DoubleStream (Java Platform SE 8 )
Keywords: java.util.stream.DoubleStream interface
Date: 2015-11-18

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

- - ### Nested Class Summary

    Nested Classes
    Modifier and Type | Interface and Description
    --- | ---
    `static interface` | `DoubleStream.Builder` A mutable builder for a `DoubleStream`.

  - ### Method Summary

    All Methods Static Methods Instance Methods Abstract Methods Default Methods
    Modifier and Type | Method and Description
    --- | ---
    `boolean` | `allMatch(DoublePredicate predicate)` Returns whether all elements of this stream match the provided predicate.
    `boolean` | `anyMatch(DoublePredicate predicate)` Returns whether any elements of this stream match the provided predicate.
    `OptionalDouble` | `average()` Returns an `OptionalDouble` describing the arithmetic mean of elements of this stream, or an empty optional if this stream is empty.
    `Stream<Double>` | `boxed()` Returns a `Stream` consisting of the elements of this stream, boxed to `Double`.
    `static DoubleStream.Builder` | `builder()` Returns a builder for a `DoubleStream`.
    `<R> R` | `collect(Supplier<R> supplier, ObjDoubleConsumer<R> accumulator, BiConsumer<R,R> combiner)` Performs a mutable reduction operation on the elements of this stream.
    `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.
    `long` | `count()` Returns the count of elements in this stream.
    `DoubleStream` | `distinct()` Returns a stream consisting of the distinct elements of this stream.
    `static DoubleStream` | `empty()` Returns an empty sequential `DoubleStream`.
    `DoubleStream` | `filter(DoublePredicate predicate)` Returns a stream consisting of the elements of this stream that match the given predicate.
    `OptionalDouble` | `findAny()` Returns an `OptionalDouble` describing some element of the stream, or an empty `OptionalDouble` if the stream is empty.
    `OptionalDouble` | `findFirst()` Returns an `OptionalDouble` describing the first element of this stream, or an empty `OptionalDouble` if the stream is empty.
    `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.
    `void` | `forEach(DoubleConsumer action)` Performs an action for each element of this stream.
    `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.
    `static DoubleStream` | `generate(DoubleSupplier s)` Returns an infinite sequential unordered stream where each element is generated by the provided `DoubleSupplier`.
    `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.
    `PrimitiveIterator.OfDouble` | `iterator()` Returns an iterator for the elements of this stream.
    `DoubleStream` | `limit(long maxSize)` Returns a stream consisting of the elements of this stream, truncated to be no longer than `maxSize` in length.
    `DoubleStream` | `map(DoubleUnaryOperator mapper)` Returns a stream consisting of the results of applying the given function to the elements of this stream.
    `IntStream` | `mapToInt(DoubleToIntFunction mapper)` Returns an `IntStream` consisting of the results of applying the given function to the elements of this stream.
    `LongStream` | `mapToLong(DoubleToLongFunction mapper)` Returns a `LongStream` consisting of the results of applying the given function to the elements of this stream.
    `<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.
    `OptionalDouble` | `max()` Returns an `OptionalDouble` describing the maximum element of this stream, or an empty OptionalDouble if this stream is empty.
    `OptionalDouble` | `min()` Returns an `OptionalDouble` describing the minimum element of this stream, or an empty OptionalDouble if this stream is empty.
    `boolean` | `noneMatch(DoublePredicate predicate)` Returns whether no elements of this stream match the provided predicate.
    `static DoubleStream` | `of(double... values)` Returns a sequential ordered stream whose elements are the specified values.
    `static DoubleStream` | `of(double t)` Returns a sequential `DoubleStream` containing a single element.
    `DoubleStream` | `parallel()` Returns an equivalent stream that is parallel.
    `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.
    `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.
    `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.
    `DoubleStream` | `sequential()` Returns an equivalent stream that is sequential.
    `DoubleStream` | `skip(long n)` Returns a stream consisting of the remaining elements of this stream after discarding the first `n` elements of the stream.
    `DoubleStream` | `sorted()` Returns a stream consisting of the elements of this stream in sorted order.
    `Spliterator.OfDouble` | `spliterator()` Returns a spliterator for the elements of this stream.
    `double` | `sum()` Returns the sum of elements in this stream.
    `DoubleSummaryStatistics` | `summaryStatistics()` Returns a `DoubleSummaryStatistics` describing various summary data about the elements of this stream.
    `double[]` | `toArray()` Returns an array containing the elements of this stream.

    - ### Methods inherited from interface java.util.stream.BaseStream

      `close, isParallel, onClose, unordered`

- - ### Method Detail

    - #### filter

          DoubleStream filter(DoublePredicate predicate)

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

      This is an intermediate operation.

      Parameters:
        `predicate` - a non-interfering, stateless predicate to apply to each element to determine if it should be included
      Returns:
        the new stream

    - #### map

          DoubleStream map(DoubleUnaryOperator 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` - a non-interfering, stateless function to apply to each element
      Returns:
        the new stream

    - #### 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

    - #### mapToInt

          IntStream mapToInt(DoubleToIntFunction 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` - a non-interfering, stateless function to apply to each element
      Returns:
        the new stream

    - #### mapToLong

          LongStream mapToLong(DoubleToLongFunction 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` - 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)`

    - #### distinct

          DoubleStream distinct()

      Returns a stream consisting of the distinct elements of this stream. The elements are compared for equality according to `Double.compare(double, double)`.

      This is a stateful intermediate operation.

      Returns:
        the result stream

    - #### sorted

          DoubleStream sorted()

      Returns a stream consisting of the elements of this stream in sorted order. The elements are compared for equality according to `Double.compare(double, double)`.

      This is a stateful intermediate operation.

      Returns:
        the result stream

    - #### 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

    - #### parallel

          DoubleStream parallel()

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

      This is an intermediate operation.

      Specified by:
        `parallel` in interface `BaseStream<Double,DoubleStream>`
      Returns:
        a parallel stream

    - #### iterator

          PrimitiveIterator.OfDouble iterator()

      Description copied from interface: `BaseStream`
      Returns an iterator for the elements of this stream.

      This is a terminal operation.

      Specified by:
        `iterator` in interface `BaseStream<Double,DoubleStream>`
      Returns:
        the element iterator for this stream

    - #### spliterator

          Spliterator.OfDouble spliterator()

      Description copied from interface: `BaseStream`
      Returns a spliterator for the elements of this stream.

      This is a terminal operation.

      Specified by:
        `spliterator` in interface `BaseStream<Double,DoubleStream>`
      Returns:
        the element spliterator for this 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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