I have this hierarchy of traits and classes in Scala:
trait A
trait B[T] extends A {
def v: T
}
case class C(v:Int) extends B[Int]
case class D(v:String) extends B[String]
val l:List[A] = C(1) :: D("a") :: Nil
l.foreach(t => println(t.asInstanceOf[B[_]].v))
I cannot change the type hierarchy or the type of the list.
Is there a better way to avoid the asInstanceOf[B[_]] statement?
您可以尝试模式匹配。
l.collect{case x :B[_] => println(x.v)}
You might try something like this:
for (x <- l.view; y <- Some(x).collect { case b: B[_] => b }) println(y.v)
It doesn't require any isInstanceOf
or asInstanceOf
, and never crashes, even if your list contains A
s that aren't B[_]
s. It also doesn't create any lengthy lists as intermediate results, only small short-lived Option
s.
Not as concise, but also much less surprising solution:
for (x <- l) {
x match {
case b: B[_] => println(b.v)
case _ => /* do nothing */
}
}
If you could change the type of l
to List[B[_]]
, this would be the preferable solution.
I think the most ideomatic way to do it would be to supply B
with an extractor object and pattern match for B
values:
object B {
def unapply[T](arg: B[T]): Some[T] = Some(arg.v)
}
l.collect{case B(x) => println(x)}
If B
is declared in a source file you can't alter you might need a different name for the extractor object.
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