what's the meaning of _ in this scala code?

Question

What's the meaning of "_" in Seq[Dependency[_]] and RDD[_] for the following code

abstract class RDD[T: ClassTag](
    @transient private var sc: SparkContext,
    @transient private var deps: Seq[Dependency[_]]
  ) extends Serializable with Logging {

  /** Construct an RDD with just a one-to-one dependency on one parent */
  def this(@transient oneParent: RDD[_]) =
    this(oneParent.context , List(new OneToOneDependency(oneParent)))

score 6 · Accepted Answer · answered Feb 09 '14 at 13:14

It's just a wildcard.
You can use a wildcard for a type parameter.

Quantification

Sometimes you do not care to be able to name a type variable, for example:
scala> def count[A](l: List[A]) = l.size
count: [A](List[A])Int
Instead you can use “wildcards”:
scala> def count(l: List[_]) = l.size
count: (List[_])Int
This is shorthand for:
scala> def count(l: List[T forSome { type T }]) = l.size
count: (List[T forSome { type T }])Int

From http://twitter.github.io/scala_school/type-basics.html

Is RDD[_] in def this(@transient oneParent: RDD[_]) the same as RDD[T]? Since class is abstract class RDD[T: ClassTag], and it defined T. — Daniel Wu, Feb 09 '14 at 13:50

what's the meaning of _ in this scala code?

1 Answers1