Question

I have the following code

object DispatchLibrary
{
  private var nodes = Map.empty[java.util.UUID, List[BigInt]]

  def addNode(uuid: java.util.UUID) = if(nodes contains uuid) nodes else (nodes += (uuid -> Nil))

  def addValue(uuid: java.util.UUID, value: BigInt) = nodes + (uuid -> (value :: (nodes get uuid getOrElse Nil)))

  //def getValue(uuid: java.util.UUID) : List[BigInt] = ???

  //def getValues() : List[BigInt] = ???

  def calculated(): Boolean = !nodes.exists(_._1 eq null)

  def main(args: Array[String]) : Unit =
  {
    val uuid = java.util.UUID.randomUUID()

    addNode(uuid)
    addValue(uuid, BigInt(999))
    addValue(uuid, BigInt(9999))

    nodes foreach {case (key, value) => println (key + "->" + value)}
  }
}

Running the above code in IntelliJ IDEA gives something similar to the following output

8b2b4a7b-3e65-4de0-9035-8ee1d2910983->List()

I am not sure why the List is not being printed.

Running a similar code from the REPL gives the expected output

scala> var nodes = Map.empty[Int, List[BigInt]]
nodes: scala.collection.immutable.Map[Int,List[BigInt]] = Map()

scala> nodes += (1->Nil)

scala> nodes += (1 -> (BigInt(999) :: (nodes get 1 getOrElse Nil)))

scala> nodes += (1 -> (BigInt(9999) :: (nodes get 1 getOrElse Nil)))

scala> nodes foreach {case (key, value) => println (key + "-->" + value )}
1-->List(9999, 999)

I would appreciate also if you could help me in writing the commented methods.

Answer 1

In your addValue method you write nodes + (uuid -> (value :: (nodes get uuid getOrElse Nil))) which does not change the list in nodes, but only creates a new copy with the value added. Since Map is immutable by default you’ll have to store it like you do with the =-sign in the addNode method.

The reason it prints anything at all is because the first entry consists of a UUID (String) and Nil (empty List). The “8b2b4a7b-3e65-4de0-9035-8ee1d2910983” makes sense then because it’s the UUID. The “List()” is a result of printing Nil (since it is an empty List).

And there you have it. Try writing node += ... in the addValue in stead.