Question

I am working on a method that has 3 possible outcomes for multiple items: Error, Invalid and Success. For each of these I need to return a json list identifying which items were in error, invalid and successful.

My current attempt follows. I have used Object to represent the class my objects are as fully explaining would take too long. The Object class has a method process which returns a boolean to indicate success or error and throws an exception when the object is invalid:

def process(list: List[Objects]) = {
    val successIds = new ListBuffer[Int]();
    val errorIds = new ListBuffer[Int]();
    val invalidIds = new ListBuffer[Int]();

    list.foreach( item => {
        try {
            if (item.process) {
                successIds ++ item.id
            } else {
                errorIds ++ item.id
            }
        } catch {
            case e: Exception => invalidIds ++ item.id
        }
    })

    JsonResult(
        Map("success" -> successIds, 
            "failed" -> errorIds, 
            "invalid" -> invalidIds)
    ) 
}

Problem is using Mutable data structures isn’t very “Scala-y”. I would prefer to build up these lists in some more functional way but I am quite new to scala. Any thoughts or hints as to how this might be done?

My though is using something like the flatMap method that takes a tuple of collections and collates them in the same way the flatMap method does for a single collection:

def process(list: List[Objects]) = {

    val (success, error, invalid) = list.flatMap( item => {
        try {
            if (item.process) {
                (List(item.id), List.empty, List.empty)
            } else {
                (List.empty, List(item.id), List.empty)
            }
        } catch {
            case e: Exception => 
                (List.empty, List.empty, List(item.id))
        }
    })

    JsonResult(
        Map("success" -> success, 
            "failed" -> error, 
            "invalid" -> invalid)
    ) 
}

Answer 1

flatMap isn’t what you need here – you need groupBy:

def process(list: List[Objects]) = {

  def result(x: Objects) =  
    try if (x.process) "success" else "failed"
    catch {case _ => "invalid"}     

  JsonResult(list groupBy result mapValues (_ map (_.id)))
}