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:

    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 annotation
    Definition Classes
    scala
  • package beans
    Definition Classes
    scala
  • package collection
    Definition Classes
    scala
  • package compat
    Definition Classes
    scala
  • 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 io
    Definition Classes
    scala
  • package jdk
    Definition Classes
    scala
  • package javaapi
  • Accumulator
  • AnyAccumulator
  • CollectionConverters
  • DoubleAccumulator
  • DurationConverters
  • FunctionConverters
  • FunctionWrappers
  • FutureConverters
  • IntAccumulator
  • LongAccumulator
  • OptionConverters
  • OptionShape
  • Priority0FunctionExtensions
  • Priority1FunctionExtensions
  • Priority2FunctionExtensions
  • Priority3FunctionExtensions
  • StreamConverters
  • package math

    The package object scala.math contains methods for performing basic numeric operations such as elementary exponential, logarithmic, root and trigonometric functions.

    The package object scala.math contains methods for performing basic numeric operations such as elementary exponential, logarithmic, root and trigonometric functions.

    All methods forward to java.lang.Math unless otherwise noted.

    Definition Classes
    scala
    See also

    java.lang.Math

  • package ref
    Definition Classes
    scala
  • package reflect
    Definition Classes
    scala
  • package sys

    The package object scala.sys contains methods for reading and altering core aspects of the virtual machine as well as the world outside of it.

    The package object scala.sys contains methods for reading and altering core aspects of the virtual machine as well as the world outside of it.

    Definition Classes
    scala
  • package util
    Definition Classes
    scala

package jdk

Content Hierarchy
Ordering
  1. Alphabetic
Visibility
  1. Public
  2. Protected

Package Members

  1. package javaapi

Type Members

  1. abstract class Accumulator[A, +CC[X] <: collection.mutable.Seq[X], +C <: collection.mutable.Seq[A]] extends collection.mutable.Seq[A] with Builder[A, C]

    Accumulators are mutable sequences with two distinct features:

    Accumulators are mutable sequences with two distinct features:

    • An accumulator can be appended efficiently to another
    • There are manually specialized Accumulators for Int, Long and Double that don't box the elements

    These two features make Accumulators a good candidate to collect the results of a parallel Java stream pipeline into a Scala collection. The scala.collection.convert.StreamExtensions.StreamHasToScala.toScala extension method on Java streams (available by importing scala.jdk.StreamConverters._) is specialized for Accumulators: they are built in parallel, the parts are merged efficiently.

    Building specialized Accumulators is handled transparently. As a user, using the Accumulator object as a factory automatically creates an IntAccumulator, LongAccumulator, DoubleAccumulator or AnyAccumulator depending on the element type.

    Note: to run the example, start the Scala REPL with scala -Yrepl-class-based to avoid deadlocks, see https://github.com/scala/bug/issues/9076.

    scala> import scala.jdk.StreamConverters._
    import scala.jdk.StreamConverters._
    
    scala> def isPrime(n: Int): Boolean = !(2 +: (3 to Math.sqrt(n).toInt by 2) exists (n % _ == 0))
    isPrime: (n: Int)Boolean
    
    scala> val intAcc = (1 to 10000).asJavaParStream.filter(isPrime).toScala(scala.jdk.Accumulator)
    intAcc: scala.jdk.IntAccumulator = IntAccumulator(1, 3, 5, 7, 11, 13, 17, 19, ...
    
    scala> val stringAcc = (1 to 100).asJavaParStream.mapToObj("<>" * _).toScala(Accumulator)
    stringAcc: scala.jdk.AnyAccumulator[String] = AnyAccumulator(<>, <><>, <><><>, ...

    There are two possibilities to process elements of a primitive Accumulator without boxing: specialized operations of the Accumulator, or the Stepper interface. The most common collection operations are overloaded or overridden in the primitive Accumulator classes, for example Int => Int)* IntAccumulator.map or IntAccumulator.exists. Thanks to Scala's function specialization, intAcc.exists(x => testOn(x)) does not incur boxing.

    The scala.collection.Stepper interface provides iterator-like hasStep and nextStep methods, and is specialized for Int, Long and Double. The intAccumulator.stepper method creates an scala.collection.IntStepper that yields the elements of the accumulator without boxing.

    Accumulators can hold more than Int.MaxValue elements. They have a sizeLong method that returns the size as a Long. Note that certain operations defined in scala.collection.Seq are implemented using length, so they will not work correctly for large accumulators.

    The Accumulator class is a base class to share code between AnyAccumulator (for reference types) and the manual specializations IntAccumulator, LongAccumulator and DoubleAccumulator.

  2. final class AnyAccumulator[A] extends Accumulator[A, AnyAccumulator, AnyAccumulator[A]] with SeqOps[A, AnyAccumulator, AnyAccumulator[A]] with IterableFactoryDefaults[A, AnyAccumulator] with Serializable

    An Accumulator for arbitrary element types, see Accumulator.

  3. final class DoubleAccumulator extends Accumulator[Double, AnyAccumulator, DoubleAccumulator] with SeqOps[Double, AnyAccumulator, DoubleAccumulator] with Serializable

    A specialized Accumulator that holds Doubles without boxing, see Accumulator.

  4. final class IntAccumulator extends Accumulator[Int, AnyAccumulator, IntAccumulator] with SeqOps[Int, AnyAccumulator, IntAccumulator] with Serializable

    A specialized Accumulator that holds Ints without boxing, see Accumulator.

  5. final class LongAccumulator extends Accumulator[Long, AnyAccumulator, LongAccumulator] with SeqOps[Long, AnyAccumulator, LongAccumulator] with Serializable

    A specialized Accumulator that holds Longs without boxing, see Accumulator.

  6. sealed abstract class OptionShape[A, O] extends AnyRef

    A type class implementing conversions from a generic Scala Option or Java Optional to a specialized Java variant (for Double, Int and Long).

    A type class implementing conversions from a generic Scala Option or Java Optional to a specialized Java variant (for Double, Int and Long).

    A

    the primitive type wrapped in an option

    O

    the specialized Java Optional wrapping an element of type A

    Annotations
    @implicitNotFound(msg = "No specialized Optional type exists for elements of type ${A}")
  7. trait Priority0FunctionExtensions extends Priority1FunctionExtensions
  8. trait Priority1FunctionExtensions extends Priority2FunctionExtensions
  9. trait Priority2FunctionExtensions extends Priority3FunctionExtensions
  10. trait Priority3FunctionExtensions extends AnyRef

Value Members

  1. object Accumulator

    Contains factory methods to build Accumulators.

    Contains factory methods to build Accumulators.

    Note that the Accumulator object itself is not a factory, but it is implicitly convert to a factory according to the element type, see Accumulator.toFactory.

    This allows passing the Accumulator object as argument when a collection.Factory, and the implicit Accumulator.AccumulatorFactoryShape instance is used to build a specialized Accumulator according to the element type:

    scala> val intAcc = Accumulator(1,2,3)
    intAcc: scala.collection.convert.IntAccumulator = IntAccumulator(1, 2, 3)
    
    scala> val anyAccc = Accumulator("K")
    anyAccc: scala.collection.convert.AnyAccumulator[String] = AnyAccumulator(K)
    
    scala> val intAcc2 = List(1,2,3).to(Accumulator)
    intAcc2: scala.jdk.IntAccumulator = IntAccumulator(1, 2, 3)
    
    scala> val anyAcc2 = List("K").to(Accumulator)
    anyAcc2: scala.jdk.AnyAccumulator[String] = AnyAccumulator(K)
  2. object AnyAccumulator extends SeqFactory[AnyAccumulator]
  3. object CollectionConverters extends AsJavaExtensions with AsScalaExtensions

    This object provides extension methods that convert between Scala and Java collections.

    This object provides extension methods that convert between Scala and Java collections.

    When writing Java code, use the explicit conversion methods defined in javaapi.CollectionConverters instead.

    Note: to create Java Streams that operate on Scala collections (sequentially or in parallel), use StreamConverters.

    import scala.jdk.CollectionConverters._
    val s: java.util.Set[String] = Set("one", "two").asJava

    The conversions return adapters for the corresponding API, i.e., the collections are wrapped, not converted. Changes to the original collection are reflected in the view, and vice versa:

    scala> import scala.jdk.CollectionConverters._
    
    scala> val s = collection.mutable.Set("one")
    s: scala.collection.mutable.Set[String] = HashSet(one)
    
    scala> val js = s.asJava
    js: java.util.Set[String] = [one]
    
    scala> js.add("two")
    
    scala> s
    res2: scala.collection.mutable.Set[String] = HashSet(two, one)

    The following conversions are supported via asScala and asJava:

    scala.collection.Iterable       <=> java.lang.Iterable
    scala.collection.Iterator       <=> java.util.Iterator
    scala.collection.mutable.Buffer <=> java.util.List
    scala.collection.mutable.Set    <=> java.util.Set
    scala.collection.mutable.Map    <=> java.util.Map
    scala.collection.concurrent.Map <=> java.util.concurrent.ConcurrentMap

    The following conversions are supported via asScala and through specially-named extension methods to convert to Java collections, as shown:

    scala.collection.Iterable    <=> java.util.Collection   (via asJavaCollection)
    scala.collection.Iterator    <=> java.util.Enumeration  (via asJavaEnumeration)
    scala.collection.mutable.Map <=> java.util.Dictionary   (via asJavaDictionary)

    In addition, the following one-way conversions are provided via asJava:

    scala.collection.Seq         => java.util.List
    scala.collection.mutable.Seq => java.util.List
    scala.collection.Set         => java.util.Set
    scala.collection.Map         => java.util.Map

    The following one way conversion is provided via asScala:

    java.util.Properties => scala.collection.mutable.Map

    In all cases, converting from a source type to a target type and back again will return the original source object. For example:

    import scala.jdk.CollectionConverters._
    
    val source = new scala.collection.mutable.ListBuffer[Int]
    val target: java.util.List[Int] = source.asJava
    val other: scala.collection.mutable.Buffer[Int] = target.asScala
    assert(source eq other)
  4. object DoubleAccumulator extends SpecificIterableFactory[Double, DoubleAccumulator] with java.io.Serializable
  5. object DurationConverters

    This object provides extension methods that convert between Scala and Java duration types.

    This object provides extension methods that convert between Scala and Java duration types.

    When writing Java code, use the explicit conversion methods defined in javaapi.DurationConverters instead.

  6. object FunctionConverters extends Priority0FunctionExtensions

    This object provides extension methods that convert between Scala and Java function types.

    This object provides extension methods that convert between Scala and Java function types.

    When writing Java code, use the explicit conversion methods defined in javaapi.FunctionConverters instead.

    Using the .asJava extension method on a Scala function produces the most specific possible Java function type:

    scala> import scala.jdk.FunctionConverters._
    scala> val f = (x: Int) => x + 1
    
    scala> val jf1 = f.asJava
    jf1: java.util.function.IntUnaryOperator = ...

    More generic Java function types can be created using the corresponding asJavaXYZ extension method:

    scala> val jf2 = f.asJavaFunction
    jf2: java.util.function.Function[Int,Int] = ...
    
    scala> val jf3 = f.asJavaUnaryOperator
    jf3: java.util.function.UnaryOperator[Int] = ...

    Converting a Java function to Scala is done using the asScala extension method:

    scala> List(1,2,3).map(jf2.asScala)
    res1: List[Int] = List(2, 3, 4)
  7. object FunctionWrappers
  8. object FutureConverters

    This object provides extension methods that convert between Scala scala.concurrent.Future and Java java.util.concurrent.CompletionStage

    This object provides extension methods that convert between Scala scala.concurrent.Future and Java java.util.concurrent.CompletionStage

    When writing Java code, use the explicit conversion methods defined in javaapi.FutureConverters instead.

    Note that the bridge is implemented at the read-only side of asynchronous handles, namely scala.concurrent.Future (instead of scala.concurrent.Promise) and java.util.concurrent.CompletionStage (instead of java.util.concurrent.CompletableFuture). This is intentional, as the semantics of bridging the write-handles would be prone to race conditions; if both ends (CompletableFuture and Promise) are completed independently at the same time, they may contain different values afterwards. For this reason, toCompletableFuture is not supported on the created CompletionStages.

  9. object IntAccumulator extends SpecificIterableFactory[Int, IntAccumulator] with java.io.Serializable
  10. object LongAccumulator extends SpecificIterableFactory[Long, LongAccumulator] with java.io.Serializable
  11. object OptionConverters

    This object provides extension methods that convert between Scala Option and Java Optional types.

    This object provides extension methods that convert between Scala Option and Java Optional types.

    When writing Java code, use the explicit conversion methods defined in javaapi.OptionConverters instead.

    Scala Option is extended with a toJava method that creates a corresponding Optional, and a toJavaPrimitive method that creates a specialized variant (e.g., OptionalInt) if applicable.

    Java Optional is extended with a toScala method and a toJavaPrimitive method.

    Finally, specialized Optional types are extended with toScala and toJavaGeneric methods.

    Example usage:

    import scala.jdk.OptionConverters._
    val a = Option("example").toJava      // Creates java.util.Optional[String] containing "example"
    val b = (None: Option[String]).toJava // Creates an empty java.util.Optional[String]
    val c = a.toScala                     // Back to Option("example")
    val d = b.toScala                     // Back to None typed as Option[String]
    val e = Option(2.7).toJava            // java.util.Optional[Double] containing boxed 2.7
    val f = Option(2.7).toJavaPrimitive   // java.util.OptionalDouble containing 2.7 (not boxed)
    val g = f.toScala                     // Back to Option(2.7)
    val h = f.toJavaGeneric               // Same as e
    val i = e.toJavaPrimitive             // Same as f
  12. object OptionShape
  13. object StreamConverters extends StreamExtensions

    This object provides extension methods to create Java Streams that operate on Scala collections (sequentially or in parallel).

    This object provides extension methods to create Java Streams that operate on Scala collections (sequentially or in parallel). For more information on Java streams, consult the documentation (https://docs.oracle.com/javase/8/docs/api/java/util/stream/package-summary.html).

    When writing Java code, use the explicit conversion methods defined in javaapi.StreamConverters instead.

    The methods asJavaSeqStream and asJavaParStream convert a collection to a Java Stream:

    scala> import scala.jdk.StreamConverters._
    
    scala> val s = (1 to 10).toList.asJavaSeqStream
    s: java.util.stream.IntStream = [email protected]7b1e5e55
    
    scala> s.map(_ * 2).filter(_ > 5).toScala(List)
    res1: List[Int] = List(6, 8, 10, 12, 14, 16, 18, 20)

    Note: using parallel streams in the Scala REPL causes deadlocks, see https://github.com/scala/bug/issues/9076. As a workaround, use scala -Yrepl-class-based.

    scala> def isPrime(n: Int): Boolean = !(2 +: (3 to Math.sqrt(n).toInt by 2) exists (n % _ == 0))
    isPrime: (n: Int)Boolean
    
    scala> (10000 to 1000000).asJavaParStream.filter(isPrime).toScala(Vector)
    res6: scala.collection.immutable.Vector[Int] = Vector(10007, 10009, 10037, 10039, ...

    A Java Stream provides operations on a sequence of elements. Streams are created from Spliterators, which are similar to Iterators with the additional capability to partition off some of their elements. This partitioning, if supported by the Spliterator, is used for parallelizing Stream operations.

    Scala collections have a method stepper that returns a scala.collection.Stepper for the collection, which in turn can be converted to a Spliterator for creating a Java Stream.

    The asJavaSeqStream extension method is available on any Scala collection. The asJavaParStream extension method can only be invoked on collections where the return type of the stepper method is marked with the scala.collection.Stepper.EfficientSplit marker trait. This trait is added to steppers that support partitioning, and therefore efficient parallel processing.

    The following extension methods are available:

    Collection Type

    Extension Methods

    IterableOnce

    asJavaSeqStream

    IndexedSeq, Arrays, BitSet, Accumulator, HashMap, HashSet, Range, TreeMap, TreeSet, Vector, Strings

    asJavaParStream

    Map

    asJavaSeqKeyStream, asJavaSeqValueStream

    HashMap, TreeMap

    asJavaParKeyStream, asJavaParValueStream

    Stepper

    asJavaSeqStream

    Stepper with EfficientSplit

    asJavaParStream

    Strings

    asJavaSeqStream, asJavaParStream, asJavaSeqCharStream, asJavaParCharStream, asJavaSeqCodePointStream, asJavaParCodePointStream

    Java streams

    toScala, asJavaPrimitiveStream

    The asJavaPrimitiveStream method converts a Stream[Int] to an IntStream. It is the dual of the boxed method defined on primitive streams (e.g., IntStream.boxed is a Stream[Integer]).

    The toScala extension methods on Java streams collects the result of a stream pipeline into a Scala collection, for example stream.toScala(List), stream.toScala(Vector). Note that transformation operations on streams are lazy (also called "intermediate"), terminal operations such as forEach, count or toScala trigger the evaluation.

    Collecting a parallel stream to a collection can be performed in parallel. This is beneficial if the target collection supports efficient merging of the segments that are built in parallel. To support this use case, the Scala standard library provides the Accumulator collection. This collection supports efficient parallel construction, and it has specialized subtypes for Int, Long and Double so that primitive Java streams can be collected to a Scala collection without boxing the elements.

Ungrouped