Packages

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.

Guide

A more detailed guide to Futures and Promises, including discussion and examples can be found at https://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 words = Files.readAllLines("/etc/dictionaries-common/words").asScala
  words.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

The jdk package contains utilities to interact with JDK classes.

This packages offers a number of converters, that are able to wrap or copy types from the scala library to equivalent types in the JDK class library and vice versa:

CollectionConverters, converting collections like scala.collection.Seq, scala.collection.Map, scala.collection.Set, scala.collection.mutable.Buffer, scala.collection.Iterator and scala.collection.Iterable to their JDK counterparts
OptionConverters, converting between Option and java.util.Optional and primitive variations
StreamConverters, to create JDK Streams from scala collections
DurationConverters, for conversions between scala scala.concurrent.duration.FiniteDuration and java.time.Duration
FunctionConverters, from scala Functions to java java.util.function.Function, java.util.function.UnaryOperator, java.util.function.Consumer and java.util.function.Predicate, as well as primitive variations and Bi-variations.

By convention, converters that wrap an object to provide a different interface to the same underlying data structure use .asScala and .asJava extension methods, whereas converters that copy the underlying data structure use .toScala and .toJava.

In the javaapi package, the same converters can be found with a java-friendly interface that don't rely on implicit enrichments.

Additionally, this package offers Accumulators, capable of efficiently traversing JDK Streams.

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.

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.

Definition Classes: scala

package util

Definition Classes: scala

#::

<:<

=:=

Any

App

Int

ScalaReflectionException

UninitializedFieldError

Unit

ValueOf

deprecated

deprecatedInheritance

ValueOf

final class ValueOf[T] extends AnyVal

ValueOf[T] provides the unique value of the type T where T is a type which has a single inhabitant. Eligible types are singleton types of the form stablePath.type, Unit and singleton types corresponding to value literals.

Instances of ValueOf[T] are provided implicitly for all eligible types. Typically an instance would be required where a runtime value corresponding to a type level computation is needed.

For example, we might define a type Residue[M <: Int] corresponding to the group of integers modulo M. We could then mandate that residues can be summed only when they are parameterized by the same modulus,

case class Residue[M <: Int](n: Int) extends AnyVal {
  def +(rhs: Residue[M])(implicit m: ValueOf[M]): Residue[M] =
    Residue((this.n + rhs.n) % valueOf[M])
}

val fiveModTen = Residue[10](5)
val nineModTen = Residue[10](9)

fiveModTen + nineModTen    // OK == Residue[10](4)

val fourModEleven = Residue[11](4)

fiveModTen + fourModEleven // compiler error: type mismatch;
                           //   found   : Residue[11]
                           //   required: Residue[10]

Notice that here the modulus is encoded in the type of the values and so does not incur any additional per-value storage cost. When a runtime value of the modulus is required in the implementation of + it is provided at the call site via the implicit argument m of type ValueOf[M].

Annotations: @implicitNotFound()
Source: ValueOf.scala

Linear Supertypes

AnyVal, Any

Type Hierarchy

Ordering

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ValueOf
AnyVal
Any

Implicitly

by any2stringadd
by StringFormat
by Ensuring
by ArrowAssoc

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Protected

Instance Constructors

new ValueOf(value: T)

Value Members

final def !=(arg0: Any): Boolean
Test two objects for inequality.
Test two objects for inequality.
returns
true if !(this == that), false otherwise.
Definition Classes
Any
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
Any
def +(other: String): String
Implicit
This member is added by an implicit conversion from ValueOf[T] toany2stringadd[ValueOf[T]] performed by method any2stringadd in scala.Predef.
Definition Classes
any2stringadd
def ->[B](y: B): (ValueOf[T], B)
Implicit
This member is added by an implicit conversion from ValueOf[T] toArrowAssoc[ValueOf[T]] performed by method ArrowAssoc in scala.Predef.
Definition Classes
ArrowAssoc
Annotations
@inline()
final def ==(arg0: Any): Boolean
Test two objects for equality.
Test two objects for equality. 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
Any
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.
def ensuring(cond: (ValueOf[T]) => Boolean, msg: => Any): ValueOf[T]
Implicit
This member is added by an implicit conversion from ValueOf[T] toEnsuring[ValueOf[T]] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
def ensuring(cond: (ValueOf[T]) => Boolean): ValueOf[T]
Implicit
This member is added by an implicit conversion from ValueOf[T] toEnsuring[ValueOf[T]] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
def ensuring(cond: Boolean, msg: => Any): ValueOf[T]
Implicit
This member is added by an implicit conversion from ValueOf[T] toEnsuring[ValueOf[T]] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
def ensuring(cond: Boolean): ValueOf[T]
Implicit
This member is added by an implicit conversion from ValueOf[T] toEnsuring[ValueOf[T]] performed by method Ensuring in scala.Predef.
Definition Classes
Ensuring
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 ValueOf[T] toStringFormat[ValueOf[T]] performed by method StringFormat in scala.Predef.
Definition Classes
StringFormat
Annotations
@inline()
def getClass(): Class[_ <: AnyVal]
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
AnyVal → Any
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
def toString(): String
Returns a string representation of the object.
Returns a string representation of the object.
The default representation is platform dependent.
returns
a string representation of the object.
Definition Classes
Any
val value: T

Deprecated Value Members

def →[B](y: B): (ValueOf[T], B)
Implicit
This member is added by an implicit conversion from ValueOf[T] toArrowAssoc[ValueOf[T]] performed by method ArrowAssoc in scala.Predef.
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.

Packages

Package structure

Automatic imports

Guide

Common Imports

Specifying Durations

Using Futures For Non-blocking Computation

Avoid Blocking

ValueOf

final class ValueOf[T] extends AnyVal

Instance Constructors

Value Members

Deprecated Value Members

Inherited from AnyVal

Inherited from Any

Inherited by implicit conversion any2stringadd fromValueOf[T] to any2stringadd[ValueOf[T]]

Inherited by implicit conversion StringFormat fromValueOf[T] to StringFormat[ValueOf[T]]

Inherited by implicit conversion Ensuring fromValueOf[T] to Ensuring[ValueOf[T]]

Inherited by implicit conversion ArrowAssoc fromValueOf[T] to ArrowAssoc[ValueOf[T]]

Ungrouped

Packages

Package structure

Automatic imports

Guide

Common Imports

Specifying Durations

Using Futures For Non-blocking Computation

Avoid Blocking

ValueOf

final class ValueOf[T] extends AnyVal

Instance Constructors

Value Members

Deprecated Value Members

Inherited from AnyVal

Inherited from Any

Inherited by implicit conversion any2stringadd fromValueOf[T] to any2stringadd[ValueOf[T]]

Inherited by implicit conversion StringFormat fromValueOf[T] to StringFormat[ValueOf[T]]

Inherited by implicit conversion Ensuring fromValueOf[T] to Ensuring[ValueOf[T]]

Inherited by implicit conversion ArrowAssoc fromValueOf[T] to ArrowAssoc[ValueOf[T]]

Ungrouped

ValueOf