Packages

  • package root

    This is the documentation for the Scala standard library.

    This is the documentation for the Scala standard library.

    Package structure

    The scala package contains core types like Int, Float, Array or Option which are accessible in all Scala compilation units without explicit qualification or imports.

    Notable packages include:

    Other packages exist. See the complete list on the right.

    Additional parts of the standard library are shipped as separate libraries. These include:

    • scala.reflect - Scala's reflection API (scala-reflect.jar)
    • scala.xml - XML parsing, manipulation, and serialization (scala-xml.jar)
    • scala.swing - A convenient wrapper around Java's GUI framework called Swing (scala-swing.jar)
    • scala.util.parsing - Parser combinators (scala-parser-combinators.jar)

    Automatic imports

    Identifiers in the scala package and the scala.Predef object are always in scope by default.

    Some of these identifiers are type aliases provided as shortcuts to commonly used classes. For example, List is an alias for scala.collection.immutable.List.

    Other aliases refer to classes provided by the underlying platform. For example, on the JVM, String is an alias for java.lang.String.

    Definition Classes
    root
  • package scala

    Core Scala types.

    Core Scala types. They are always available without an explicit import.

    Definition Classes
    root
  • package concurrent

    This package object contains primitives for concurrent and parallel programming.

    This package object contains primitives for concurrent and parallel programming.

    Guide

    A more detailed guide to Futures and Promises, including discussion and examples can be found at http://docs.scala-lang.org/overviews/core/futures.html.

    Common Imports

    When working with Futures, you will often find that importing the whole concurrent package is convenient:

    import scala.concurrent._

    When using things like Futures, it is often required to have an implicit ExecutionContext in scope. The general advice for these implicits are as follows.

    If the code in question is a class or method definition, and no ExecutionContext is available, request one from the caller by adding an implicit parameter list:

    def myMethod(myParam: MyType)(implicit ec: ExecutionContext) = …
    //Or
    class MyClass(myParam: MyType)(implicit ec: ExecutionContext) { … }

    This allows the caller of the method, or creator of the instance of the class, to decide which ExecutionContext should be used.

    For typical REPL usage and experimentation, importing the global ExecutionContext is often desired.

    import scala.concurrent.ExcutionContext.Implicits.global

    Specifying Durations

    Operations often require a duration to be specified. A duration DSL is available to make defining these easier:

    import scala.concurrent.duration._
    val d: Duration = 10.seconds

    Using Futures For Non-blocking Computation

    Basic use of futures is easy with the factory method on Future, which executes a provided function asynchronously, handing you back a future result of that function without blocking the current thread. In order to create the Future you will need either an implicit or explicit ExecutionContext to be provided:

    import scala.concurrent._
    import ExecutionContext.Implicits.global  // implicit execution context
    
    val firstZebra: Future[Int] = Future {
      val source = scala.io.Source.fromFile("/etc/dictionaries-common/words")
      source.toSeq.indexOfSlice("zebra")
    }

    Avoid Blocking

    Although blocking is possible in order to await results (with a mandatory timeout duration):

    import scala.concurrent.duration._
    Await.result(firstZebra, 10.seconds)

    and although this is sometimes necessary to do, in particular for testing purposes, blocking in general is discouraged when working with Futures and concurrency in order to avoid potential deadlocks and improve performance. Instead, use callbacks or combinators to remain in the future domain:

    val animalRange: Future[Int] = for {
      aardvark <- firstAardvark
      zebra <- firstZebra
    } yield zebra - aardvark
    
    animalRange.onSuccess {
      case x if x > 500000 => println("It's a long way from Aardvark to Zebra")
    }
    Definition Classes
    scala
  • package duration
    Definition Classes
    concurrent
  • package forkjoin
    Definition Classes
    concurrent
  • Await
  • Awaitable
  • BlockContext
  • CanAwait
  • Channel
  • DelayedLazyVal
  • ExecutionContext
  • ExecutionContextExecutor
  • ExecutionContextExecutorService
  • Future
  • JavaConversions
  • Lock
  • OnCompleteRunnable
  • Promise
  • SyncChannel
  • SyncVar

object Future

Future companion object.

Source
Future.scala
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Value Members

  1. final def !=(arg0: Any): Boolean

    Test two objects for inequality.

    Test two objects for inequality.

    returns

    true if !(this == that), false otherwise.

    Definition Classes
    AnyRef → Any
  2. final def ##(): Int

    Equivalent to x.hashCode except for boxed numeric types and null.

    Equivalent to x.hashCode except for boxed numeric types and null. For numerics, it returns a hash value which is consistent with value equality: if two value type instances compare as true, then ## will produce the same hash value for each of them. For null returns a hashcode where null.hashCode throws a NullPointerException.

    returns

    a hash value consistent with ==

    Definition Classes
    AnyRef → Any
  3. final def ==(arg0: Any): Boolean

    The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).

    The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).

    returns

    true if the receiver object is equivalent to the argument; false otherwise.

    Definition Classes
    AnyRef → Any
  4. def apply[T](body: ⇒ T)(implicit executor: ExecutionContext): Future[T]

    Starts an asynchronous computation and returns a Future instance with the result of that computation.

    Starts an asynchronous computation and returns a Future instance with the result of that computation.

    The following expressions are equivalent:

    val f1 = Future(expr)
    val f2 = Future.unit.map(_ => expr)

    The result becomes available once the asynchronous computation is completed.

    T

    the type of the result

    body

    the asynchronous computation

    executor

    the execution context on which the future is run

    returns

    the Future holding the result of the computation

  5. final def asInstanceOf[T0]: T0

    Cast the receiver object to be of type T0.

    Cast the receiver object to be of type T0.

    Note that the success of a cast at runtime is modulo Scala's erasure semantics. Therefore the expression 1.asInstanceOf[String] will throw a ClassCastException at runtime, while the expression List(1).asInstanceOf[List[String]] will not. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested type.

    returns

    the receiver object.

    Definition Classes
    Any
    Exceptions thrown

    ClassCastException if the receiver object is not an instance of the erasure of type T0.

  6. def clone(): AnyRef

    Create a copy of the receiver object.

    Create a copy of the receiver object.

    The default implementation of the clone method is platform dependent.

    returns

    a copy of the receiver object.

    Attributes
    protected[lang]
    Definition Classes
    AnyRef
    Annotations
    @throws( ... ) @native()
    Note

    not specified by SLS as a member of AnyRef

  7. final def eq(arg0: AnyRef): Boolean

    Tests whether the argument (that) is a reference to the receiver object (this).

    Tests whether the argument (that) is a reference to the receiver object (this).

    The eq method implements an equivalence relation on non-null instances of AnyRef, and has three additional properties:

    • It is consistent: for any non-null instances x and y of type AnyRef, multiple invocations of x.eq(y) consistently returns true or consistently returns false.
    • For any non-null instance x of type AnyRef, x.eq(null) and null.eq(x) returns false.
    • null.eq(null) returns true.

    When overriding the equals or hashCode methods, it is important to ensure that their behavior is consistent with reference equality. Therefore, if two objects are references to each other (o1 eq o2), they should be equal to each other (o1 == o2) and they should hash to the same value (o1.hashCode == o2.hashCode).

    returns

    true if the argument is a reference to the receiver object; false otherwise.

    Definition Classes
    AnyRef
  8. def equals(arg0: Any): Boolean

    The equality method for reference types.

    The equality method for reference types. Default implementation delegates to eq.

    See also equals in scala.Any.

    returns

    true if the receiver object is equivalent to the argument; false otherwise.

    Definition Classes
    AnyRef → Any
  9. def failed[T](exception: Throwable): Future[T]

    Creates an already completed Future with the specified exception.

    Creates an already completed Future with the specified exception.

    T

    the type of the value in the future

    exception

    the non-null instance of Throwable

    returns

    the newly created Future instance

  10. def finalize(): Unit

    Called by the garbage collector on the receiver object when there are no more references to the object.

    Called by the garbage collector on the receiver object when there are no more references to the object.

    The details of when and if the finalize method is invoked, as well as the interaction between finalize and non-local returns and exceptions, are all platform dependent.

    Attributes
    protected[lang]
    Definition Classes
    AnyRef
    Annotations
    @throws( classOf[java.lang.Throwable] )
    Note

    not specified by SLS as a member of AnyRef

  11. def find[T](futures: collection.immutable.Iterable[Future[T]])(p: (T) ⇒ Boolean)(implicit executor: ExecutionContext): Future[Option[T]]

    Asynchronously and non-blockingly returns a Future that will hold the optional result of the first Future with a result that matches the predicate, failed Futures will be ignored.

    Asynchronously and non-blockingly returns a Future that will hold the optional result of the first Future with a result that matches the predicate, failed Futures will be ignored.

    T

    the type of the value in the future

    futures

    the scala.collection.immutable.Iterable of Futures to search

    p

    the predicate which indicates if it's a match

    returns

    the Future holding the optional result of the search

  12. def firstCompletedOf[T](futures: TraversableOnce[Future[T]])(implicit executor: ExecutionContext): Future[T]

    Asynchronously and non-blockingly returns a new Future to the result of the first future in the list that is completed.

    Asynchronously and non-blockingly returns a new Future to the result of the first future in the list that is completed. This means no matter if it is completed as a success or as a failure.

    T

    the type of the value in the future

    futures

    the TraversableOnce of Futures in which to find the first completed

    returns

    the Future holding the result of the future that is first to be completed

  13. def foldLeft[T, R](futures: collection.immutable.Iterable[Future[T]])(zero: R)(op: (R, T) ⇒ R)(implicit executor: ExecutionContext): Future[R]

    A non-blocking, asynchronous left fold over the specified futures, with the start value of the given zero.

    A non-blocking, asynchronous left fold over the specified futures, with the start value of the given zero. The fold is performed asynchronously in left-to-right order as the futures become completed. The result will be the first failure of any of the futures, or any failure in the actual fold, or the result of the fold.

    Example:

    val futureSum = Future.foldLeft(futures)(0)(_ + _)
    T

    the type of the value of the input Futures

    R

    the type of the value of the returned Future

    futures

    the scala.collection.immutable.Iterable of Futures to be folded

    zero

    the start value of the fold

    op

    the fold operation to be applied to the zero and futures

    returns

    the Future holding the result of the fold

  14. def fromTry[T](result: Try[T]): Future[T]

    Creates an already completed Future with the specified result or exception.

    Creates an already completed Future with the specified result or exception.

    T

    the type of the value in the Future

    result

    the result of the returned Future instance

    returns

    the newly created Future instance

  15. final def getClass(): Class[_]

    Returns the runtime class representation of the object.

    Returns the runtime class representation of the object.

    returns

    a class object corresponding to the runtime type of the receiver.

    Definition Classes
    AnyRef → Any
    Annotations
    @native()
  16. def hashCode(): Int

    The hashCode method for reference types.

    The hashCode method for reference types. See hashCode in scala.Any.

    returns

    the hash code value for this object.

    Definition Classes
    AnyRef → Any
    Annotations
    @native()
  17. final def isInstanceOf[T0]: Boolean

    Test whether the dynamic type of the receiver object is T0.

    Test whether the dynamic type of the receiver object is T0.

    Note that the result of the test is modulo Scala's erasure semantics. Therefore the expression 1.isInstanceOf[String] will return false, while the expression List(1).isInstanceOf[List[String]] will return true. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the specified type.

    returns

    true if the receiver object is an instance of erasure of type T0; false otherwise.

    Definition Classes
    Any
  18. final def ne(arg0: AnyRef): Boolean

    Equivalent to !(this eq that).

    Equivalent to !(this eq that).

    returns

    true if the argument is not a reference to the receiver object; false otherwise.

    Definition Classes
    AnyRef
  19. final def notify(): Unit

    Wakes up a single thread that is waiting on the receiver object's monitor.

    Wakes up a single thread that is waiting on the receiver object's monitor.

    Definition Classes
    AnyRef
    Annotations
    @native()
    Note

    not specified by SLS as a member of AnyRef

  20. final def notifyAll(): Unit

    Wakes up all threads that are waiting on the receiver object's monitor.

    Wakes up all threads that are waiting on the receiver object's monitor.

    Definition Classes
    AnyRef
    Annotations
    @native()
    Note

    not specified by SLS as a member of AnyRef

  21. def reduceLeft[T, R >: T](futures: collection.immutable.Iterable[Future[T]])(op: (R, T) ⇒ R)(implicit executor: ExecutionContext): Future[R]

    Initiates a non-blocking, asynchronous, left reduction over the supplied futures where the zero is the result value of the first Future.

    Initiates a non-blocking, asynchronous, left reduction over the supplied futures where the zero is the result value of the first Future.

    Example:

    val futureSum = Future.reduceLeft(futures)(_ + _)
    T

    the type of the value of the input Futures

    R

    the type of the value of the returned Future

    futures

    the scala.collection.immutable.Iterable of Futures to be reduced

    op

    the reduce operation which is applied to the results of the futures

    returns

    the Future holding the result of the reduce

  22. def sequence[A, M[X] <: TraversableOnce[X]](in: M[Future[A]])(implicit cbf: CanBuildFrom[M[Future[A]], A, M[A]], executor: ExecutionContext): Future[M[A]]

    Simple version of Future.traverse.

    Simple version of Future.traverse. Asynchronously and non-blockingly transforms a TraversableOnce[Future[A]] into a Future[TraversableOnce[A]]. Useful for reducing many Futures into a single Future.

    A

    the type of the value inside the Futures

    M

    the type of the TraversableOnce of Futures

    in

    the TraversableOnce of Futures which will be sequenced

    returns

    the Future of the TraversableOnce of results

  23. def successful[T](result: T): Future[T]

    Creates an already completed Future with the specified result.

    Creates an already completed Future with the specified result.

    T

    the type of the value in the future

    result

    the given successful value

    returns

    the newly created Future instance

  24. final def synchronized[T0](arg0: ⇒ T0): T0
    Definition Classes
    AnyRef
  25. def toString(): String

    Creates a String representation of this object.

    Creates a String representation of this object. The default representation is platform dependent. On the java platform it is the concatenation of the class name, "@", and the object's hashcode in hexadecimal.

    returns

    a String representation of the object.

    Definition Classes
    AnyRef → Any
  26. def traverse[A, B, M[X] <: TraversableOnce[X]](in: M[A])(fn: (A) ⇒ Future[B])(implicit cbf: CanBuildFrom[M[A], B, M[B]], executor: ExecutionContext): Future[M[B]]

    Asynchronously and non-blockingly transforms a TraversableOnce[A] into a Future[TraversableOnce[B]] using the provided function A => Future[B].

    Asynchronously and non-blockingly transforms a TraversableOnce[A] into a Future[TraversableOnce[B]] using the provided function A => Future[B]. This is useful for performing a parallel map. For example, to apply a function to all items of a list in parallel:

    val myFutureList = Future.traverse(myList)(x => Future(myFunc(x)))
    A

    the type of the value inside the Futures in the TraversableOnce

    B

    the type of the value of the returned Future

    M

    the type of the TraversableOnce of Futures

    in

    the TraversableOnce of Futures which will be sequenced

    fn

    the function to apply to the TraversableOnce of Futures to produce the results

    returns

    the Future of the TraversableOnce of results

  27. val unit: Future[Unit]

    A Future which is always completed with the Unit value.

  28. final def wait(): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws( ... )
  29. final def wait(arg0: Long, arg1: Int): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws( ... )
  30. final def wait(arg0: Long): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws( ... ) @native()
  31. object never extends Future[Nothing]

    A Future which is never completed.

Deprecated Value Members

  1. def find[T](futures: TraversableOnce[Future[T]])(p: (T) ⇒ Boolean)(implicit executor: ExecutionContext): Future[Option[T]]

    Asynchronously and non-blockingly returns a Future that will hold the optional result of the first Future with a result that matches the predicate.

    Asynchronously and non-blockingly returns a Future that will hold the optional result of the first Future with a result that matches the predicate.

    T

    the type of the value in the future

    futures

    the TraversableOnce of Futures to search

    p

    the predicate which indicates if it's a match

    returns

    the Future holding the optional result of the search

    Annotations
    @deprecated
    Deprecated

    (Since version 2.12.0) use the overloaded version of this method that takes a scala.collection.immutable.Iterable instead

  2. def fold[T, R](futures: TraversableOnce[Future[T]])(zero: R)(op: (R, T) ⇒ R)(implicit executor: ExecutionContext): Future[R]

    A non-blocking, asynchronous fold over the specified futures, with the start value of the given zero.

    A non-blocking, asynchronous fold over the specified futures, with the start value of the given zero. The fold is performed on the thread where the last future is completed, the result will be the first failure of any of the futures, or any failure in the actual fold, or the result of the fold.

    Example:

    val futureSum = Future.fold(futures)(0)(_ + _)
    T

    the type of the value of the input Futures

    R

    the type of the value of the returned Future

    futures

    the TraversableOnce of Futures to be folded

    zero

    the start value of the fold

    op

    the fold operation to be applied to the zero and futures

    returns

    the Future holding the result of the fold

    Annotations
    @deprecated
    Deprecated

    (Since version 2.12.0) use Future.foldLeft instead

  3. def reduce[T, R >: T](futures: TraversableOnce[Future[T]])(op: (R, T) ⇒ R)(implicit executor: ExecutionContext): Future[R]

    Initiates a non-blocking, asynchronous, fold over the supplied futures where the fold-zero is the result value of the first Future in the collection.

    Initiates a non-blocking, asynchronous, fold over the supplied futures where the fold-zero is the result value of the first Future in the collection.

    Example:

    val futureSum = Future.reduce(futures)(_ + _)
    T

    the type of the value of the input Futures

    R

    the type of the value of the returned Future

    futures

    the TraversableOnce of Futures to be reduced

    op

    the reduce operation which is applied to the results of the futures

    returns

    the Future holding the result of the reduce

    Annotations
    @deprecated
    Deprecated

    (Since version 2.12.0) use Future.reduceLeft instead

Inherited from AnyRef

Inherited from Any

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