Question

The following code claims that Jack is employed in construction, but Jane is yet another victim of the rough economy:

abstract class Person(name: String) {

  case class Student(name: String, major: String) extends Person(name)

  override def toString(): String = this match {
    case Student(name, major) => name + " studies " + major
    case Worker(name, occupation) => name + " does " + occupation
    case _ => name + " is unemployed"
  }
}

case class Worker(name: String, job: String) extends Person(name)

object Narrator extends Person("Jake") {
  def main(args: Array[String]) {
    var friend: Person = new Student("Jane", "biology")
    println("My friend " + friend) //outputs "Jane is unemployed"
    friend = new Worker("Jack", "construction")
    println("My friend " + friend) //outputs "Jack does construction"
  }
}

Why does the match fail to recognize Jane as a Student?

Answer 1

Emil is entirely correct, but here’s an example to make it clear:

scala> case class A(a: String) {
     |   case class B(b: String)
     |   def who(obj: Any) = obj match {
     |     case B(b) => println("I'm A("+a+").B("+b+").")
     |     case b: A#B => println("I'm B("+b+") from some A")
     |     case other => println("Who am I?")
     |   }
     | }
defined class A

scala> val a1 = A("a1")
a1: A = A(a1)

scala> val a2 = A("a2")
a2: A = A(a2)

scala> val b1= a1.B("b1")
b1: a1.B = B(b1)

scala> val b2 = a2.B("b2")
b2: a2.B = B(b2)

scala> a1 who b1
I'm A(a1).B(b1).

scala> a1 who b2
I'm B(B(b2)) from some A

To be more precise, this line:

case Student(name, major) => name + " studies " + major

really means

case this.Student(name, major) => name + " studies " + major

Unfortunately, while Jane was instantiated on Jake, this in Jane’s case is pointing to Jane herself.