Scala Map to Case Class conversion

Question

I'm trying to convert a Scala Map[String, Any] to a case class using Scala reflection (Scala 2.11) as follows -

val m = Map("name" -> "ABC", "age" -> 7, "gender" -> "male")
case class someCC(name: String, age: Int, gender: String)

import scala.reflect.ClassTag

   def createCaseClass[T](someMap : Map[String, Any])(implicit someClassTag : ClassTag[T]) = {

    val ctor = someClassTag.runtimeClass.getConstructors.head
    val args = someClassTag.runtimeClass.getDeclaredFields.map(x => someMap(x.getName))

       ctor.newInstance(args: _*).asInstanceOf[T]
  }

this unfortunately results in a compile error -

Name: Unknown Error
Message: <console>:106: error: type mismatch;
 found   : Array[Any]
 required: Array[_ <: Object]
Note: Any >: Object, but class Array is invariant in type T.
You may wish to investigate a wildcard type such as `_ >: Object`. (SLS 3.2.10)
              ctor.newInstance(args: _*).asInstanceOf[T]
                           ^

I'm fairly new to using ClassTags and I understand that this error is primarily because java.lang.Object is a subset of Any and Any could include non-java objects.

When I tried to replace Any with AnyRef (which corresponds to java.lang.Object in JRE), the function call results in a type mismatch error.

import scala.reflect.ClassTag

   def createCaseClass[T](someMap : Map[String, AnyRef])(implicit someClassTag : ClassTag[T]) = {

    val ctor = someClassTag.runtimeClass.getConstructors.head
    val args = someClassTag.runtimeClass.getDeclaredFields.map(x => someMap(x.getName))

       ctor.newInstance(args: _*).asInstanceOf[T]
  }

val someCC = createCaseClass[someCC](m)

Name: Unknown Error
Message: <console>:106: error: type mismatch;
 found   : scala.collection.immutable.Map[String,Any]
 required: Map[String,AnyRef]
       val someCC = createCaseClass[someCC](m)

What's the best way to resolve this error? Suggestions appreciated. Thanks!

Update 1 - Updating this to implicitly cast an Any to AnyRef leads to an error 'java.util.NoSuchElementException' on function call.

import scala.reflect.ClassTag
import scala.reflect.runtime.universe._

   def createMyClass[T](someMap : Map[String, Any])(implicit someClassTag : ClassTag[T]) = {

       val ctor = someClassTag.runtimeClass.getConstructors.head

       val args = someClassTag.runtimeClass.getDeclaredFields.map(x => someMap(x.getName))

       ctor.newInstance(args.asInstanceOf[Seq[AnyRef]]: _*).asInstanceOf[T]

}

val m = Map("name" -> "ABC", "age" -> 7, "gender" -> "male")
case class someCC(name: String, age: Int, gender: String)

createMyClass[someCC](m)

Name: java.util.NoSuchElementException
Message: key not found: $outer
StackTrace:   at scala.collection.MapLike$class.default(MapLike.scala:228)
  at scala.collection.AbstractMap.default(Map.scala:59)
  at scala.collection.MapLike$class.apply(MapLike.scala:141)
  at scala.collection.AbstractMap.apply(Map.scala:59)
  at $$$e75186ae1b35495ffea8e318378149a$$$$anonfun$1.apply(<console>:135)
  at $$$e75186ae1b35495ffea8e318378149a$$$$anonfun$1.apply(<console>:135)
  at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
  at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
  at scala.collection.IndexedSeqOptimized$class.foreach(IndexedSeqOptimized.scala:33)
  at scala.collection.mutable.ArrayOps$ofRef.foreach(ArrayOps.scala:186)
  at scala.collection.TraversableLike$class.map(TraversableLike.scala:234)
  at scala.collection.mutable.ArrayOps$ofRef.map(ArrayOps.scala:186)
  at createMyClass(<console>:135)

What am I doing wrong here?

If you do it the Java way, you need to pass in the outer class instance in the parameter $outer. — HTNW

HTNW HTNW · Accepted Answer · 2017-08-14T21:35:23

Use a cast:

ctor.newInstance(args.asInstanceOf[Seq[AnyRef]]: _*).asInstanceOf[T]

Note that you map every field of a case class to a constructor param. This is incorrect, because a case class can have fields that aren't in the constructor, and your code will break.

A better idea is to use Scala reflection:

import reflect.runtime.universe._

def mkClassInstance[T: TypeTag](args: Map[String, Any]): T = {
  val rMirror = runtimeMirror(getClass.getClassLoader)
  val cMirror = rMirror.reflectClass(typeOf[T].typeSymbol.asClass)
  // The primary constructor is the first one
  val ctor = typeOf[T].decl(termNames.CONSTRUCTOR).asTerm.alternatives.head.asMethod
  val argList = ctor.paramLists.flatten.map(param => args(param.name.toString))
  cMirror.reflectConstructor(ctor)(argList: _*).asInstanceOf[T]
}
def mkInnerClassInstance[T: TypeTag](outer: Any)(args: Map[String, Any]): T = {
  val rMirror = runtimeMirror(getClass.getClassLoader)
  val cMirror = rMirror.reflect(outer).reflectClass(typeOf[T].typeSymbol.asClass)
  // The primary constructor is the first one
  val ctor = typeOf[T].decl(termNames.CONSTRUCTOR).asTerm.alternatives.head.asMethod
  val argList = ctor.paramLists.flatten.map(param => args(param.name.toString))
  cMirror.reflectConstructor(ctor)(argList: _*).asInstanceOf[T]
}

Scala Map to Case Class conversion

1 Answers