This "virtual" case class represents the reflection interface for literal expressions which can not be further broken down or evaluated, such as "true", "0", "classOf[List]".
This "virtual" case class represents the reflection interface for literal expressions which can not be further broken down or evaluated, such as "true", "0", "classOf[List]". Such values become parts of the Scala abstract syntax tree representing the program. The constants correspond to section 6.24 "Constant Expressions" of the Scala Language Specification.
Such constants are used to represent literals in abstract syntax trees (the scala.reflect.api.Trees#Literal node) and literal arguments for Java class file annotations (the scala.reflect.api.Annotations#LiteralArgument class).
Constants can be matched against and can be constructed directly, as if they were case classes:
assert(Constant(true).value == true) Constant(true) match { case Constant(s: String) => println("A string: " + s) case Constant(b: Boolean) => println("A boolean value: " + b) case Constant(x) => println("Something else: " + x) }
Constant
instances can wrap certain kinds of these expressions:
Byte
, Short
, Int
, Long
, Float
, Double
, Char
, Boolean
and Unit
) - represented directly as the corresponding typeString
. Class references are represented as instances of scala.reflect.api.Types#Type
(because when the Scala compiler processes a class reference, the underlying runtime class might not yet have
been compiled). To convert such a reference to a runtime class, one should use the runtimeClass
method of a
mirror such as RuntimeMirror
(the simplest way to get such a mirror is using
scala.reflect.runtime.currentMirror
).
Enumeration value references are represented as instances of scala.reflect.api.Symbols#Symbol, which on JVM point to methods that return underlying enum values. To inspect an underlying enumeration or to get runtime value of a reference to an enum, one should use a scala.reflect.api.Mirrors#RuntimeMirror (the simplest way to get such a mirror is again scala.reflect.runtime.package#currentMirror).
Usage example:
enum JavaSimpleEnumeration { FOO, BAR } import java.lang.annotation.*; @Retention(RetentionPolicy.RUNTIME) @Target({ElementType.TYPE}) public @interface JavaSimpleAnnotation { Class<?> classRef(); JavaSimpleEnumeration enumRef(); } @JavaSimpleAnnotation( classRef = JavaAnnottee.class, enumRef = JavaSimpleEnumeration.BAR ) public class JavaAnnottee {}
import scala.reflect.runtime.universe._ import scala.reflect.runtime.{currentMirror => cm} object Test extends App { val jann = typeOf[JavaAnnottee].typeSymbol.annotations(0).javaArgs def jarg(name: String) = jann(TermName(name)) match { // Constant is always wrapped into a Literal or LiteralArgument tree node case LiteralArgument(ct: Constant) => value case _ => sys.error("Not a constant") } val classRef = jarg("classRef").value.asInstanceOf[Type] // ideally one should match instead of casting println(showRaw(classRef)) // TypeRef(ThisType(), JavaAnnottee, List()) println(cm.runtimeClass(classRef)) // class JavaAnnottee val enumRef = jarg("enumRef").value.asInstanceOf[Symbol] // ideally one should match instead of casting println(enumRef) // value BAR val siblings = enumRef.owner.info.decls val enumValues = siblings.filter(sym => sym.isVal && sym.isPublic) println(enumValues) // Scope{ // final val FOO: JavaSimpleEnumeration; // final val BAR: JavaSimpleEnumeration // } // doesn't work because of https://issues.scala-lang.org/browse/SI-6459 // val enumValue = mirror.reflectField(enumRef.asTerm).get val enumClass = cm.runtimeClass(enumRef.owner.asClass) val enumValue = enumClass.getDeclaredField(enumRef.name.toString).get(null) println(enumValue) // BAR }
The API of Constant instances.
An extractor class to create and pattern match with syntax Constant(value)
where value
is the Scala value of the constant.
The constructor/extractor for Constant
instances.
Test two objects for inequality.
Test two objects for inequality.
true
if !(this == that), false otherwise.
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
.
a hash value consistent with ==
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)
.
true
if the receiver object is equivalent to the argument; false
otherwise.
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.
the receiver object.
ClassCastException
if the receiver object is not an instance of the erasure of type T0
.
Create a copy of the receiver object.
Create a copy of the receiver object.
The default implementation of the clone
method is platform dependent.
a copy of the receiver object.
not specified by SLS as a member of AnyRef
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:
x
and y
of type AnyRef
, multiple invocations of
x.eq(y)
consistently returns true
or consistently returns false
.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
).
true
if the argument is a reference to the receiver object; false
otherwise.
The equality method for reference types.
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.
not specified by SLS as a member of AnyRef
A representation that corresponds to the dynamic class of the receiver object.
A representation that corresponds to the dynamic class of the receiver object.
The nature of the representation is platform dependent.
a representation that corresponds to the dynamic class of the receiver object.
not specified by SLS as a member of AnyRef
The hashCode method for reference types.
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.
true
if the receiver object is an instance of erasure of type T0
; false
otherwise.
Equivalent to !(this eq that)
.
Equivalent to !(this eq that)
.
true
if the argument is not a reference to the receiver object; false
otherwise.
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.
not specified by SLS as a member of AnyRef
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.
not specified by SLS as a member of AnyRef
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.
a String representation of the object.
The methods available for each reflection entity, without the implementation. Since the reflection entities are later overridden by runtime reflection and macros, their API counterparts guarantee a minimum set of methods that are implemented.
Extractors provide the machinery necessary to allow pattern matching and construction of reflection entities that is similar to case classes, although the entities are only abstract types that are later overridden.
EXPERIMENTAL
According to the section 6.24 "Constant Expressions" of the Scala language specification, certain expressions (dubbed constant expressions) can be evaluated by the Scala compiler at compile-time.
scala.reflect.api.Constants#Constant instances represent certain kinds of these expressions (with values stored in the
value
field and its strongly-typed views namedbooleanValue
,intValue
etc.), namely:Such constants are used to represent literals in abstract syntax trees (the scala.reflect.api.Trees#Literal node) and literal arguments for Java class file annotations (the scala.reflect.api.Annotations#LiteralArgument class).
Example
The
value
field deserves some explanation. Primitive and string values are represented as themselves, whereas references to classes and enums are a bit roundabout.Class references are represented as instances of scala.reflect.api.Types#Type (because when the Scala compiler processes a class reference, the underlying runtime class might not yet have been compiled). To convert such a reference to a runtime class, one should use the
runtimeClass
method of a mirror such as scala.reflect.api.Mirrors#RuntimeMirror (the simplest way to get such a mirror is using scala.reflect.runtime.package#currentMirror).Enumeration value references are represented as instances of scala.reflect.api.Symbols#Symbol, which on JVM point to methods that return underlying enum values. To inspect an underlying enumeration or to get runtime value of a reference to an enum, one should use a scala.reflect.api.Mirrors#RuntimeMirror (the simplest way to get such a mirror is again scala.reflect.runtime.package#currentMirror).