Maintain order in .groupBy() operation of Scala collection - scala

Question

I have the following code snippet:
object Main1 extends App {
sealed trait XYZ
{
def id : String
}
class Test() extends XYZ {
override def id: String = "Test"
}
class ABC() extends XYZ {
override def id: String = "ABC"
}
val a: List[XYZ] = List(new Test(), new ABC())
val b: Map[String, List[XYZ]] = a.groupBy(_.id)
println(b)
val c: List[XYZ] = List(new ABC(), new Test())
val d: Map[String, List[XYZ]] = c.groupBy(_.id)
println(d)
}
The output is as follows:
Map(Test -> List(Main1$Test#7dc5e7b4), ABC -> List(Main1$ABC#1ee0005))
Map(Test -> List(Main1$Test#3d012ddd), ABC -> List(Main1$ABC#6f2b958e))
How can I maintain the order in the result like in the input collection?

Answer 1

If you are not limited to use immutable collections only then it can be easily done with mutable.LinkedHashMap:
import scala.collection.mutable
import scala.collection.mutable.ArrayBuffer
object Main1 extends App {
sealed trait XYZ
{
def id : String
}
class Test() extends XYZ {
override def id: String = "Test"
}
class ABC() extends XYZ {
override def id: String = "ABC"
}
val a: List[XYZ] = List(new Test(), new ABC())
val b: mutable.LinkedHashMap[String, ArrayBuffer[XYZ]] = a.foldLeft(mutable.LinkedHashMap.empty[String, ArrayBuffer[XYZ]]) { (m, x) =>
m.getOrElseUpdate(x.id, new ArrayBuffer[XYZ]) += x
m
}
println(b)
val c: List[XYZ] = List(new ABC(), new Test())
val d: mutable.LinkedHashMap[String, ArrayBuffer[XYZ]] = c.foldLeft(mutable.LinkedHashMap.empty[String, ArrayBuffer[XYZ]]) { (m, x) =>
m.getOrElseUpdate(x.id, new ArrayBuffer[XYZ]) += x
m
}
println(d)
}
Output maintains the order of the input collection:
LinkedHashMap(Test -> ArrayBuffer(com.github.plokhotnyuk.jsoniter_scala.examples.Main1$Test#56cbfb61), ABC -> ArrayBuffer(com.github.plokhotnyuk.jsoniter_scala.examples.Main1$ABC#1134affc))
LinkedHashMap(ABC -> ArrayBuffer(com.github.plokhotnyuk.jsoniter_scala.examples.Main1$ABC#1b0375b3), Test -> ArrayBuffer(com.github.plokhotnyuk.jsoniter_scala.examples.Main1$Test#2f7c7260))