more compactly:
             double sum = numbers.reduce(0, Double::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 - the identity value for the accumulating function
        op - anassociative,non-interfering,stateless function for combining two values
        Returns:
        the result of the reduction
        See Also:
        sum(),min(),max(),average()

      • reduce

        OptionalDouble reduce​(DoubleBinaryOperator op)
        Performs areduction on the elements of this stream, using anassociative accumulation function, and returns anOptionalDouble describing the reduced value, if any. This is equivalent to:
             boolean foundAny = false;     double result = null;     for (double element : this stream) {         if (!foundAny) {             foundAny = true;             result = element;         }         else             result = accumulator.applyAsDouble(result, element);     }     return foundAny ? OptionalDouble.of(result) : OptionalDouble.empty();
        but is not constrained to execute sequentially.

        Theaccumulator function must be anassociative function.

        This is aterminal operation.

        Parameters:
        op - anassociative,non-interfering,stateless function for combining two values
        Returns:
        the result of the reduction
        See Also:
        reduce(double, DoubleBinaryOperator)

      • collect

        <R> R collect​(Supplier<R> supplier,ObjDoubleConsumer<R> accumulator,BiConsumer<R,​R> combiner)
        Performs amutable 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 anArrayList, 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 (double element : this stream)         accumulator.accept(result, element);     return result;

        Likereduce(double, DoubleBinaryOperator),collect operations can be parallelized without requiring additional synchronization.

        This is aterminal operation.

        Type Parameters:
        R - the type of the mutable result container
        Parameters:
        supplier - a function that creates a new mutable result container. For a parallel execution, this function may be called multiple times and must return a fresh value each time.
        accumulator - anassociative,non-interfering,stateless function that must fold an element into a result container.
        combiner - anassociative,non-interfering,stateless function that accepts two partial result containers and merges them, which must be compatible with the accumulator function. The combiner function must fold the elements from the second result container into the first result container.
        Returns:
        the result of the reduction
        See Also:
        Stream.collect(Supplier, BiConsumer, BiConsumer)

      • sum

        double sum()
        Returns the sum of elements in this stream. Summation is a special case of areduction. If floating-point summation were exact, this method would be equivalent to:
             return reduce(0, Double::sum);
        However, since floating-point summation is not exact, the above code is not necessarily equivalent to the summation computation done by this method.

        The value of a floating-point sum is a function both of the input values as well as the order of addition operations. The order of addition operations of this method is intentionally not defined to allow for implementation flexibility to improve the speed and accuracy of the computed result. In particular, this method may be implemented using compensated summation or other technique to reduce the error bound in the numerical sum compared to a simple summation ofdouble values. Because of the unspecified order of operations and the possibility of using differing summation schemes, the output of this method may vary on the same input elements.

        Various conditions can result in a non-finite sum being computed. This can occur even if the all the elements being summed are finite. If any element is non-finite, the sum will be non-finite:

        • If any element is a NaN, then the final sum will be NaN.
        • If the elements contain one or more infinities, the sum will be infinite or NaN.
          • If the elements contain infinities of opposite sign, the sum will be NaN.
          • If the elements contain infinities of one sign and an intermediate sum overflows to an infinity of the opposite sign, the sum may be NaN.
        It is possible for intermediate sums of finite values to overflow into opposite-signed infinities; if that occurs, the final sum will be NaN even if the elements are all finite. If all the elements are zero, the sign of zero isnot guaranteed to be preserved in the final sum.

        This is aterminal operation.

        API Note:
        Elements sorted by increasing absolute magnitude tend to yield more accurate results.
        Returns:
        the sum of elements in this stream

      • min

        OptionalDouble min()
        Returns anOptionalDouble describing the minimum element of this stream, or an empty OptionalDouble if this stream is empty. The minimum element will beDouble.NaN if any stream element was NaN. Unlike the numerical comparison operators, this method considers negative zero to be strictly smaller than positive zero. This is a special case of areduction and is equivalent to:
             return reduce(Double::min);

        This is aterminal operation.

        Returns:
        anOptionalDouble containing the minimum element of this stream, or an empty optional if the stream is empty

      • max

        OptionalDouble max()
        Returns anOptionalDouble describing the maximum element of this stream, or an empty OptionalDouble if this stream is empty. The maximum element will beDouble.NaN if any stream element was NaN. Unlike the numerical comparison operators, this method considers negative zero to be strictly smaller than positive zero. This is a special case of areduction and is equivalent to:
             return reduce(Double::max);

        This is aterminal operation.

        Returns:
        anOptionalDouble containing the maximum element of this stream, or an empty optional if the stream is empty

      • count

        long count()
        Returns the count of elements in this stream. This is a special case of areduction and is equivalent to:
             return mapToLong(e -> 1L).sum();

        This is aterminal operation.

        API Note:
        An implementation may choose to not execute the stream pipeline (either sequentially or in parallel) if it is capable of computing the count directly from the stream source. In such cases no source elements will be traversed and no intermediate operations will be evaluated. Behavioral parameters with side-effects, which are strongly discouraged except for harmless cases such as debugging, may be affected. For example, consider the following stream:
             DoubleStream s = DoubleStream.of(1, 2, 3, 4);     long count = s.peek(System.out::println).count();
        The number of elements covered by the stream source is known and the intermediate operation,peek, does not inject into or remove elements from the stream (as may be the case forflatMap orfilter operations). Thus the count is 4 and there is no need to execute the pipeline and, as a side-effect, print out the elements.
        Returns:
        the count of elements in this stream

      • average

        OptionalDouble average()
        Returns anOptionalDouble describing the arithmetic mean of elements of this stream, or an empty optional if this stream is empty.

        The computed average can vary numerically and have the special case behavior as computing the sum; seesum() for details.

        The average is a special case of areduction.

        This is aterminal operation.

        API Note:
        Elements sorted by increasing absolute magnitude tend to yield more accurate results.
        Returns:
        anOptionalDouble containing the average element of this stream, or an empty optional if the stream is empty

      • summaryStatistics

        DoubleSummaryStatistics summaryStatistics()
        Returns aDoubleSummaryStatistics describing various summary data about the elements of this stream. This is a special case of areduction.

        This is aterminal operation.

        Returns:
        aDoubleSummaryStatistics describing various summary data about the elements of this stream

      • anyMatch

        boolean anyMatch​(DoublePredicate 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 thenfalse is returned and the predicate is not evaluated.

        This is ashort-circuiting terminal operation.

        API Note:
        This method evaluates theexistential quantification of the predicate over the elements of the stream (for some x P(x)).
        Parameters:
        predicate - anon-interfering,stateless predicate to apply to elements of this stream
        Returns:
        true if any elements of the stream match the provided predicate, otherwisefalse

      • allMatch

        boolean allMatch​(DoublePredicate 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 thentrue is returned and the predicate is not evaluated.

        This is ashort-circuiting terminal operation.

        API Note:
        This method evaluates theuniversal 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 bevacuously satisfied and is alwaystrue (regardless of P(x)).
        Parameters:
        predicate - anon-interfering,stateless predicate to apply to elements of this stream
        Returns:
        true if either all elements of the stream match the provided predicate or the stream is empty, otherwisefalse

      • noneMatch

        boolean noneMatch​(DoublePredicate 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 thentrue is returned and the predicate is not evaluated.

        This is ashort-circuiting terminal operation.

        API Note:
        This method evaluates theuniversal 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 alwaystrue, regardless of P(x).
        Parameters:
        predicate - anon-interfering,stateless predicate to apply to elements of this stream
        Returns:
        true if either no elements of the stream match the provided predicate or the stream is empty, otherwisefalse

      • findFirst

        OptionalDouble findFirst()
        Returns anOptionalDouble describing the first element of this stream, or an emptyOptionalDouble if the stream is empty. If the stream has no encounter order, then any element may be returned.

        This is ashort-circuiting terminal operation.

        Returns:
        anOptionalDouble describing the first element of this stream, or an emptyOptionalDouble if the stream is empty

      • findAny

        OptionalDouble findAny()
        Returns anOptionalDouble describing some element of the stream, or an emptyOptionalDouble if the stream is empty.

        This is ashort-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, usefindFirst() instead.)

        Returns:
        anOptionalDouble describing some element of this stream, or an emptyOptionalDouble if the stream is empty
        See Also:
        findFirst()

      • boxed

        Stream<Double> boxed()
        Returns aStream consisting of the elements of this stream, boxed toDouble.

        This is anintermediate operation.

        Returns:
        aStream consistent of the elements of this stream, each boxed to aDouble

      • builder

        static DoubleStream.Builder builder()
        Returns a builder for aDoubleStream.
        Returns:
        a stream builder

      • empty

        static DoubleStream empty()
        Returns an empty sequentialDoubleStream.
        Returns:
        an empty sequential stream

      • of

        static DoubleStream of​(double t)
        Returns a sequentialDoubleStream containing a single element.
        Parameters:
        t - the single element
        Returns:
        a singleton sequential stream

      • of

        static DoubleStream of​(double... values)
        Returns a sequential ordered stream whose elements are the specified values.
        Parameters:
        values - the elements of the new stream
        Returns:
        the new stream

      • iterate

        static DoubleStream iterate​(double seed,DoubleUnaryOperator f)
        Returns an infinite sequential orderedDoubleStream produced by iterative application of a functionf to an initial elementseed, producing aStream consisting ofseed,f(seed),f(f(seed)), etc.

        The first element (position0) in theDoubleStream will be the providedseed. Forn > 0, the element at positionn, will be the result of applying the functionf to the element at positionn - 1.

        The action of applyingf for one elementhappens-before the action of applyingf for subsequent elements. For any given element the action may be performed in whatever thread the library chooses.

        Parameters:
        seed - the initial element
        f - a function to be applied to the previous element to produce a new element
        Returns:
        a new sequentialDoubleStream

      • iterate

        static DoubleStream iterate​(double seed,DoublePredicate hasNext,DoubleUnaryOperator next)
        Returns a sequential orderedDoubleStream produced by iterative application of the givennext function to an initial element, conditioned on satisfying the givenhasNext predicate. The stream terminates as soon as thehasNext predicate returns false.

        DoubleStream.iterate should produce the same sequence of elements as produced by the corresponding for-loop:

             for (double index=seed; hasNext.test(index); index = next.applyAsDouble(index)) {         ...     }

        The resulting sequence may be empty if thehasNext predicate does not hold on the seed value. Otherwise the first element will be the suppliedseed value, the next element (if present) will be the result of applying thenext function to theseed value, and so on iteratively until thehasNext predicate indicates that the stream should terminate.

        The action of applying thehasNext predicate to an elementhappens-before the action of applying thenext function to that element. The action of applying thenext function for one elementhappens-before the action of applying thehasNext predicate for subsequent elements. For any given element an action may be performed in whatever thread the library chooses.

        Parameters:
        seed - the initial element
        hasNext - a predicate to apply to elements to determine when the stream must terminate.
        next - a function to be applied to the previous element to produce a new element
        Returns:
        a new sequentialDoubleStream
        Since:
        9

      • generate

        static DoubleStream generate​(DoubleSupplier s)
        Returns an infinite sequential unordered stream where each element is generated by the providedDoubleSupplier. This is suitable for generating constant streams, streams of random elements, etc.
        Parameters:
        s - theDoubleSupplier for generated elements
        Returns:
        a new infinite sequential unorderedDoubleStream

      • concat

        static DoubleStream concat​(DoubleStream a,DoubleStream 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.

        This method operates on the two input streams and binds each stream to its source. As a result subsequent modifications to an input stream source may not be reflected in the concatenated stream result.

        API Note:
        To preserve optimization opportunities this method binds each stream to its source and accepts only two streams as parameters. For example, the exact size of the concatenated stream source can be computed if the exact size of each input stream source is known. To concatenate more streams without binding, or without nested calls to this method, try creating a stream of streams and flat-mapping with the identity function, for example:
             DoubleStream concat = Stream.of(s1, s2, s3, s4).flatMapToDouble(s -> s);
        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 evenStackOverflowError.
        Parameters:
        a - the first stream
        b - the second stream
        Returns:
        the concatenation of the two input streams