Question

以下代码声称杰克受雇于建筑业，但简是另一个粗暴经济的受害者：

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"
  }
}

为什么匹配无法将Jane识别为学生？

Answer 1

我认为这里发生的是Student案例类是在Person内声明的。因此，case Student中的toString仅匹配属于特定Student实例的Person。

如果您将case class Student移至与case class Worker平行（然后从extends Person("Jake")移除不必要的object Narrator ...只有那样new Student {1}}成为杰克特有的Person$Student，你会发现简确实在研究生物学。

Answer 2

Emil是完全正确的，但这是一个明确的例子：

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

更确切地说，这一行：

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

真的意味着

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

不幸的是，虽然Jane在Jake身上实例化，但Jane的案例中this指向Jane本人。

Scala：匹配案例类

2 个答案: