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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 and scala.collection.immutable.Set.

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
Ordering
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Inherited
  1. Predef
  2. LowPriorityImplicits
  3. LowPriorityImplicits2
  4. AnyRef
  5. Any
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Visibility
  1. Public
  2. Protected

Type Members

  1. final class ArrayCharSequence extends CharSequence

  2. implicit final class ArrowAssoc[A] extends AnyVal

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

  4. implicit final class Ensuring[A] extends AnyVal

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

  6. type Manifest[T] = reflect.Manifest[T]
    Annotations
    @implicitNotFound()
  7. type Map[K, +V] = collection.immutable.Map[K, V]

  8. type OptManifest[T] = reflect.OptManifest[T]
  9. final class SeqCharSequence extends CharSequence

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

  11. 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.

    The String type in Scala has all the methods of the underlying java.lang.String, of which it is just an alias.

    In addition, extension methods in scala.collection.StringOps are added implicitly through the conversion augmentString.

  12. implicit final class StringFormat[A] extends AnyVal

Deprecated Type Members

  1. implicit final class any2stringadd[A] extends AnyVal

    Injects String concatenation operator + to any classes.

    Injects String concatenation operator + to any classes.

    Annotations
    @deprecated
    Deprecated

    (Since version 2.13.0) Implicit injection of + is deprecated. Convert to String to call +

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. 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.

    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).

  4. val ->: Tuple2.type

    Allows destructuring tuples with the same syntax as constructing them.

    Allows destructuring tuples with the same syntax as constructing them.

    Example:
    1. val tup = "foobar" -> 3
      
      val c = tup match {
        case str -> i => str.charAt(i)
      }
  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 ???: Nothing

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

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

    Exceptions thrown

    NotImplementedError when ??? is invoked.

  7. def ArrayCharSequence(arrayOfChars: Array[Char]): ArrayCharSequence

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

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

  10. implicit def Character2char(x: Character): Char

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

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

  13. implicit def Integer2int(x: Integer): Int

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

  15. val Manifest: reflect.Manifest.type
  16. val Map: collection.immutable.Map.type

  17. val NoManifest: reflect.NoManifest.type
  18. def SeqCharSequence(sequenceOfChars: collection.IndexedSeq[Char]): SeqCharSequence

  19. val Set: collection.immutable.Set.type

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

  21. 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.

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

    Tests an expression, throwing an AssertionError if false.

    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() @inline()
    See also

    elidable

  23. def assert(assertion: Boolean): Unit

    Tests an expression, throwing an AssertionError if false.

    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()
    See also

    elidable

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

    Tests an expression, throwing an AssertionError if false.

    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() @inline()
    See also

    elidable

  25. def assume(assumption: Boolean): Unit

    Tests an expression, throwing an AssertionError if false.

    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()
    See also

    elidable

  26. implicit def augmentString(x: String): StringOps

    Annotations
    @inline()
  27. implicit def boolean2Boolean(x: Boolean): java.lang.Boolean

  28. implicit def booleanArrayOps(xs: Array[Boolean]): ArrayOps[Boolean]
    Annotations
    @inline()
  29. implicit def booleanWrapper(x: Boolean): RichBoolean
    Definition Classes
    LowPriorityImplicits
    Annotations
    @inline()
  30. implicit def byte2Byte(x: Byte): java.lang.Byte

  31. implicit def byteArrayOps(xs: Array[Byte]): ArrayOps[Byte]
    Annotations
    @inline()
  32. implicit def byteWrapper(x: Byte): RichByte

    We prefer the java.lang.* boxed types to these wrappers in any potential conflicts.

    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()
  33. implicit def char2Character(x: Char): Character

  34. implicit def charArrayOps(xs: Array[Char]): ArrayOps[Char]
    Annotations
    @inline()
  35. implicit def charWrapper(c: Char): RichChar
    Definition Classes
    LowPriorityImplicits
    Annotations
    @inline()
  36. def classOf[T]: Class[T]

    Retrieve the runtime representation of a class type.

    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:
    1. 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
  37. 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

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

  39. implicit def doubleArrayOps(xs: Array[Double]): ArrayOps[Double]
    Annotations
    @inline()
  40. implicit def doubleWrapper(x: Double): RichDouble
    Definition Classes
    LowPriorityImplicits
    Annotations
    @inline()
  41. 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
  42. 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
  43. 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

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

  45. implicit def floatArrayOps(xs: Array[Float]): ArrayOps[Float]
    Annotations
    @inline()
  46. implicit def floatWrapper(x: Float): RichFloat
    Definition Classes
    LowPriorityImplicits
    Annotations
    @inline()
  47. implicit def genericArrayOps[T](xs: Array[T]): ArrayOps[T]
    Annotations
    @inline()
  48. implicit def genericWrapArray[T](xs: Array[T]): ArraySeq[T]

    Definition Classes
    LowPriorityImplicits
  49. final def getClass(): java.lang.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()
  50. 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()
  51. def identity[A](x: A): A

    A method that returns its input value.

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

    Summon an implicit value of type 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()
  53. implicit def int2Integer(x: Int): Integer

  54. implicit def intArrayOps(xs: Array[Int]): ArrayOps[Int]
    Annotations
    @inline()
  55. implicit def intWrapper(x: Int): RichInt
    Definition Classes
    LowPriorityImplicits
    Annotations
    @inline()
  56. final def isInstanceOf[T0]: Boolean

    Test whether the dynamic type of the receiver object has the same erasure as T0.

    Test whether the dynamic type of the receiver object has the same erasure as T0.

    Depending on what T0 is, the test is done in one of the below ways:

    • T0 is a non-parameterized class type, e.g. BigDecimal: this method returns true if the value of the receiver object is a BigDecimal or a subtype of BigDecimal.
    • T0 is a parameterized class type, e.g. List[Int]: this method returns true if the value of the receiver object is some List[X] for any X. For example, List(1, 2, 3).isInstanceOf[List[String]] will return true.
    • T0 is some singleton type x.type or literal x: this method returns this.eq(x). For example, x.isInstanceOf[1] is equivalent to x.eq(1)
    • T0 is an intersection X with Y or X & Y: this method is equivalent to x.isInstanceOf[X] && x.isInstanceOf[Y]
    • T0 is a union X | Y: this method is equivalent to x.isInstanceOf[X] || x.isInstanceOf[Y]
    • T0 is a type parameter or an abstract type member: this method is equivalent to isInstanceOf[U] where U is T0's upper bound, Any if T0 is unbounded. For example, x.isInstanceOf[A] where A is an unbounded type parameter will return true for any value of x.

    This is exactly equivalent to the type pattern _: T0

    returns

    true if the receiver object is an instance of erasure of type T0; false otherwise.

    Definition Classes
    Any
    Note

    due to the unexpectedness of List(1, 2, 3).isInstanceOf[List[String]] returning true and x.isInstanceOf[A] where A is a type parameter or abstract member returning true, these forms issue a warning.

  57. 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.

    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:
    1. 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
  58. implicit def long2Long(x: Long): java.lang.Long

  59. implicit def longArrayOps(xs: Array[Long]): ArrayOps[Long]
    Annotations
    @inline()
  60. implicit def longWrapper(x: Long): RichLong
    Definition Classes
    LowPriorityImplicits
    Annotations
    @inline()
  61. def manifest[T](implicit m: Manifest[T]): Manifest[T]
  62. 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
  63. 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

  64. 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

  65. def optManifest[T](implicit m: OptManifest[T]): OptManifest[T]
  66. def print(x: Any): Unit

    Prints an object to out using its toString method.

    Prints an object to out using its toString method.

    x

    the object to print; may be null.

  67. 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).

    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.

    xs

    the arguments used to instantiate the pattern.

    Exceptions thrown

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

    See also

    StringContext.f

  68. def println(x: Any): Unit

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

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

    x

    the object to print.

  69. def println(): Unit

    Prints a newline character on the default output.

  70. implicit def refArrayOps[T <: AnyRef](xs: Array[T]): ArrayOps[T]
    Annotations
    @inline()
  71. final def require(requirement: Boolean, message: => Any): Unit

    Tests an expression, throwing an IllegalArgumentException if false.

    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()
  72. def require(requirement: Boolean): Unit

    Tests an expression, throwing an IllegalArgumentException if false.

    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

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

  74. implicit def shortArrayOps(xs: Array[Short]): ArrayOps[Short]
    Annotations
    @inline()
  75. implicit def shortWrapper(x: Short): RichShort
    Definition Classes
    LowPriorityImplicits
    Annotations
    @inline()
  76. final def synchronized[T0](arg0: => T0): T0

    Executes the code in body with an exclusive lock on this.

    Executes the code in body with an exclusive lock on this.

    returns

    the result of body

    Definition Classes
    AnyRef
  77. def toString(): java.lang.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
  78. implicit def tuple2ToZippedOps[T1, T2](x: (T1, T2)): Ops[T1, T2]
  79. implicit def tuple3ToZippedOps[T1, T2, T3](x: (T1, T2, T3)): Ops[T1, T2, T3]
  80. implicit def unitArrayOps(xs: Array[Unit]): ArrayOps[Unit]
    Annotations
    @inline()
  81. def valueOf[T](implicit vt: ValueOf[T]): T

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

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

    Annotations
    @inline()
    Example:
    1. object Foo
      val foo = valueOf[Foo.type]
      // foo is Foo.type = Foo
      
      val bar = valueOf[23]
      // bar is 23.type = 23
  82. final def wait(): Unit

    See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait--.

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

    not specified by SLS as a member of AnyRef

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

    See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait-long-int-

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

    not specified by SLS as a member of AnyRef

  84. final def wait(arg0: Long): Unit

    See https://docs.oracle.com/javase/8/docs/api/java/lang/Object.html#wait-long-.

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

    not specified by SLS as a member of AnyRef

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

    Definition Classes
    LowPriorityImplicits
  86. implicit def wrapByteArray(xs: Array[Byte]): ofByte

    Definition Classes
    LowPriorityImplicits
  87. implicit def wrapCharArray(xs: Array[Char]): ofChar

    Definition Classes
    LowPriorityImplicits
  88. implicit def wrapDoubleArray(xs: Array[Double]): ofDouble

    Definition Classes
    LowPriorityImplicits
  89. implicit def wrapFloatArray(xs: Array[Float]): ofFloat

    Definition Classes
    LowPriorityImplicits
  90. implicit def wrapIntArray(xs: Array[Int]): ofInt

    Definition Classes
    LowPriorityImplicits
  91. implicit def wrapLongArray(xs: Array[Long]): ofLong

    Definition Classes
    LowPriorityImplicits
  92. implicit def wrapRefArray[T <: AnyRef](xs: Array[T]): ofRef[T]

    Definition Classes
    LowPriorityImplicits
  93. implicit def wrapShortArray(xs: Array[Short]): ofShort

    Definition Classes
    LowPriorityImplicits
  94. implicit def wrapString(s: String): WrappedString

    Definition Classes
    LowPriorityImplicits
  95. implicit def wrapUnitArray(xs: Array[Unit]): ofUnit

    Definition Classes
    LowPriorityImplicits

Deprecated Value Members

  1. implicit def copyArrayToImmutableIndexedSeq[T](xs: Array[T]): IndexedSeq[T]
    Definition Classes
    LowPriorityImplicits2
    Annotations
    @deprecated
    Deprecated

    (Since version 2.13.0) Implicit conversions from Array to immutable.IndexedSeq are implemented by copying; Use the more efficient non-copying ArraySeq.unsafeWrapArray or an explicit toIndexedSeq call

Inherited from LowPriorityImplicits

Inherited from LowPriorityImplicits2

Inherited from AnyRef

Inherited from Any

Utility Methods

Assertions

These methods support program verification and runtime correctness.

Console Output

These methods provide output via the console.

Aliases

These aliases bring selected immutable types into scope without any imports.

String Conversions

Conversions from String to StringOps or WrappedString.

Implicit Classes

These implicit classes add useful extension methods to every type.

CharSequence Wrappers

Wrappers that implements CharSequence and were implicit classes.

Java to Scala

Implicit conversion from Java primitive wrapper types to Scala equivalents.

Scala to Java

Implicit conversion from Scala AnyVals to Java primitive wrapper types equivalents.

Array to ArraySeq

Conversions from Arrays to ArraySeqs.

Ungrouped