object language
The scala.language object controls the language features available to the programmer, as proposed in the
SIP-18 document.
Each of these features has to be explicitly imported into the current scope to become available:
import language.postfixOps // or language._ List(1, 2, 3) reverse
The language features are:
dynamicsenables defining calls rewriting using theDynamictraitexistentialsenables writing existential typeshigherKindsenables writing higher-kinded typesimplicitConversionsenables defining implicit methods and memberspostfixOpsenables postfix operators (not recommended)reflectiveCallsenables using structural typesexperimentalcontains newer features that have not yet been tested in production
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- language.scala
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Value Members
- final def !=(arg0: Any): Boolean
Test two objects for inequality.
Test two objects for inequality.
- returns
trueif !(this == that), false otherwise.
- Definition Classes
- AnyRef → Any
- final def ##(): Int
Equivalent to
x.hashCodeexcept for boxed numeric types andnull.Equivalent to
x.hashCodeexcept for boxed numeric types andnull. 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. Fornullreturns a hashcode wherenull.hashCodethrows aNullPointerException.- returns
a hash value consistent with ==
- Definition Classes
- AnyRef → Any
- final def ==(arg0: Any): Boolean
The expression
x == thatis equivalent toif (x eq null) that eq null else x.equals(that).The expression
x == thatis equivalent toif (x eq null) that eq null else x.equals(that).- returns
trueif the receiver object is equivalent to the argument;falseotherwise.
- Definition Classes
- AnyRef → 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 aClassCastExceptionat runtime, while the expressionList(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
ClassCastExceptionif the receiver object is not an instance of the erasure of typeT0.
- def clone(): AnyRef
Create a copy of the receiver object.
Create a copy of the receiver object.
The default implementation of the
clonemethod is platform dependent.- returns
a copy of the receiver object.
- implicit lazy val dynamics: dynamics
Only where this feature is enabled, can direct or indirect subclasses of trait scala.Dynamic be defined.
Only where this feature is enabled, can direct or indirect subclasses of trait scala.Dynamic be defined. If
dynamicsis not enabled, a definition of a class, trait, or object that hasDynamicas a base trait is rejected by the compiler.Selections of dynamic members of existing subclasses of trait
Dynamicare unaffected; they can be used anywhere.Why introduce the feature? To enable flexible DSLs and convenient interfacing with dynamic languages.
Why control it? Dynamic member selection can undermine static checkability of programs. Furthermore, dynamic member selection often relies on reflection, which is not available on all platforms.
- 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
eqmethod implements an equivalence relation on non-null instances ofAnyRef, and has three additional properties:- It is consistent: for any non-null instances
xandyof typeAnyRef, multiple invocations ofx.eq(y)consistently returnstrueor consistently returnsfalse. - For any non-null instance
xof typeAnyRef,x.eq(null)andnull.eq(x)returnsfalse. null.eq(null)returnstrue.
When overriding the
equalsorhashCodemethods, 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
trueif the argument is a reference to the receiver object;falseotherwise.
- Definition Classes
- AnyRef
- It is consistent: for any non-null instances
- def equals(arg0: AnyRef): Boolean
The equality method for reference types.
- implicit lazy val existentials: existentials
Where this feature is enabled, existential types that cannot be expressed as wildcard types can be written and are allowed in inferred types of values or return types of methods.
Where this feature is enabled, existential types that cannot be expressed as wildcard types can be written and are allowed in inferred types of values or return types of methods. If
existentialsis not enabled, those cases will trigger a warning from the compiler.Existential types with wildcard type syntax such as
List[_], orMap[String, _]are not affected.Why keep the feature? Existential types are needed to make sense of Java’s wildcard types and raw types and the erased types of run-time values.
Why control it? Having complex existential types in a code base usually makes application code very brittle, with a tendency to produce type errors with obscure error messages. Therefore, going overboard with existential types is generally perceived not to be a good idea. Also, complicated existential types might be no longer supported in a future simplification of the language.
- 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
finalizemethod is invoked, as well as the interaction betweenfinalizeand non-local returns and exceptions, are all platform dependent. - final def getClass(): Class[_ <: AnyRef]
Returns the runtime class representation of the object.
- def hashCode(): Int
The hashCode method for reference types.
- implicit lazy val higherKinds: higherKinds
Where this feature is enabled, higher-kinded types can be written.
Where this feature is enabled, higher-kinded types can be written. If
higherKindsis not enabled, a higher-kinded type such asF[A]will trigger a warning from the compiler.Why keep the feature? Higher-kinded types enable the definition of very general abstractions such as functor, monad, or arrow. A significant set of advanced libraries relies on them. Higher-kinded types are also at the core of the scala-virtualized effort to produce high-performance parallel DSLs through staging.
Why control it? Higher kinded types in Scala lead to a Turing-complete type system, where compiler termination is no longer guaranteed. They tend to be useful mostly for type-level computation and for highly generic design patterns. The level of abstraction implied by these design patterns is often a barrier to understanding for newcomers to a Scala codebase. Some syntactic aspects of higher-kinded types are hard to understand for the uninitiated and type inference is less effective for them than for normal types. Because we are not completely happy with them yet, it is possible that some aspects of higher-kinded types will change in future versions of Scala. So an explicit enabling also serves as a warning that code involving higher-kinded types might have to be slightly revised in the future.
- implicit lazy val implicitConversions: implicitConversions
Where this feature is enabled, definitions of implicit conversions are allowed.
Where this feature is enabled, definitions of implicit conversions are allowed. If
implicitConversionsis not enabled, the definition of an implicit conversion will trigger a warning from the compiler.An implicit conversion is an implicit value of unary function type
A => B, or an implicit method that has in its first parameter section a single, non-implicit parameter. Examples:implicit def stringToInt(s: String): Int = s.length implicit val conv = (s: String) => s.length implicit def listToX(xs: List[T])(implicit f: T => X): X = ...
Implicit classes and implicit values of other types are not governed by this language feature.
Why keep the feature? Implicit conversions are central to many aspects of Scala’s core libraries.
Why control it? Implicit conversions are known to cause many pitfalls if over-used. And there is a tendency to over-use them because they look very powerful and their effects seem to be easy to understand. Also, in most situations using implicit parameters leads to a better design than implicit conversions.
- 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 returnfalse, while the expressionList(1).isInstanceOf[List[String]]will returntrue. 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
trueif the receiver object is an instance of erasure of typeT0;falseotherwise.
- Definition Classes
- Any
- final def ne(arg0: AnyRef): Boolean
Equivalent to
!(this eq that).Equivalent to
!(this eq that).- returns
trueif the argument is not a reference to the receiver object;falseotherwise.
- Definition Classes
- AnyRef
- 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
- 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
- implicit lazy val postfixOps: postfixOps
Only where this feature is enabled, is postfix operator notation
(expr op)permitted.Only where this feature is enabled, is postfix operator notation
(expr op)permitted. IfpostfixOpsis not enabled, an expression using postfix notation is rejected by the compiler.Why keep the feature? Postfix notation is preserved for backward compatibility only. Historically, several DSLs written in Scala need the notation.
Why control it? Postfix operators interact poorly with semicolon inference. Most programmers avoid them for this reason alone. Postfix syntax is associated with an abuse of infix notation,
a op1 b op2 c op3, that can be harder to read than ordinary method invocation with judicious use of parentheses. It is recommended not to enable this feature except for legacy code. - implicit lazy val reflectiveCalls: reflectiveCalls
Where this feature is enabled, accesses to members of structural types that need reflection are supported.
Where this feature is enabled, accesses to members of structural types that need reflection are supported. If
reflectiveCallsis not enabled, an expression requiring reflection will trigger a warning from the compiler.A structural type is a type of the form
Parents { Decls }whereDeclscontains declarations of new members that do not override any member inParents. To access one of these members, a reflective call is needed.Why keep the feature? Structural types provide great flexibility because they avoid the need to define inheritance hierarchies a priori. Besides, their definition falls out quite naturally from Scala’s concept of type refinement.
Why control it? Reflection is not available on all platforms. Popular tools such as ProGuard have problems dealing with it. Even where reflection is available, reflective dispatch can lead to surprising performance degradations.
- final def synchronized[T0](arg0: => T0): T0
- Definition Classes
- AnyRef
- 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
- AnyRef → Any
- 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
- object experimental
The experimental object contains features that are known to have unstable API or behavior that may change in future releases.
The experimental object contains features that are known to have unstable API or behavior that may change in future releases.
Experimental features may undergo API changes in future releases, so production code should not rely on them.
Programmers are encouraged to try out experimental features and report any bugs or API inconsistencies they encounter so they can be improved in future releases.
This is the documentation for the Scala standard library.
Package structure
The scala package contains core types like
Int,Float,ArrayorOptionwhich are accessible in all Scala compilation units without explicit qualification or imports.Notable packages include:
scala.collectionand its sub-packages contain Scala's collections frameworkscala.collection.immutable- Immutable, sequential data-structures such asVector,List,Range,HashMaporHashSetscala.collection.mutable- Mutable, sequential data-structures such asArrayBuffer,StringBuilder,HashMaporHashSetscala.collection.concurrent- Mutable, concurrent data-structures such asTrieMapscala.concurrent- Primitives for concurrent programming such asFuturesandPromisesscala.io- Input and output operationsscala.math- Basic math functions and additional numeric types likeBigIntandBigDecimalscala.sys- Interaction with other processes and the operating systemscala.util.matching- Regular expressionsOther 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.Predefobject are always in scope by default.Some of these identifiers are type aliases provided as shortcuts to commonly used classes. For example,
Listis an alias forscala.collection.immutable.List.Other aliases refer to classes provided by the underlying platform. For example, on the JVM,
Stringis an alias forjava.lang.String.