Scala Standard Library2.12.21 < Back
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# 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:

  - `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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-  package annotation
  Definition Classes
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-  package beans
  Definition Classes
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-  package collection

  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 `Future`s, 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

-  package ref
  Definition Classes
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-  package reflect
  Definition Classes
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- AnyValManifest
- ClassManifestDeprecatedApis
- ClassManifestFactory
- ClassTag
- Manifest
- ManifestFactory
- NameTransformer
- NoManifest
- OptManifest
-  package runtime
  Definition Classes
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-  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
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  Since

    2.9

-  package text
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-  package util
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p

scala

# reflect 

#### package reflect

Source
  package.scala

Linear Supertypes
AnyRef, Any
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### Type Members

1.  abstract class AnyValManifest[T <: AnyVal] extends Manifest[T] with Equals
   Annotations
     @SerialVersionUID()
2.  trait ClassTag[T] extends ClassManifestDeprecatedApis[T] with Equals with Serializable

   A `ClassTag[T]` stores the erased class of a given type `T`, accessible via the `runtimeClass` field.

   A `ClassTag[T]` stores the erased class of a given type `T`, accessible via the `runtimeClass` field. This is particularly useful for instantiating `Array`s whose element types are unknown at compile time.

   `ClassTag`s are a weaker special case of scala.reflect.api.TypeTags#TypeTags, in that they wrap only the runtime class of a given type, whereas a `TypeTag` contains all static type information. That is, `ClassTag`s are constructed from knowing only the top-level class of a type, without necessarily knowing all of its argument types. This runtime information is enough for runtime `Array` creation.

   For example:

       scala> def mkArray[T : ClassTag](elems: T*) = Array[T](elems: _*)
       mkArray: [T](elems: T*)(implicit evidence$1: scala.reflect.ClassTag[T])Array[T]

       scala> mkArray(42, 13)
       res0: Array[Int] = Array(42, 13)

       scala> mkArray("Japan","Brazil","Germany")
       res1: Array[String] = Array(Japan, Brazil, Germany)

   See scala.reflect.api.TypeTags for more examples, or the Reflection Guide: TypeTags for more details.

   Annotations
     @implicitNotFound( msg = "No ClassTag available for ${T}" )
3.  trait Manifest[T] extends ClassManifest[T] with Equals

   A `Manifest[T]` is an opaque descriptor for type T.

   A `Manifest[T]` is an opaque descriptor for type T. Its supported use is to give access to the erasure of the type as a `Class` instance, as is necessary for the creation of native `Arrays` if the class is not known at compile time.

   The type-relation operators `<:<` and `=:=` should be considered approximations only, as there are numerous aspects of type conformance which are not yet adequately represented in manifests.

   Example usages:

       def arr[T] = new Array[T](0)                          // does not compile
       def arr[T](implicit m: Manifest[T]) = new Array[T](0) // compiles
       def arr[T: Manifest] = new Array[T](0)                // shorthand for the preceding

       // Methods manifest, classManifest, and optManifest are in [[scala.Predef]].
       def isApproxSubType[T: Manifest, U: Manifest] = manifest[T] <:< manifest[U]
       isApproxSubType[List[String], List[AnyRef]] // true
       isApproxSubType[List[String], List[Int]]    // false

       def methods[T: ClassManifest] = classManifest[T].erasure.getMethods
       def retType[T: ClassManifest](name: String) =
         methods[T] find (_.getName == name) map (_.getGenericReturnType)

       retType[Map[_, _]]("values")  // Some(scala.collection.Iterable<B>)

   Annotations
     @implicitNotFound( msg = "No Manifest available for ${T}." )
4.  trait OptManifest[+T] extends Serializable

   A `OptManifest[T]` is an optional scala.reflect.Manifest.

   A `OptManifest[T]` is an optional scala.reflect.Manifest.

   It is either a `Manifest` or the value `NoManifest`.

5.  type ClassManifest[T] = ClassTag[T]

   A `ClassManifest[T]` is an opaque descriptor for type `T`.

   A `ClassManifest[T]` is an opaque descriptor for type `T`. It is used by the compiler to preserve information necessary for instantiating `Arrays` in those cases where the element type is unknown at compile time.

   The type-relation operators make an effort to present a more accurate picture than can be realized with erased types, but they should not be relied upon to give correct answers. In particular they are likely to be wrong when variance is involved or when a subtype has a different number of type arguments than a supertype.

   Annotations
     @deprecated @implicitNotFound( msg = ... )
   Deprecated

     (Since version 2.10.0) use scala.reflect.ClassTag instead

6.  trait ClassManifestDeprecatedApis[T] extends OptManifest[T]
   Annotations
     @deprecated
   Deprecated

     (Since version 2.10.0) use scala.reflect.ClassTag instead

### Value Members

1.  val Manifest: ManifestFactory.type

   The object `Manifest` defines factory methods for manifests.

   The object `Manifest` defines factory methods for manifests. It is intended for use by the compiler and should not be used in client code.

2.  def classTag[T](implicit ctag: ClassTag[T]): ClassTag[T]
3.  def ensureAccessible[T <: AccessibleObject](m: T): T

   Make a java reflection object accessible, if it is not already and it is possible to do so.

   Make a java reflection object accessible, if it is not already and it is possible to do so. If a SecurityException is thrown in the attempt, it is caught and discarded.

4.  object ClassManifestFactory

   `ClassManifestFactory` defines factory methods for manifests.

   `ClassManifestFactory` defines factory methods for manifests. It is intended for use by the compiler and should not be used in client code.

   Unlike `ClassManifest`, this factory isn't annotated with a deprecation warning. This is done to prevent avalanches of deprecation warnings in the code that calls methods with manifests.

   In a perfect world, we would just remove the @deprecated annotation from `ClassManifest` the object and then delete it in 2.11. After all, that object is explicitly marked as internal, so no one should use it. However a lot of existing libraries disregarded the Scaladoc that comes with `ClassManifest`, so we need to somehow nudge them into migrating prior to removing stuff out of the blue. Hence we've introduced this design decision as the lesser of two evils.

5.  object ClassTag extends Serializable

   Class tags corresponding to primitive types and constructor/extractor for ClassTags.

6.  object ManifestFactory

   `ManifestFactory` defines factory methods for manifests.

   `ManifestFactory` defines factory methods for manifests. It is intended for use by the compiler and should not be used in client code.

   Unlike `Manifest`, this factory isn't annotated with a deprecation warning. This is done to prevent avalanches of deprecation warnings in the code that calls methods with manifests. Why so complicated? Read up the comments for `ClassManifestFactory`.

7.  object NameTransformer

   Provides functions to encode and decode Scala symbolic names.

   Provides functions to encode and decode Scala symbolic names. Also provides some constants.

8.  object NoManifest extends OptManifest[Nothing] with Serializable

   One of the branches of an scala.reflect.OptManifest.

### Deprecated Value Members

1.  val ClassManifest: ClassManifestFactory.type

   The object `ClassManifest` defines factory methods for manifests.

   The object `ClassManifest` defines factory methods for manifests. It is intended for use by the compiler and should not be used in client code.

   Annotations
     @deprecated
   Deprecated

     (Since version 2.10.0) use scala.reflect.ClassTag instead

### Inherited from AnyRef

### Inherited from Any

### Ungrouped