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Java Standard Edition (SE)

DoubleStream


compact1, compact2, compact3
java.util.stream

Interface DoubleStream

  • All Superinterfaces:
    AutoCloseable, BaseStream<Double,DoubleStream>


    public interface DoubleStream
    extends BaseStream<Double,DoubleStream>
    A sequence of primitive double-valued elements supporting sequential and parallel aggregate operations. This is the double primitive specialization of Stream.

    The following example illustrates an aggregate operation using Stream and DoubleStream, 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 for Stream and the package documentation for java.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-valued Stream 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 - a non-interfering, stateless function to apply to each element
        Returns:
        the new stream
      • flatMap

        DoubleStream flatMap(DoubleFunction<? extends DoubleStream> 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.

        Parameters:
        mapper - a non-interfering, stateless function to apply to each element which produces a DoubleStream 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 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:
        
             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();
         
        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 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(DoubleSupplier)) or removing the ordering constraint with BaseStream.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 - the number of elements the stream should be limited to
        Returns:
        the new stream
        Throws:
        IllegalArgumentException - if maxSize is negative
      • skip

        DoubleStream 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(DoubleSupplier)) or removing the ordering constraint with BaseStream.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 - the number of leading elements to skip
        Returns:
        the new stream
        Throws:
        IllegalArgumentException - if n is negative
      • forEach

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

        This is a terminal operation.

        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 - 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 a terminal 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 a terminal operation.

        Returns:
        an array containing the elements of this stream
      • reduce

        double reduce(double identity,
                      DoubleBinaryOperator op)
        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:
        
             double result = identity;
             for (double element : this stream)
                 result = accumulator.applyAsDouble(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 x, accumulator.apply(identity, x) is equal to x. The accumulator function must be an associative function.

        This is a terminal 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 - an associative, 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 a reduction on the elements of this stream, using an associative accumulation function, and returns an OptionalDouble 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.

        The accumulator function must be an associative function.

        This is a terminal operation.

        Parameters:
        op - an associative, 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 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 (double element : this stream)
                 accumulator.accept(result, element);
             return result;
         

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

        This is a terminal operation.

        Type Parameters:
        R - type of the result
        Parameters:
        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 - an associative, non-interfering, stateless function for incorporating an additional element into a result
        combiner - an associative, non-interfering, stateless function for combining two values, which must be compatible with the accumulator function
        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 a reduction. 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.

        If any stream element is a NaN or the sum is at any point a NaN then the sum will be NaN. 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 of double values.

        This is a terminal 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 an OptionalDouble describing the minimum element of this stream, or an empty OptionalDouble if this stream is empty. The minimum element will be Double.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 a reduction and is equivalent to:
        
             return reduce(Double::min);
         

        This is a terminal operation.

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

        OptionalDouble max()
        Returns an OptionalDouble describing the maximum element of this stream, or an empty OptionalDouble if this stream is empty. The maximum element will be Double.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 a reduction and is equivalent to:
        
             return reduce(Double::max);
         

        This is a terminal operation.

        Returns:
        an OptionalDouble 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 a reduction and is equivalent to:
        
             return mapToLong(e -> 1L).sum();
         

        This is a terminal operation.

        Returns:
        the count of elements in this stream
      • average

        OptionalDouble average()
        Returns an OptionalDouble describing the arithmetic mean of elements of this stream, or an empty optional if this stream is empty. If any recorded value is a NaN or the sum is at any point a NaN then the average will be NaN.

        The average returned can vary depending upon the order in which values are recorded. This method may be implemented using compensated summation or other technique to reduce the error bound in the numerical sum used to compute the average.

        The average is a special case of a reduction.

        This is a terminal operation.

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

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

        This is a terminal operation.

        Returns:
        a DoubleSummaryStatistics 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 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 - a non-interfering, stateless predicate to apply to elements of this stream
        Returns:
        true if any elements of the stream match the provided predicate, otherwise false
      • 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 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 - a non-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, otherwise false
      • 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 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 - a non-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, otherwise false
      • findFirst

        OptionalDouble findFirst()
        Returns an OptionalDouble describing the first element of this stream, or an empty OptionalDouble 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:
        an OptionalDouble describing the first element of this stream, or an empty OptionalDouble if the stream is empty
      • findAny

        OptionalDouble findAny()
        Returns an OptionalDouble describing some element of the stream, or an empty OptionalDouble 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:
        an OptionalDouble describing some element of this stream, or an empty OptionalDouble if the stream is empty
        See Also:
        findFirst()
      • boxed

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

        This is an intermediate operation.

        Returns:
        a Stream consistent of the elements of this stream, each boxed to a Double
      • sequential

        DoubleStream sequential()
        Description copied from interface: BaseStream
        Returns an equivalent stream that is sequential. May return itself, either because the stream was already sequential, or because the underlying stream state was modified to be sequential.

        This is an intermediate operation.

        Specified by:
        sequential in interface BaseStream<Double,DoubleStream>
        Returns:
        a sequential stream
      • builder

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

        static DoubleStream empty()
        Returns an empty sequential DoubleStream.
        Returns:
        an empty sequential stream
      • of

        static DoubleStream of(double t)
        Returns a sequential DoubleStream 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 ordered DoubleStream 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 DoubleStream 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 - the initial element
        f - a function to be applied to to the previous element to produce a new element
        Returns:
        a new sequential DoubleStream
      • generate

        static DoubleStream generate(DoubleSupplier s)
        Returns an infinite sequential unordered stream where each element is generated by the provided DoubleSupplier. This is suitable for generating constant streams, streams of random elements, etc.
        Parameters:
        s - the DoubleSupplier for generated elements
        Returns:
        a new infinite sequential unordered DoubleStream
      • 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.
        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 - the first stream
        b - the second stream
        Returns:
        the concatenation of the two input streams


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