# The Scala Standard Library

The Scala standard library consists of the package `scala`

with a
number of classes and modules. Some of these classes are described in
the following.

## Root Classes

The root of this hierarchy is formed by class `Any`

.
Every class in a Scala execution environment inherits directly or
indirectly from this class. Class `Any`

has two direct
subclasses: `AnyRef`

and `AnyVal`

.

The subclass `AnyRef`

represents all values which are represented
as objects in the underlying host system. Classes written in other languages
inherit from `scala.AnyRef`

.

The predefined subclasses of class `AnyVal`

describe
values which are not implemented as objects in the underlying host
system.

User-defined Scala classes which do not explicitly inherit from
`AnyVal`

inherit directly or indirectly from `AnyRef`

. They can
not inherit from both `AnyRef`

and `AnyVal`

.

Classes `AnyRef`

and `AnyVal`

are required to provide only
the members declared in class `Any`

, but implementations may add
host-specific methods to these classes (for instance, an
implementation may identify class `AnyRef`

with its own root
class for objects).

The signatures of these root classes are described by the following definitions.

The type test `$x$.isInstanceOf[$T$]`

is equivalent to a typed
pattern match

where the type $T'$ is the same as $T$ except if $T$ is
of the form $D$ or $D[\mathit{tps}]$ where $D$ is a type member of some outer class $C$.
In this case $T'$ is `$C$#$D$`

(or `$C$#$D[tps]$`

, respectively), whereas $T$ itself would expand to `$C$.this.$D[tps]$`

.
In other words, an `isInstanceOf`

test does not check that types have the same enclosing instance.

The test `$x$.asInstanceOf[$T$]`

is treated specially if $T$ is a
numeric value type. In this case the cast will
be translated to an application of a conversion method
`x.to$T$`

. For non-numeric values $x$ the operation will raise a
`ClassCastException`

.

## Value Classes

Value classes are classes whose instances are not represented as
objects by the underlying host system. All value classes inherit from
class `AnyVal`

. Scala implementations need to provide the
value classes `Unit`

, `Boolean`

, `Double`

, `Float`

,
`Long`

, `Int`

, `Char`

, `Short`

, and `Byte`

(but are free to provide others as well).
The signatures of these classes are defined in the following.

### Numeric Value Types

Classes `Double`

, `Float`

,
`Long`

, `Int`

, `Char`

, `Short`

, and `Byte`

are together called *numeric value types*. Classes `Byte`

,
`Short`

, or `Char`

are called *subrange types*.
Subrange types, as well as `Int`

and `Long`

are called *integer types*, whereas `Float`

and `Double`

are called *floating point types*.

Numeric value types are ranked in the following partial order:

`Byte`

and `Short`

are the lowest-ranked types in this order,
whereas `Double`

is the highest-ranked. Ranking does *not*
imply a conformance relationship; for
instance `Int`

is not a subtype of `Long`

. However, object
`Predef`

defines views
from every numeric value type to all higher-ranked numeric value types.
Therefore, lower-ranked types are implicitly converted to higher-ranked types
when required by the context.

Given two numeric value types $S$ and $T$, the *operation type* of
$S$ and $T$ is defined as follows: If both $S$ and $T$ are subrange
types then the operation type of $S$ and $T$ is `Int`

. Otherwise
the operation type of $S$ and $T$ is the larger of the two types wrt
ranking. Given two numeric values $v$ and $w$ the operation type of
$v$ and $w$ is the operation type of their run-time types.

Any numeric value type $T$ supports the following methods.

- Comparison methods for equals (
`==`

), not-equals (`!=`

), less-than (`<`

), greater-than (`>`

), less-than-or-equals (`<=`

), greater-than-or-equals (`>=`

), which each exist in 7 overloaded alternatives. Each alternative takes a parameter of some numeric value type. Its result type is type`Boolean`

. The operation is evaluated by converting the receiver and its argument to their operation type and performing the given comparison operation of that type. - Arithmetic methods addition (
`+`

), subtraction (`-`

), multiplication (`*`

), division (`/`

), and remainder (`%`

), which each exist in 7 overloaded alternatives. Each alternative takes a parameter of some numeric value type $U$. Its result type is the operation type of $T$ and $U$. The operation is evaluated by converting the receiver and its argument to their operation type and performing the given arithmetic operation of that type. - Parameterless arithmetic methods identity (
`+`

) and negation (`-`

), with result type $T$. The first of these returns the receiver unchanged, whereas the second returns its negation. - Conversion methods
`toByte`

,`toShort`

,`toChar`

,`toInt`

,`toLong`

,`toFloat`

,`toDouble`

which convert the receiver object to the target type, using the rules of Java's numeric type cast operation. The conversion might truncate the numeric value (as when going from`Long`

to`Int`

or from`Int`

to`Byte`

) or it might lose precision (as when going from`Double`

to`Float`

or when converting between`Long`

and`Float`

).

Integer numeric value types support in addition the following operations:

Bit manipulation methods bitwise-and (

`&`

), bitwise-or {`|`

}, and bitwise-exclusive-or (`^`

), which each exist in 5 overloaded alternatives. Each alternative takes a parameter of some integer numeric value type. Its result type is the operation type of $T$ and $U$. The operation is evaluated by converting the receiver and its argument to their operation type and performing the given bitwise operation of that type.A parameterless bit-negation method (

`~`

). Its result type is the receiver type $T$ or`Int`

, whichever is larger. The operation is evaluated by converting the receiver to the result type and negating every bit in its value.Bit-shift methods left-shift (

`<<`

), arithmetic right-shift (`>>`

), and unsigned right-shift (`>>>`

). Each of these methods has two overloaded alternatives, which take a parameter $n$ of type`Int`

, respectively`Long`

. The result type of the operation is the receiver type $T$, or`Int`

, whichever is larger. The operation is evaluated by converting the receiver to the result type and performing the specified shift by $n$ bits.

Numeric value types also implement operations `equals`

,
`hashCode`

, and `toString`

from class `Any`

.

The `equals`

method tests whether the argument is a numeric value
type. If this is true, it will perform the `==`

operation which
is appropriate for that type. That is, the `equals`

method of a
numeric value type can be thought of being defined as follows:

The `hashCode`

method returns an integer hashcode that maps equal
numeric values to equal results. It is guaranteed to be the identity for
for type `Int`

and for all subrange types.

The `toString`

method displays its receiver as an integer or
floating point number.

###### Example

This is the signature of the numeric value type `Int`

:

### Class `Boolean`

Class `Boolean`

has only two values: `true`

and
`false`

. It implements operations as given in the following
class definition.

The class also implements operations `equals`

, `hashCode`

,
and `toString`

from class `Any`

.

The `equals`

method returns `true`

if the argument is the
same boolean value as the receiver, `false`

otherwise. The
`hashCode`

method returns a fixed, implementation-specific hash-code when invoked on `true`

,
and a different, fixed, implementation-specific hash-code when invoked on `false`

. The `toString`

method
returns the receiver converted to a string, i.e. either `"true"`

or `"false"`

.

### Class `Unit`

Class `Unit`

has only one value: `()`

. It implements only
the three methods `equals`

, `hashCode`

, and `toString`

from class `Any`

.

The `equals`

method returns `true`

if the argument is the
unit value `()`

, `false`

otherwise. The
`hashCode`

method returns a fixed, implementation-specific hash-code,
The `toString`

method returns `"()"`

.

## Standard Reference Classes

This section presents some standard Scala reference classes which are treated in a special way by the Scala compiler – either Scala provides syntactic sugar for them, or the Scala compiler generates special code for their operations. Other classes in the standard Scala library are documented in the Scala library documentation by HTML pages.

### Class `String`

Scala's `String`

class is usually derived from the standard String
class of the underlying host system (and may be identified with
it). For Scala clients the class is taken to support in each case a
method

which concatenates its left operand with the textual representation of its right operand.

### The `Tuple`

classes

Scala defines tuple classes `Tuple$n$`

for $n = 2 , \ldots , 22$.
These are defined as follows.

### The `Function`

Classes

Scala defines function classes `Function$n$`

for $n = 1 , \ldots , 22$.
These are defined as follows.

The `PartialFunction`

subclass of `Function1`

represents functions that (indirectly) specify their domain.
Use the `isDefined`

method to query whether the partial function is defined for a given input (i.e., whether the input is part of the function's domain).

The implicitly imported `Predef`

object defines the name
`Function`

as an alias of `Function1`

.

### Class `Array`

All operations on arrays desugar to the corresponding operations of the underlying platform. Therefore, the following class definition is given for informational purposes only:

If $T$ is not a type parameter or abstract type, the type `Array[T]`

is represented as the array type `|T|[]`

in the
underlying host system, where `|T|`

is the erasure of `T`

.
If $T$ is a type parameter or abstract type, a different representation might be
used (it is `Object`

on the Java platform).

#### Operations

`length`

returns the length of the array, `apply`

means subscripting,
and `update`

means element update.

Because of the syntactic sugar for `apply`

and `update`

operations,
we have the following correspondences between Scala and Java code for
operations on an array `xs`

:

Scala |
Java |
---|---|

`xs.length` |
`xs.length` |

`xs(i)` |
`xs[i]` |

`xs(i) = e` |
`xs[i] = e` |

Two implicit conversions exist in `Predef`

that are frequently applied to arrays:
a conversion to `scala.collection.mutable.ArrayOps`

and a conversion to
`scala.collection.mutable.ArraySeq`

(a subtype of `scala.collection.Seq`

).

Both types make many of the standard operations found in the Scala
collections API available. The conversion to `ArrayOps`

is temporary, as all operations
defined on `ArrayOps`

return a value of type `Array`

, while the conversion to `ArraySeq`

is permanent as all operations return a value of type `ArraySeq`

.
The conversion to `ArrayOps`

takes priority over the conversion to `ArraySeq`

.

Because of the tension between parametrized types in Scala and the ad-hoc implementation of arrays in the host-languages, some subtle points need to be taken into account when dealing with arrays. These are explained in the following.

#### Variance

Unlike arrays in Java, arrays in Scala are *not*
co-variant; That is, $S <: T$ does not imply
`Array[$S$] $<:$ Array[$T$]`

in Scala.
However, it is possible to cast an array
of $S$ to an array of $T$ if such a cast is permitted in the host
environment.

For instance `Array[String]`

does not conform to
`Array[Object]`

, even though `String`

conforms to `Object`

.
However, it is possible to cast an expression of type
`Array[String]`

to `Array[Object]`

, and this
cast will succeed without raising a `ClassCastException`

. Example:

The instantiation of an array with a polymorphic element type $T$ requires
information about type $T$ at runtime.
This information is synthesized by adding a context bound
of `scala.reflect.ClassTag`

to type $T$.
An example is the
following implementation of method `mkArray`

, which creates
an array of an arbitrary type $T$, given a sequence of $T$`s which
defines its elements:

If type $T$ is a type for which the host platform offers a specialized array representation, this representation is used.

###### Example

On the Java Virtual Machine, an invocation of `mkArray(List(1,2,3))`

will return a primitive array of `int`

s, written as `int[]`

in Java.

#### Companion object

`Array`

's companion object provides various factory methods for the
instantiation of single- and multi-dimensional arrays, an extractor method
`unapplySeq`

which enables pattern matching
over arrays and additional utility methods:

## Class Node

## The `Predef`

Object

The `Predef`

object defines standard functions and type aliases
for Scala programs. It is implicitly imported, as described in
the chapter on name binding,
so that all its defined members are available without qualification.
Its definition for the JVM environment conforms to the following signature:

### Predefined Implicit Definitions

The `Predef`

object also contains a number of implicit definitions, which are available by default (because `Predef`

is implicitly imported).
Implicit definitions come in two priorities. High-priority implicits are defined in the `Predef`

class itself whereas low priority implicits are defined in a class inherited by `Predef`

. The rules of
static overloading resolution
stipulate that, all other things being equal, implicit resolution
prefers high-priority implicits over low-priority ones.

The available low-priority implicits include definitions falling into the following categories.

For every primitive type, a wrapper that takes values of that type to instances of a

`runtime.Rich*`

class. For instance, values of type`Int`

can be implicitly converted to instances of class`runtime.RichInt`

.For every array type with elements of primitive type, a wrapper that takes the arrays of that type to instances of a

`ArraySeq`

class. For instance, values of type`Array[Float]`

can be implicitly converted to instances of class`ArraySeq[Float]`

. There are also generic array wrappers that take elements of type`Array[T]`

for arbitrary`T`

to`ArraySeq`

s.An implicit conversion from

`String`

to`WrappedString`

.

The available high-priority implicits include definitions falling into the following categories.

An implicit wrapper that adds

`ensuring`

methods with the following overloaded variants to type`Any`

.An implicit wrapper that adds a

`->`

method with the following implementation to type`Any`

.For every array type with elements of primitive type, a wrapper that takes the arrays of that type to instances of a

`runtime.ArrayOps`

class. For instance, values of type`Array[Float]`

can be implicitly converted to instances of class`runtime.ArrayOps[Float]`

. There are also generic array wrappers that take elements of type`Array[T]`

for arbitrary`T`

to`ArrayOps`

s.An implicit wrapper that adds

`+`

and`formatted`

method with the following implementations to type`Any`

.Numeric primitive conversions that implement the transitive closure of the following mappings:

Boxing and unboxing conversions between primitive types and their boxed versions:

An implicit definition that generates instances of type

`T <:< T`

, for any type`T`

. Here,`<:<`

is a class defined as follows.Implicit parameters of

`<:<`

types are typically used to implement type constraints.