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/Scala 2.13 Library

Object scala.Predef

object Predef extends LowPriorityImplicits

The Predef object provides definitions that are accessible in all Scala compilation units without explicit qualification.

Commonly Used Types

Predef provides type aliases for types which are commonly used, such as the immutable collection types scala.collection.immutable.Map, scala.collection.immutable.Set, and the scala.collection.immutable.List constructors (scala.collection.immutable.:: and scala.collection.immutable.Nil).

Console Output

For basic console output, Predef provides convenience methods print and println, which are aliases of the methods in the object scala.Console.

Assertions

A set of assert functions are provided for use as a way to document and dynamically check invariants in code. Invocations of assert can be elided at compile time by providing the command line option -Xdisable-assertions, which raises -Xelide-below above elidable.ASSERTION, to the scalac command.

Variants of assert intended for use with static analysis tools are also provided: assume, require and ensuring. require and ensuring are intended for use as a means of design-by-contract style specification of pre- and post-conditions on functions, with the intention that these specifications could be consumed by a static analysis tool. For instance,

def addNaturals(nats: List[Int]): Int = {
  require(nats forall (_ >= 0), "List contains negative numbers")
  nats.foldLeft(0)(_ + _)
} ensuring(_ >= 0)

The declaration of addNaturals states that the list of integers passed should only contain natural numbers (i.e. non-negative), and that the result returned will also be natural. require is distinct from assert in that if the condition fails, then the caller of the function is to blame rather than a logical error having been made within addNaturals itself. ensuring is a form of assert that declares the guarantee the function is providing with regards to its return value.

Implicit Conversions

A number of commonly applied implicit conversions are also defined here, and in the parent type scala.LowPriorityImplicits. Implicit conversions are provided for the "widening" of numeric values, for instance, converting a Short value to a Long value as required, and to add additional higher-order functions to Array values. These are described in more detail in the documentation of scala.Array.

Source
Predef.scala
Linear Supertypes
LowPriorityImplicits, LowPriorityImplicits2, AnyRef, Any

Type Members

implicit final class ArrayCharSequence extends CharSequence

implicit final class ArrowAssoc[A] extends AnyVal

type Class[T] = java.lang.Class[T]

implicit final class Ensuring[A] extends AnyVal

type Function[-A, +B] = (A) => B

type Manifest[T] = reflect.Manifest[T]

Annotations
@implicitNotFound("No Manifest available for ${T}.")

type Map[K, +V] = collection.immutable.Map[K, V]

type OptManifest[T] = reflect.OptManifest[T]

implicit final class SeqCharSequence extends CharSequence

type Set[A] = collection.immutable.Set[A]

type String = java.lang.String

The String type in Scala has all the methods of the underlying java.lang.String, of which it is just an alias. (See the documentation corresponding to your Java version, for example http://docs.oracle.com/javase/8/docs/api/java/lang/String.html.) In addition, extension methods in scala.collection.StringOps are added implicitly through the conversion augmentString.

implicit final class StringFormat[A] extends AnyVal

Value Members

final def !=(arg0: Any): Boolean

Test two objects for inequality.

returns

true if !(this == that), false otherwise.

Definition Classes
AnyRef → Any

final def ##(): Int

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

implicit def $conforms[A]: (A) => A

An implicit of type A => A is available for all A because it can always be implemented using the identity function. This also means that an implicit of type A => B is always available when A <: B, because (A => A) <: (A => B).

val ->: Tuple2.type

Allows destructuring tuples with the same syntax as constructing them.

Example:
    val tup = "foobar" -> 3
    
    val c = tup match {
      case str -> i => str.charAt(i)
    }

final def ==(arg0: Any): Boolean

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

def ???: Nothing

??? can be used for marking methods that remain to be implemented.

Exceptions thrown

NotImplementedError when ??? is invoked.

implicit def Boolean2boolean(x: java.lang.Boolean): Boolean

implicit def Byte2byte(x: java.lang.Byte): Byte

implicit def Character2char(x: Character): Char

implicit def Double2double(x: java.lang.Double): Double

implicit def Float2float(x: java.lang.Float): Float

implicit def Integer2int(x: Integer): Int

implicit def Long2long(x: java.lang.Long): Long

val Manifest: reflect.Manifest.type

val Map: collection.immutable.Map.type

val NoManifest: reflect.NoManifest.type

val Set: collection.immutable.Set.type

implicit def Short2short(x: java.lang.Short): Short

final def asInstanceOf[T0]: 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.

final def assert(assertion: Boolean, message: => Any): Unit

Tests an expression, throwing an AssertionError if false. Calls to this method will not be generated if -Xelide-below is greater than ASSERTION.

assertion

the expression to test

message

a String to include in the failure message

Annotations
@elidable(ASSERTION) @inline()
See also

elidable

def assert(assertion: Boolean): Unit

Tests an expression, throwing an AssertionError if false. Calls to this method will not be generated if -Xelide-below is greater than ASSERTION.

assertion

the expression to test

Annotations
@elidable(ASSERTION)
See also

elidable

final def assume(assumption: Boolean, message: => Any): Unit

Tests an expression, throwing an AssertionError if false. This method differs from assert only in the intent expressed: assert contains a predicate which needs to be proven, while assume contains an axiom for a static checker. Calls to this method will not be generated if -Xelide-below is greater than ASSERTION.

assumption

the expression to test

message

a String to include in the failure message

Annotations
@elidable(ASSERTION) @inline()
See also

elidable

def assume(assumption: Boolean): Unit

Tests an expression, throwing an AssertionError if false. This method differs from assert only in the intent expressed: assert contains a predicate which needs to be proven, while assume contains an axiom for a static checker. Calls to this method will not be generated if -Xelide-below is greater than ASSERTION.

assumption

the expression to test

Annotations
@elidable(ASSERTION)
See also

elidable

implicit def augmentString(x: String): StringOps

Annotations
@inline()

implicit def boolean2Boolean(x: Boolean): java.lang.Boolean

implicit def booleanArrayOps(xs: Array[Boolean]): ArrayOps[Boolean]

Annotations
@inline()

implicit def booleanWrapper(x: Boolean): RichBoolean

Definition Classes
LowPriorityImplicits
Annotations
@inline()

implicit def byte2Byte(x: Byte): java.lang.Byte

implicit def byteArrayOps(xs: Array[Byte]): ArrayOps[Byte]

Annotations
@inline()

implicit def byteWrapper(x: Byte): RichByte

We prefer the java.lang.* boxed types to these wrappers in any potential conflicts. Conflicts do exist because the wrappers need to implement ScalaNumber in order to have a symmetric equals method, but that implies implementing java.lang.Number as well.

Note - these are inlined because they are value classes, but the call to xxxWrapper is not eliminated even though it does nothing. Even inlined, every call site does a no-op retrieval of Predef's MODULE$ because maybe loading Predef has side effects!

Definition Classes
LowPriorityImplicits
Annotations
@inline()

implicit def char2Character(x: Char): Character

implicit def charArrayOps(xs: Array[Char]): ArrayOps[Char]

Annotations
@inline()

implicit def charWrapper(c: Char): RichChar

Definition Classes
LowPriorityImplicits
Annotations
@inline()

def classOf[T]: Class[T]

Retrieve the runtime representation of a class type. classOf[T] is equivalent to the class literal T.class in Java.

returns

The runtime Class representation of type T.

Example:
    val listClass = classOf[List[_]]
    // listClass is java.lang.Class[List[_]] = class scala.collection.immutable.List
    
    val mapIntString = classOf[Map[Int,String]]
    // mapIntString is java.lang.Class[Map[Int,String]] = interface scala.collection.immutable.Map

def clone(): AnyRef

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

implicit def double2Double(x: Double): java.lang.Double

implicit def doubleArrayOps(xs: Array[Double]): ArrayOps[Double]

Annotations
@inline()

implicit def doubleWrapper(x: Double): RichDouble

Definition Classes
LowPriorityImplicits
Annotations
@inline()

final def eq(arg0: AnyRef): Boolean

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

def equals(arg0: AnyRef): Boolean

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

def finalize(): Unit

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

implicit def float2Float(x: Float): java.lang.Float

implicit def floatArrayOps(xs: Array[Float]): ArrayOps[Float]

Annotations
@inline()

implicit def floatWrapper(x: Float): RichFloat

Definition Classes
LowPriorityImplicits
Annotations
@inline()

implicit def genericArrayOps[T](xs: Array[T]): ArrayOps[T]

Annotations
@inline()

implicit def genericWrapArray[T](xs: Array[T]): ArraySeq[T]

Definition Classes
LowPriorityImplicits

final def getClass(): java.lang.Class[_ <: AnyRef]

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()

def hashCode(): Int

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()

def identity[A](x: A): A

A method that returns its input value.

A

type of the input value x.

x

the value of type A to be returned.

returns

the value x.

Annotations
@inline()

def implicitly[T](implicit e: T): T

Summon an implicit value of type T. Usually, the argument is not passed explicitly.

T

the type of the value to be summoned

returns

the implicit value of type T

Annotations
@inline()

implicit def int2Integer(x: Int): Integer

implicit def intArrayOps(xs: Array[Int]): ArrayOps[Int]

Annotations
@inline()

implicit def intWrapper(x: Int): RichInt

Definition Classes
LowPriorityImplicits
Annotations
@inline()

final def isInstanceOf[T0]: Boolean

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 locally[T](x: T): T

Used to mark code blocks as being expressions, instead of being taken as part of anonymous classes and the like. This is just a different name for identity.

Annotations
@inline()
Example:

    Separating code blocks from new:

    val x = new AnyRef
    {
      val y = ...
      println(y)
    }
    // the { ... } block is seen as the body of an anonymous class
    
    val x = new AnyRef
    
    {
      val y = ...
      println(y)
    }
    // an empty line is a brittle "fix"
    
    val x = new AnyRef
    locally {
      val y = ...
      println(y)
    }
    // locally guards the block and helps communicate intent

implicit def long2Long(x: Long): java.lang.Long

implicit def longArrayOps(xs: Array[Long]): ArrayOps[Long]

Annotations
@inline()

implicit def longWrapper(x: Long): RichLong

Definition Classes
LowPriorityImplicits
Annotations
@inline()

def manifest[T](implicit m: Manifest[T]): Manifest[T]

final def ne(arg0: AnyRef): Boolean

Equivalent to !(this eq that).

returns

true if the argument is not a reference to the receiver object; false otherwise.

Definition Classes
AnyRef

final def notify(): Unit

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

final def notifyAll(): Unit

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

def optManifest[T](implicit m: OptManifest[T]): OptManifest[T]

def print(x: Any): Unit

Prints an object to out using its toString method.

x

the object to print; may be null.

def printf(text: String, xs: Any*): Unit

Prints its arguments as a formatted string to the default output, based on a string pattern (in a fashion similar to printf in C).

The interpretation of the formatting patterns is described in java.util.Formatter.

Consider using the f interpolator as more type safe and idiomatic.

text

the pattern for formatting the arguments.

Exceptions thrown

java.lang.IllegalArgumentException if there was a problem with the format string or arguments

See also

StringContext.f

def println(x: Any): Unit

Prints out an object to the default output, followed by a newline character.

x

the object to print.

def println(): Unit

implicit def refArrayOps[T <: AnyRef](xs: Array[T]): ArrayOps[T]

Annotations
@inline()

final def require(requirement: Boolean, message: => Any): Unit

Tests an expression, throwing an IllegalArgumentException if false. This method is similar to assert, but blames the caller of the method for violating the condition.

requirement

the expression to test

message

a String to include in the failure message

Annotations
@inline()

def require(requirement: Boolean): Unit

Tests an expression, throwing an IllegalArgumentException if false. This method is similar to assert, but blames the caller of the method for violating the condition.

requirement

the expression to test

implicit def short2Short(x: Short): java.lang.Short

implicit def shortArrayOps(xs: Array[Short]): ArrayOps[Short]

Annotations
@inline()

implicit def shortWrapper(x: Short): RichShort

Definition Classes
LowPriorityImplicits
Annotations
@inline()

final def synchronized[T0](arg0: => T0): T0

Definition Classes
AnyRef

def toString(): java.lang.String

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

implicit def unitArrayOps(xs: Array[Unit]): ArrayOps[Unit]

Annotations
@inline()

def valueOf[T](implicit vt: ValueOf[T]): T

Retrieve the single value of a type with a unique inhabitant.

Annotations
@inline()
Example:
    object Foo
    val foo = valueOf[Foo.type]
    // foo is Foo.type = Foo
    
    val bar = valueOf[23]
    // bar is 23.type = 23

final def wait(): Unit

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

final def wait(arg0: Long, arg1: Int): Unit

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

final def wait(arg0: Long): Unit

Definition Classes
AnyRef
Annotations
@throws(classOf[java.lang.InterruptedException]) @native()

implicit def wrapBooleanArray(xs: Array[Boolean]): ofBoolean

Definition Classes
LowPriorityImplicits

implicit def wrapByteArray(xs: Array[Byte]): ofByte

Definition Classes
LowPriorityImplicits

implicit def wrapCharArray(xs: Array[Char]): ofChar

Definition Classes
LowPriorityImplicits

implicit def wrapDoubleArray(xs: Array[Double]): ofDouble

Definition Classes
LowPriorityImplicits

implicit def wrapFloatArray(xs: Array[Float]): ofFloat

Definition Classes
LowPriorityImplicits

implicit def wrapIntArray(xs: Array[Int]): ofInt

Definition Classes
LowPriorityImplicits

implicit def wrapLongArray(xs: Array[Long]): ofLong

Definition Classes
LowPriorityImplicits

implicit def wrapRefArray[T <: AnyRef](xs: Array[T]): ofRef[T]

Definition Classes
LowPriorityImplicits

implicit def wrapShortArray(xs: Array[Short]): ofShort

Definition Classes
LowPriorityImplicits

implicit def wrapString(s: String): WrappedString

Definition Classes
LowPriorityImplicits

implicit def wrapUnitArray(xs: Array[Unit]): ofUnit

Definition Classes
LowPriorityImplicits

© 2002-2019 EPFL, with contributions from Lightbend.
Licensed under the Apache License, Version 2.0.
https://www.scala-lang.org/api/2.13.0/scala/Predef$.html