Packages

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  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:

  • scala.collection and its sub-packages contain Scala's collections framework
    • scala.collection.immutable - Immutable, sequential data-structures such as Vector, List, Range, HashMap or HashSet
    • scala.collection.mutable - Mutable, sequential data-structures such as ArrayBuffer, StringBuilder, HashMap or HashSet
    • scala.collection.concurrent - Mutable, concurrent data-structures such as TrieMap
    • scala.collection.parallel.immutable - Immutable, parallel data-structures such as ParVector, ParRange, ParHashMap or ParHashSet
    • scala.collection.parallel.mutable - Mutable, parallel data-structures such as ParArray, ParHashMap, ParTrieMap or ParHashSet
  • scala.concurrent - Primitives for concurrent programming such as Futures and Promises
  • scala.io - Input and output operations
  • scala.math - Basic math functions and additional numeric types like BigInt and BigDecimal
  • scala.sys - Interaction with other processes and the operating system
  • scala.util.matching - Regular expressions

  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

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•  package scala

  Core Scala types.

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

  Definition Classes

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  Definition Classes

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  Contains the base traits and objects needed to use and extend Scala's collection library.

  Contains the base traits and objects needed to use and extend Scala's collection library.

  Guide

  A detailed guide for using the collections library is available at http://docs.scala-lang.org/overviews/collections/introduction.html. Developers looking to extend the collections library can find a description of its architecture at http://docs.scala-lang.org/overviews/core/architecture-of-scala-collections.html.

  Using Collections

  It is convenient to treat all collections as either a scala.collection.Traversable or scala.collection.Iterable, as these traits define the vast majority of operations on a collection.

  Collections can, of course, be treated as specifically as needed, and the library is designed to ensure that the methods that transform collections will return a collection of the same type:

  scala> val array = Array(1,2,3,4,5,6)
  array: Array[Int] = Array(1, 2, 3, 4, 5, 6)
  
  scala> array map { _.toString }
  res0: Array[String] = Array(1, 2, 3, 4, 5, 6)
  
  scala> val list = List(1,2,3,4,5,6)
  list: List[Int] = List(1, 2, 3, 4, 5, 6)
  
  scala> list map { _.toString }
  res1: List[String] = List(1, 2, 3, 4, 5, 6)

  Creating Collections

  The most common way to create a collection is to use its companion object as a factory. The three most commonly used collections are scala.collection.Seq, scala.collection.immutable.Set, and scala.collection.immutable.Map. They can be used directly as shown below since their companion objects are all available as type aliases in either the scala package or in scala.Predef. New collections are created like this:

  scala> val seq = Seq(1,2,3,4,1)
  seq: Seq[Int] = List(1, 2, 3, 4, 1)
  
  scala> val set = Set(1,2,3,4,1)
  set: scala.collection.immutable.Set[Int] = Set(1, 2, 3, 4)
  
  scala> val map = Map(1 -> "one", 2 -> "two", 3 -> "three", 2 -> "too")
  map: scala.collection.immutable.Map[Int,String] = Map(1 -> one, 2 -> too, 3 -> three)

  It is also typical to prefer the scala.collection.immutable collections over those in scala.collection.mutable; the types aliased in the scala.Predef object are the immutable versions.

  Also note that the collections library was carefully designed to include several implementations of each of the three basic collection types. These implementations have specific performance characteristics which are described in the guide.

  The concrete parallel collections also have specific performance characteristics which are described in the parallel collections guide

  Converting to and from Java Collections

  The scala.collection.JavaConverters object provides a collection of decorators that allow converting between Scala and Java collections using asScala and asJava methods.

  Definition Classes

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•  package compat

  Definition Classes

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•  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

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•  package io

  Definition Classes

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•  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

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  See also

  java.lang.Math

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  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.

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