java.util.stream

Interface Stream<T>

  • Type Parameters:
    T - the type of the stream elements
    All Superinterfaces:
    AutoCloseable, BaseStream<T,Stream<T>> 

    public interface Stream<T>
extends BaseStream<T,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 BaseStream.iterator() and BaseStream.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 BaseStream.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 (such as those returned by Files.lines(Path, Charset)) 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 BaseStream.sequential() or BaseStream.parallel() methods, and may be queried with the BaseStream.isParallel() method.

    Since:
    1.8
    See Also:
    IntStream, LongStream, DoubleStream, java.util.stream

    • Nested Class Summary

      Nested Classes

      Modifier and Type Interface and Description
      static interface  Stream.Builder<T>
      A mutable builder for a Stream.

    • Method Summary

      Modifier and Type Method and Description
      boolean allMatch(Predicate<? super T> predicate)
      Returns whether all elements of this stream match the provided predicate.
      boolean anyMatch(Predicate<? super T> predicate)
      Returns whether any elements of this stream match the provided predicate.
      static <T> Stream.Builder<T> builder()
      Returns a builder for a Stream.
      <R,A> R collect(Collector<? super T,A,R> collector)
      Performs a mutable reduction operation on the elements of this stream using a Collector.
      <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.
      long count()
      Returns the count of elements in this stream.
      Stream<T> distinct()
      Returns a stream consisting of the distinct elements (according to Object.equals(Object)) of this stream.
      static <T> Stream<T> empty()
      Returns an empty sequential Stream.
      Stream<T> filter(Predicate<? super T> predicate)
      Returns a stream consisting of the elements of this stream that match the given predicate.
      Optional<T> findAny()
      Returns an Optional describing some element of the stream, or an empty Optional if the stream is empty.
      Optional<T> findFirst()
      Returns an Optional describing the first element of this stream, or an empty Optional if the stream is empty.
      <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.
      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.
      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.
      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.
      void forEach(Consumer<? super T> action)
      Performs an action for each element of this stream.
      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.
      Stream<T> limit(long maxSize)
      Returns a stream consisting of the elements of this stream, truncated to be no longer than maxSize in length.
      <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.
      DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper)
      Returns a DoubleStream consisting of the results of applying the given function to the elements of this stream.
      IntStream mapToInt(ToIntFunction<? super T> mapper)
      Returns an IntStream consisting of the results of applying the given function to the elements of this stream.
      LongStream mapToLong(ToLongFunction<? super T> mapper)
      Returns a LongStream consisting of the results of applying the given function to the elements of this stream.
      Optional<T> max(Comparator<? super T> comparator)
      Returns the maximum element of this stream according to the provided Comparator.
      Optional<T> min(Comparator<? super T> comparator)
      Returns the minimum element of this stream according to the provided Comparator.
      boolean noneMatch(Predicate<? super T> predicate)
      Returns whether no elements of this stream match the provided predicate.
      static <T> Stream<T> of(T... values)
      Returns a sequential ordered stream whose elements are the specified values.
      static <T> Stream<T> of(T t)
      Returns a sequential Stream containing a single element.
      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.
      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.
      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.
      <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.
      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.
      Stream<T> sorted()
      Returns a stream consisting of the elements of this stream, sorted according to natural order.
      Stream<T> sorted(Comparator<? super T> comparator)
      Returns a stream consisting of the elements of this stream, sorted according to the provided Comparator.
      Object[] toArray()
      Returns an array containing the elements of this stream.
      <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.

    • Method Detail

      • filter

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

      • map

        <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.

        This is an intermediate operation.

        Type Parameters:
        R - The element type of the new stream
        Parameters:
        mapper - a non-interfering, stateless function to apply to each element
        Returns:
        the new stream

      • flatMap

        <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. 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.

        Type Parameters:
        R - The element type of the new stream
        Parameters:
        mapper - a non-interfering, stateless function to apply to each element which produces a stream of new values
        Returns:
        the new stream

      • flatMapToInt

        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 - a non-interfering, stateless function to apply to each element which produces a stream of new values
        Returns:
        the new stream
        See Also:
        flatMap(Function)

      • flatMapToLong

        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 - a non-interfering, stateless function to apply to each element which produces a stream of new values
        Returns:
        the new stream
        See Also:
        flatMap(Function)

      • flatMapToDouble

        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 - a non-interfering, stateless function to apply to each element which produces a stream of new values
        Returns:
        the new stream
        See Also:
        flatMap(Function)

      • distinct

        Stream<T> distinct()
        Returns a stream consisting of the distinct elements (according to Object.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.

        Returns:
        the new stream

      • sorted

        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:
        the new stream

      • sorted

        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 - a non-interfering, stateless Comparator to be used to compare stream elements
        Returns:
        the new stream

      • peek

        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.

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

      • skip

        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.

        Parameters:
        n - the number of leading elements to skip
        Returns:
        the new stream
        Throws:
        IllegalArgumentException - if n is negative

      • forEach

        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 - a non-interfering action to perform on the elements

      • forEachOrdered

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

      • toArray

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

        This is a terminal operation.

        Returns:
        an array containing the elements of this stream

      • toArray

        <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.

        This is a terminal operation.

        Type Parameters:
        A - the element type of the resulting array
        Parameters:
        generator - a function which produces a new array of the desired type and the provided length
        Returns:
        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

      • reduce

        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.

        Parameters:
        identity - the identity value for the accumulating function
        accumulator - an associative, non-interfering, stateless function for combining two values
        Returns:
        the result of the reduction

      • reduce

        <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. 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.

        Type Parameters:
        U - The type of the result
        Parameters:
        identity - the identity value for the combiner function
        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:
        reduce(BinaryOperator), reduce(Object, BinaryOperator)

      • collect

        <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. 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.

        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

      • collect

        <R,A> 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.

        Type Parameters:
        R - the type of the result
        A - the intermediate accumulation type of the Collector
        Parameters:
        collector - the Collector describing the reduction
        Returns:
        the result of the reduction
        See Also:
        collect(Supplier, BiConsumer, BiConsumer), Collectors

      • min

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

      • max

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

      • 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

      • anyMatch

        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.

        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(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.

        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(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.

        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

        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:
        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

      • findAny

        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:
        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()

      • builder

        static <T> Stream.Builder<T> builder()
        Returns a builder for a Stream.
        Type Parameters:
        T - type of elements
        Returns:
        a stream builder

      • empty

        static <T> Stream<T> empty()
        Returns an empty sequential Stream.
        Type Parameters:
        T - the type of stream elements
        Returns:
        an empty sequential stream

      • of

        static <T> Stream<T> of(T t)
        Returns a sequential Stream containing a single element.
        Type Parameters:
        T - the type of stream elements
        Parameters:
        t - the single element
        Returns:
        a singleton sequential stream

      • of

        @SafeVarargs
static <T> Stream<T> of(T... values)
        Returns a sequential ordered stream whose elements are the specified values.
        Type Parameters:
        T - the type of stream elements
        Parameters:
        values - the elements of the new stream
        Returns:
        the new stream

      • iterate

        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.

        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.

        Type Parameters:
        T - the type of stream elements
        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 Stream

      • generate

        static <T> 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.
        Type Parameters:
        T - the type of stream elements
        Parameters:
        s - the Supplier of generated elements
        Returns:
        a new infinite sequential unordered Stream

      • concat

        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. 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.
        Type Parameters:
        T - The type of stream elements
        Parameters:
        a - the first stream
        b - the second stream
        Returns:
        the concatenation of the two input streams