Converting from Java List to Scala mutable Buffer

Question

I'm having some issues converting from Scala to Java, and then back to Scala. I'm trying to convert from a Scala mutable buffer to a Java List, and then back to a Scala mutable buffer after I apply Java's nifty shuffling function.

I tried using Scala's Random library's shuffling function (even when the buffer is converted to a Scala list) however it's not working for me, as the type of buffer is of type "Card" which is an Object type that I have set up for the project that I am working on. The code in question looks like this:

def shuffleDeck() {
  val list: java.util.List[Card] = cards
  val newList = java.util.Collections.shuffle(list)
  asScalaBuffer(newList)
}

in the Scala IDE I'm using, the error given to me is:

type mismatch; found : Unit required: java.util.List[?]

I'm not really sure what to do. Any and all help would be greatly appreciated!

Answer 1

The line that's causing the error is likely this one:

val newList = java.util.Collections.shuffle(list)

As Collections.shuffle(..) (in java.util) has return type void - the passed in list becomes shuffled, a new list is not created. With that in mind, your code should be:

def shuffleDeck() {
  val list: java.util.List[Card] = cards
  java.util.Collections.shuffle(list)
  asScalaBuffer(list)
}

Answer 2

The java.util.Collections.shuffle will shuffle the list in place and it does not return a new list. The return type is void. Try calling asScalaBuffer with the original reference list.