Scala Language Instance type checking


Example

Type check: variable.isInstanceOf[Type]

With pattern matching (not so useful in this form):

variable match {
  case _: Type => true
  case _ => false
}

Both isInstanceOf and pattern matching are checking only the object's type, not its generic parameter (no type reification), except for arrays:

val list: List[Any] = List(1, 2, 3)             //> list  : List[Any] = List(1, 2, 3)

val upcasting = list.isInstanceOf[Seq[Int]]     //> upcasting  : Boolean = true

val shouldBeFalse = list.isInstanceOf[List[String]]
                                                //> shouldBeFalse  : Boolean = true

But

val chSeqArray: Array[CharSequence] = Array("a") //> chSeqArray  : Array[CharSequence] = Array(a)
val correctlyReified = chSeqArray.isInstanceOf[Array[String]]
                                              //> correctlyReified  : Boolean = false


val stringIsACharSequence: CharSequence = ""    //> stringIsACharSequence  : CharSequence = ""
  
val sArray = Array("a")                         //> sArray  : Array[String] = Array(a)
val correctlyReified = sArray.isInstanceOf[Array[String]]
                                                //> correctlyReified  : Boolean = true

//val arraysAreInvariantInScala: Array[CharSequence] = sArray
//Error: type mismatch;  found   : Array[String]  required: Array[CharSequence]
//Note: String <: CharSequence, but class Array is invariant in type T.
//You may wish to investigate a wildcard type such as `_ <: CharSequence`. (SLS 3.2.10)
//Workaround:
val arraysAreInvariantInScala: Array[_ <: CharSequence] = sArray
                                                //> arraysAreInvariantInScala  : Array[_ <: CharSequence] = Array(a)
  

val arraysAreCovariantOnJVM = sArray.isInstanceOf[Array[CharSequence]]
                                                //> arraysAreCovariantOnJVM  : Boolean = true

Type casting: variable.asInstanceOf[Type]

With pattern matching:

variable match {
  case _: Type => true
}

Examples:

  val x = 3                                       //> x  : Int = 3
  x match {
    case _: Int => true//better: do something
    case _ => false
  }                                               //> res0: Boolean = true
  
  x match {
    case _: java.lang.Integer => true//better: do something
    case _ => false
  }                                               //> res1: Boolean = true
  
  x.isInstanceOf[Int]                             //> res2: Boolean = true
  
  //x.isInstanceOf[java.lang.Integer]//fruitless type test: a value of type Int cannot also be a Integer
  
  trait Valuable { def value: Int}
  case class V(val value: Int) extends Valuable
  
  val y: Valuable = V(3)                          //> y  : Valuable = V(3)
  y.isInstanceOf[V]                               //> res3: Boolean = true
  y.asInstanceOf[V]                               //> res4: V = V(3)

Remark: This is only about the behaviour on the JVM, on other platforms (JS, native) type casting/checking might behave differently.