Scala Standard Library2.13.2 < Back


# 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
-  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
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c

scala

# <:<

###
Companion object <:<

#### sealed abstract class <:<[-From, +To] extends (From) => To with Serializable

An instance of `A <:< B` witnesses that `A` is a subtype of `B`. Requiring an implicit argument of the type `A <:< B` encodes the generalized constraint `A <: B`.

To constrain any abstract type `T` that's in scope in a method's argument list (not just the method's own type parameters) simply add an implicit argument of type `T <:< U`, where `U` is the required upper bound; or for lower-bounds, use: `L <:< T`, where `L` is the required lower bound.

In case of any confusion over which method goes in what direction, all the "Co" methods (including apply) go from left to right in the type ("with" the type), and all the "Contra" methods go from right to left ("against" the type). E.g., apply turns a `From` into a `To`, and substituteContra replaces the `To`s in a type with `From`s.

In part contributed by Jason Zaugg.

From

  a type which is proved a subtype of `To`

To

  a type which is proved a supertype of `From`

Annotations
  @implicitNotFound(msg = "Cannot prove that ${From} <:< ${To}.")
Source
  typeConstraints.scala
Example:

1. scala.Option#flatten

       sealed trait Option[+A] {
         // def flatten[B, A <: Option[B]]: Option[B] = ...
         // won't work, since the A in flatten shadows the class-scoped A.
         def flatten[B](implicit ev: A <:< Option[B]): Option[B]
           = if(isEmpty) None else ev(get)
         // Because (A <:< Option[B]) <: (A => Option[B]), ev can be called to turn the
         // A from get into an Option[B], and because ev is implicit, that call can be
         // left out and inserted automatically.
       }

See also

  =:= for expressing equality constraints

Linear Supertypes
java.io.Serializable, (From) => To, AnyRef, Any
Known Subclasses
=:=
Type Hierarchy

Ordering

1. Alphabetic
2. By Inheritance

Inherited

1. <:<
2. Serializable
3. Function1
4. AnyRef
5. Any

Implicitly

1. by UnliftOps
2. by any2stringadd
3. by StringFormat
4. by Ensuring
5. by ArrowAssoc

1. Hide All
2. Show All

Visibility

1. Public
2. Protected

### Abstract Value Members

1.  abstract def substituteBoth[F[-_, +_]](ftf: F[To, From]): F[From, To]

   Substitute `To` for `From` and `From` for `To` in the type `F[To, From]`, given that `F` is contravariant in the first argument and covariant in the second.

   Substitute `To` for `From` and `From` for `To` in the type `F[To, From]`, given that `F` is contravariant in the first argument and covariant in the second. Essentially swaps `To` and `From` in `ftf`'s type.

   Equivalent in power to each of substituteCo and substituteContra.

   This method is impossible to implement without `throw`ing or otherwise "cheating" unless `From <: To`, so it ensures that this really represents a subtyping relationship.

   returns

     `ftf`, but with a (potentially) different type

### Concrete 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.  def +(other: String): String
   Implicit
     This member is added by an implicit conversion from <:<[From, To] toany2stringadd[<:<[From, To]] performed by method any2stringadd in scala.Predef.
   Definition Classes
     any2stringadd
4.  def ->[B](y: B): (<:<[From, To], B)
   Implicit
     This member is added by an implicit conversion from <:<[From, To] toArrowAssoc[<:<[From, To]] performed by method ArrowAssoc in scala.Predef.This conversion will take place only if To is a subclass of Option[Nothing] (To <: Option[Nothing]).
   Definition Classes
     ArrowAssoc
   Annotations
     @inline()
5.  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
6.  def andThen[C](r: <:<[To, C]): <:<[From, C]

   If `From <: To` and `To <: C`, then `From <: C` (subtyping is transitive)

7.  def andThen[C](r: (To) => C): (From) => C

   Composes two instances of Function1 in a new Function1, with this function applied first.

   Composes two instances of Function1 in a new Function1, with this function applied first.

   returns

     a new function `f` such that `f(x) == g(apply(x))`

   Definition Classes
     <:< → Function1
8.  def apply(f: From): To

   Coerce a `From` into a `To`.

   Coerce a `From` into a `To`. This is guaranteed to be the identity function.

   This method is often called implicitly as an implicit `A <:< B` doubles as an implicit view `A => B`.

   f

     some value of type `From`

   returns

     `f`, but with a (potentially) different type

   Definition Classes
     <:< → Function1
9.  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`.

10.  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(classOf[java.lang.CloneNotSupportedException]) @native()
    Note

      not specified by SLS as a member of AnyRef

11.  def compose[C](r: <:<[C, From]): <:<[C, To]

    If `From <: To` and `C <: From`, then `C <: To` (subtyping is transitive)

12.  def compose[C](r: (C) => From): (C) => To

    Composes two instances of Function1 in a new Function1, with this function applied last.

    Composes two instances of Function1 in a new Function1, with this function applied last.

    returns

      a new function `f` such that `f(x) == apply(g(x))`

    Definition Classes
      <:< → Function1
13.  def ensuring(cond: (<:<[From, To]) => Boolean, msg: => Any): <:<[From, To]
    Implicit
      This member is added by an implicit conversion from <:<[From, To] toEnsuring[<:<[From, To]] performed by method Ensuring in scala.Predef.
    Definition Classes
      Ensuring
14.  def ensuring(cond: (<:<[From, To]) => Boolean): <:<[From, To]
    Implicit
      This member is added by an implicit conversion from <:<[From, To] toEnsuring[<:<[From, To]] performed by method Ensuring in scala.Predef.
    Definition Classes
      Ensuring
15.  def ensuring(cond: Boolean, msg: => Any): <:<[From, To]
    Implicit
      This member is added by an implicit conversion from <:<[From, To] toEnsuring[<:<[From, To]] performed by method Ensuring in scala.Predef.
    Definition Classes
      Ensuring
16.  def ensuring(cond: Boolean): <:<[From, To]
    Implicit
      This member is added by an implicit conversion from <:<[From, To] toEnsuring[<:<[From, To]] performed by method Ensuring in scala.Predef.
    Definition Classes
      Ensuring
17.  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
18.  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
19.  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

20.  def formatted(fmtstr: String): String

    Returns string formatted according to given `format` string.

    Returns string formatted according to given `format` string. Format strings are as for `String.format` (@see java.lang.String.format).

    Implicit
      This member is added by an implicit conversion from <:<[From, To] toStringFormat[<:<[From, To]] performed by method StringFormat in scala.Predef.
    Definition Classes
      StringFormat
    Annotations
      @inline()
21.  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()
22.  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()
23.  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
24.  def liftCo[F[+_]]: <:<[F[From], F[To]]

    Lift this evidence over a covariant type constructor `F`.

25.  def liftContra[F[-_]]: <:<[F[To], F[From]]

    Lift this evidence over a contravariant type constructor `F`.

26.  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
27.  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

28.  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

29.  def substituteCo[F[+_]](ff: F[From]): F[To]

    Substitute the `From` in the type `F[From]`, where `F` is a covariant type constructor, for `To`.

    Substitute the `From` in the type `F[From]`, where `F` is a covariant type constructor, for `To`.

    Equivalent in power to each of substituteBoth and substituteContra.

    This method is impossible to implement without `throw`ing or otherwise "cheating" unless `From <: To`, so it ensures that this really represents a subtyping relationship.

    returns

      `ff`, but with a (potentially) different type

30.  def substituteContra[F[-_]](ft: F[To]): F[From]

    Substitute the `To` in the type `F[To]`, where `F` is a contravariant type constructor, for `From`.

    Substitute the `To` in the type `F[To]`, where `F` is a contravariant type constructor, for `From`.

    Equivalent in power to each of substituteBoth and substituteCo.

    This method is impossible to implement without `throw`ing or otherwise "cheating" unless `From <: To`, so it ensures that this really represents a subtyping relationship.

    returns

      `ft`, but with a (potentially) different type

31.  final def synchronized[T0](arg0: => T0): T0
    Definition Classes
      AnyRef
32.  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
      Function1 → AnyRef → Any
33.  def unlift: PartialFunction[From, B]

    Converts an optional function to a partial function.

    Converts an optional function to a partial function.

    Implicit
      This member is added by an implicit conversion from <:<[From, To] toUnliftOps[From, B] performed by method UnliftOps in scala.Function1.This conversion will take place only if To is a subclass of Option[B] (To <: Option[B]).
    Definition Classes
      UnliftOps
    Example:

    1. Unlike Function.unlift, this UnliftOps.unlift method can be used in extractors.

           val of: Int => Option[String] = { i =>
             if (i == 2) {
               Some("matched by an optional function")
             } else {
               None
             }
           }

           util.Random.nextInt(4) match {
             case of.unlift(m) => // Convert an optional function to a pattern
               println(m)
             case _ =>
               println("Not matched")
           }

34.  final def wait(): Unit
    Definition Classes
      AnyRef
    Annotations
      @throws(classOf[java.lang.InterruptedException])
35.  final def wait(arg0: Long, arg1: Int): Unit
    Definition Classes
      AnyRef
    Annotations
      @throws(classOf[java.lang.InterruptedException])
36.  final def wait(arg0: Long): Unit
    Definition Classes
      AnyRef
    Annotations
      @throws(classOf[java.lang.InterruptedException]) @native()

### Deprecated Value Members

1.  def →[B](y: B): (<:<[From, To], B)
   Implicit
     This member is added by an implicit conversion from <:<[From, To] toArrowAssoc[<:<[From, To]] performed by method ArrowAssoc in scala.Predef.This conversion will take place only if To is a subclass of Option[Nothing] (To <: Option[Nothing]).
   Definition Classes
     ArrowAssoc
   Annotations
     @deprecated
   Deprecated

     (Since version 2.13.0) Use `->` instead. If you still wish to display it as one character, consider using a font with programming ligatures such as Fira Code.

### Inherited from java.io.Serializable

### Inherited from (From) => To

### Inherited from AnyRef

### Inherited from Any

### Inherited by implicit conversion UnliftOps from<:<[From, To] to UnliftOps[From, B]

### Inherited by implicit conversion any2stringadd from<:<[From, To] to any2stringadd[<:<[From, To]]

### Inherited by implicit conversion StringFormat from<:<[From, To] to StringFormat[<:<[From, To]]

### Inherited by implicit conversion Ensuring from<:<[From, To] to Ensuring[<:<[From, To]]

### Inherited by implicit conversion ArrowAssoc from<:<[From, To] to ArrowAssoc[<:<[From, To]]

### Ungrouped