scala - 为什么 asInstanceOf 不抛出 ClassCastException？

Question

为什么 asInstanceOf 不抛出 ClassCastException ？

scala> List("a").asInstanceOf[List[Int]]
res34: List[Int] = List(a)

Answer 1

来自 scaladoc :

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.

如果您尝试访问变量或映射转换，它会正确地抛出异常:

scala> List("a").asInstanceOf[List[Int]]
res0: List[Int] = List(a)

scala> res0
res1: List[Int] = List(a)

scala> res0(0)
java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer

List("a").map(_.asInstanceOf[Int])
java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer