Scala: Casting and Assignment

Question

In Scala/Spark DataFrame

    dfReduced.schema.fieldNames 

is a java String array (String[]). However,

    dfReduced.schema.fieldNames.asInstanceOf[Seq[String]]

throws

java.lang.ClassCastException: [Ljava.lang.String; cannot be cast to scala.collection.Seq

Assigning same array to a Seq[String] is fine.

   val f3:Seq[String]=dfReduced.schema.fieldNames

As a Java programmer this surprises me as both would require casting in Java. Can someone explain why there is this distinction in Scala

(Note, I'm not being critical, I just want to understand Scala better)

Answer 1

The reason why val f3:Seq[String]=dfReduced.schema.fieldNames this is working is because In Scala there is implicit conversion available than can cast the Array[T] to Seq[T] implicitly

In Java there is no such type of implicit casting available.

As Leo C mention in comment The difference is run-time type cast versus compile-time type ascription. For more info you can refer to this link.

Hope this clears your dough

Thanks