Scala Standard Library2.13.0 < 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.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.collection.parallel` - Parallel collections (scala-parallel-collections.jar)
  - `scala.util.parsing` - Parser combinators (scala-parser-combinators.jar)
  - `scala.swing` - A convenient wrapper around Java's GUI framework called Swing (scala-swing.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 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 `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
    scala
-  package io
  Definition Classes
    scala
-  package jdk
  Definition Classes
    scala
- #::
- <:<
- =:=
- Any
- AnyVal
- App
- Array
- Boolean
- Byte
- Char
- Console
- DelayedInit
- Double
- DummyImplicit
- Dynamic
- Enumeration
- Equals
- Float
- Function
- Function0
- Function1
- Function10
- Function11
- Function12
- Function13
- Function14
- Function15
- Function16
- Function17
- Function18
- Function19
- Function2
- Function20
- Function21
- Function22
- Function3
- Function4
- Function5
- Function6
- Function7
- Function8
- Function9
- Int
- Long
- MatchError
- None
- NotImplementedError
- Nothing
- Null
- Option
- PartialFunction
- Predef
- Product
- Product1
- Product10
- Product11
- Product12
- Product13
- Product14
- Product15
- Product16
- Product17
- Product18
- Product19
- Product2
- Product20
- Product21
- Product22
- Product3
- Product4
- Product5
- Product6
- Product7
- Product8
- Product9
- Proxy
- ScalaReflectionException
- SerialVersionUID
- Short
- Singleton
- Some
- Specializable
- StringContext
- Symbol
- Tuple1
- Tuple10
- Tuple11
- Tuple12
- Tuple13
- Tuple14
- Tuple15
- Tuple16
- Tuple17
- Tuple18
- Tuple19
- Tuple2
- Tuple20
- Tuple21
- Tuple22
- Tuple3
- Tuple4
- Tuple5
- Tuple6
- Tuple7
- Tuple8
- Tuple9
- UninitializedError
- UninitializedFieldError
- Unit
- ValueOf
- deprecated
- deprecatedInheritance
- deprecatedName
- deprecatedOverriding
- inline
- language
- languageFeature
- native
- noinline
- specialized
- throws
- transient
- unchecked
- volatile
-  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
  Since

    2.9

-  package util
  Definition Classes
    scala

o

scala

# language

###

#### object language

The `scala.language` object controls the language features available to the programmer, as proposed in the SIP-18 document.

Each of these features has to be explicitly imported into the current scope to become available:

    import language.postfixOps // or language._
    List(1, 2, 3) reverse

The language features are:

- `dynamics` enables defining calls rewriting using the `Dynamic` trait
- `existentials` enables writing existential types
- `higherKinds` enables writing higher-kinded types
- `implicitConversions` enables defining implicit methods and members
- `postfixOps` enables postfix operators (not recommended)
- `reflectiveCalls` enables using structural types
- `experimental` contains newer features that have not yet been tested in production

Source
  language.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.  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`.

5.  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[java.lang]
   Definition Classes
     AnyRef
   Annotations
     @throws(classOf[java.lang.CloneNotSupportedException]) @native()
   Note

     not specified by SLS as a member of AnyRef

6.  implicit lazy val dynamics: dynamics

   Only where this feature is enabled, can direct or indirect subclasses of trait scala.Dynamic be defined.

   Only where this feature is enabled, can direct or indirect subclasses of trait scala.Dynamic be defined. If `dynamics` is not enabled, a definition of a class, trait, or object that has `Dynamic` as a base trait is rejected by the compiler.

   Selections of dynamic members of existing subclasses of trait `Dynamic` are unaffected; they can be used anywhere.

   Why introduce the feature? To enable flexible DSLs and convenient interfacing with dynamic languages.

   Why control it? Dynamic member selection can undermine static checkability of programs. Furthermore, dynamic member selection often relies on reflection, which is not available on all platforms.

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: AnyRef): 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.  implicit lazy val existentials: existentials

   Where this feature is enabled, existential types that cannot be expressed as wildcard types can be written and are allowed in inferred types of values or return types of methods.

   Where this feature is enabled, existential types that cannot be expressed as wildcard types can be written and are allowed in inferred types of values or return types of methods. If `existentials` is not enabled, those cases will trigger a warning from the compiler.

   Existential types with wildcard type syntax such as `List[_]`, or `Map[String, _]` are not affected.

   Why keep the feature? Existential types are needed to make sense of Java’s wildcard types and raw types and the erased types of run-time values.

   Why control it? Having complex existential types in a code base usually makes application code very brittle, with a tendency to produce type errors with obscure error messages. Therefore, going overboard with existential types is generally perceived not to be a good idea. Also, complicated existential types might be no longer supported in a future simplification of the language.

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

      not specified by SLS as a member of AnyRef

11.  final def getClass(): Class[_ <: AnyRef]

    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()
12.  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()
13.  implicit lazy val higherKinds: higherKinds

    Where this feature is enabled, higher-kinded types can be written.

    Where this feature is enabled, higher-kinded types can be written. If `higherKinds` is not enabled, a higher-kinded type such as `F[A]` will trigger a warning from the compiler.

    Why keep the feature? Higher-kinded types enable the definition of very general abstractions such as functor, monad, or arrow. A significant set of advanced libraries relies on them. Higher-kinded types are also at the core of the scala-virtualized effort to produce high-performance parallel DSLs through staging.

    Why control it? Higher kinded types in Scala lead to a Turing-complete type system, where compiler termination is no longer guaranteed. They tend to be useful mostly for type-level computation and for highly generic design patterns. The level of abstraction implied by these design patterns is often a barrier to understanding for newcomers to a Scala codebase. Some syntactic aspects of higher-kinded types are hard to understand for the uninitiated and type inference is less effective for them than for normal types. Because we are not completely happy with them yet, it is possible that some aspects of higher-kinded types will change in future versions of Scala. So an explicit enabling also serves as a warning that code involving higher-kinded types might have to be slightly revised in the future.

14.  implicit lazy val implicitConversions: implicitConversions

    Where this feature is enabled, definitions of implicit conversions are allowed.

    Where this feature is enabled, definitions of implicit conversions are allowed. If `implicitConversions` is not enabled, the definition of an implicit conversion will trigger a warning from the compiler.

    An implicit conversion is an implicit value of unary function type `A => B`, or an implicit method that has in its first parameter section a single, non-implicit parameter. Examples:

        implicit def stringToInt(s: String): Int = s.length
        implicit val conv = (s: String) => s.length
        implicit def listToX(xs: List[T])(implicit f: T => X): X = ...

    Implicit classes and implicit values of other types are not governed by this language feature.

    Why keep the feature? Implicit conversions are central to many aspects of Scala’s core libraries.

    Why control it? Implicit conversions are known to cause many pitfalls if over-used. And there is a tendency to over-use them because they look very powerful and their effects seem to be easy to understand. Also, in most situations using implicit parameters leads to a better design than implicit conversions.

15.  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
16.  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
17.  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

18.  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

19.  implicit lazy val postfixOps: postfixOps

    Only where this feature is enabled, is postfix operator notation `(expr op)` permitted.

    Only where this feature is enabled, is postfix operator notation `(expr op)` permitted. If `postfixOps` is not enabled, an expression using postfix notation is rejected by the compiler.

    Why keep the feature? Postfix notation is preserved for backward compatibility only. Historically, several DSLs written in Scala need the notation.

    Why control it? Postfix operators interact poorly with semicolon inference. Most programmers avoid them for this reason alone. Postfix syntax is associated with an abuse of infix notation, `a op1 b op2 c op3`, that can be harder to read than ordinary method invocation with judicious use of parentheses. It is recommended not to enable this feature except for legacy code.

20.  implicit lazy val reflectiveCalls: reflectiveCalls

    Where this feature is enabled, accesses to members of structural types that need reflection are supported.

    Where this feature is enabled, accesses to members of structural types that need reflection are supported. If `reflectiveCalls` is not enabled, an expression requiring reflection will trigger a warning from the compiler.

    A structural type is a type of the form `Parents { Decls }` where `Decls` contains declarations of new members that do not override any member in `Parents`. To access one of these members, a reflective call is needed.

    Why keep the feature? Structural types provide great flexibility because they avoid the need to define inheritance hierarchies a priori. Besides, their definition falls out quite naturally from Scala’s concept of type refinement.

    Why control it? Reflection is not available on all platforms. Popular tools such as ProGuard have problems dealing with it. Even where reflection is available, reflective dispatch can lead to surprising performance degradations.

21.  final def synchronized[T0](arg0: => T0): T0
    Definition Classes
      AnyRef
22.  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
23.  final def wait(): Unit
    Definition Classes
      AnyRef
    Annotations
      @throws(classOf[java.lang.InterruptedException])
24.  final def wait(arg0: Long, arg1: Int): Unit
    Definition Classes
      AnyRef
    Annotations
      @throws(classOf[java.lang.InterruptedException])
25.  final def wait(arg0: Long): Unit
    Definition Classes
      AnyRef
    Annotations
      @throws(classOf[java.lang.InterruptedException]) @native()
26.  object experimental

    The experimental object contains features that are known to have unstable API or behavior that may change in future releases.

    The experimental object contains features that are known to have unstable API or behavior that may change in future releases.

    Experimental features may undergo API changes in future releases, so production code should not rely on them.

    Programmers are encouraged to try out experimental features and report any bugs or API inconsistencies they encounter so they can be improved in future releases.

### Inherited from AnyRef

### Inherited from Any

### Language Features

### Experimental Language Features

### Ungrouped