scala.MatchError：在scala上为null

Question

I am getting the following error .. scala.MatchError: null

Is there something wrong with my code ? And if so, how do I fix it ?

val a : Option[String] = {
  company.customUrl match {
    case Some(a) => company.a
    case None => null
  }
}

val b :Option[String] = {
  company.b match {
    case Some(b) => company.b
    case _ => null
  }
}

val c : Option[Long] = {
  company.c match {
    case Some(c) => Option(company.c.get.toLong)
    case _ => null
  }
}

Answer 1

The return types of a,b,c are all Òption , but the raw type null in every second case is not. Try to return None instead.

a second case should catch all with _

b can also be simplified to val b :Option[String] = company.b

Answer 2

In your first case, you must have a null value for customUrl :

scala> (null: Any) match { case Some(x) => x ; case None => ??? }
scala.MatchError: null
  ... 29 elided

It's considered better to wrap nullable values in Option as soon as possible and not to unwrap them. In particular, don't set Options to null.

Answer 3

Option is meant to completely avoid null . Something that is Option should never be null ; instead, it should be None . Therefore, you should not be doing this strange dance:

val a: Option[String] = company.customUrl match {
  case Some(a) => company.a
  case None => null
}

Just val a = company.customUrl will do.

This is also likely the cause of the MatchError . One or more of company.customUrl , company.b , or company.c is null , due to some other piece of your code.

Reiterating: Option should never be null . Furthermore, in Scala, always try to avoid null and prefer Option .