How to get the class of Option[_] on a None with Reflection - scala

Question

I have this setup where I want to populate an object fields from an XML NodeSeq. A cutdown version of my setup is shown. It works well, but when I put None in testObj, I get a java.util.NoSuchElementException: None.get.
The problem I believe is that I cannot find a way to get the class of the Option[_] when it is None. I've been stuck on this for ages and I cannot see a way out. I feel there must be a way to branch and populate the object field when the original value is None, but how?
import xml.NodeSeq
object test {
case class TestObject(name: Option[String] = None, value: Option[Double] = None)
def main(args: Array[String]): Unit = {
val testObj = TestObject(Some("xxx"), Some(12.0))
// val testObj = TestObject(None, Some(12.0))
val nodeSeq = <test> <name> yyy </name> <value> 34.0 </value> </test>
val result = getObject[TestObject](nodeSeq, Option(testObj))
println("result = " + result) // result = Some(TestObject(Some(yyy),Some(34.0)))
}
def getObject[A](nodeSeq: NodeSeq, obj: Option[A]): Option[A] = {
if ((nodeSeq.isEmpty) || (!obj.isDefined)) None else {
for (field <- obj.get.getClass.getDeclaredFields) {
field.setAccessible(true)
val fieldValue = field.get(obj.get)
if (fieldValue == null || !fieldValue.isInstanceOf[Option[_]]) None
var newValue: Option[_] = None
fieldValue.asInstanceOf[Option[_]].get match { // <---- None.get is the error here
case x: Double => newValue = Some((nodeSeq \ field.getName).text.trim.toDouble)
case x: String => newValue = Some((nodeSeq \ field.getName).text.trim)
}
val decField = obj.get.getClass.getDeclaredField(field.getName)
decField.setAccessible(true)
decField.set(obj.get, newValue)
}
obj
}
}
}

Answer 1

I'm ignoring some other parts of this code that I don't like, however, you can't call .get on a None. You can call map and it will return what I assume you want. Here's the modified code:
def getObject[A](nodeSeq: NodeSeq, obj: Option[A]): Option[A] = {
if (nodeSeq.isEmpty || obj.isEmpty) {
None
}
else {
for (field <- obj.get.getClass.getDeclaredFields) {
field.setAccessible(true)
val fieldValue = field.get(obj.get)
if (fieldValue == null || !fieldValue.isInstanceOf[Option[_]]) None
val newValue = fieldValue.asInstanceOf[Option[_]].map(a => {
a match {
case x: Double => Some((nodeSeq \ field.getName).text.trim.toDouble)
case x: String => Some((nodeSeq \ field.getName).text.trim)
}
})
val decField = obj.get.getClass.getDeclaredField(field.getName)
decField.setAccessible(true)
decField.set(obj.get, newValue)
}
obj
}
}
In practice, I usually avoid calling .get on options as it results in bugs quite often.
UPDATE:
This version is a little more functional, though it still alters the obj that you send in:
def getObject[A](nodeSeq: NodeSeq, obj: Option[A]): Option[A] = {
obj.map(o => {
for {
field <- o.getClass.getDeclaredFields
_ = field.setAccessible(true)
fieldValue <- Option(field.get(o))
} yield {
val newValue = fieldValue match {
case Some(a) => {
a match {
case x: Double => Some((nodeSeq \ field.getName).text.trim.toDouble)
case x: String => Some((nodeSeq \ field.getName).text.trim)
}
}
case _ => None
}
val decField = o.getClass.getDeclaredField(field.getName)
decField.setAccessible(true)
decField.set(o, newValue)
}
o
})
}

Answer 2

Actually, this might be closer to what you want: check the type of the field instead.
import xml.NodeSeq
object test {
case class TestObject(name: Option[String] = None, value: Option[Double] = None)
def main(args: Array[String]): Unit = {
val testObj = TestObject(Some("xxx"), Some(12.0))
// val testObj = TestObject(None, Some(12.0))
val nodeSeq = <test> <name> yyy </name> <value> 34.0 </value> </test>
val result = getObject[TestObject](nodeSeq, Option(testObj))
println("result = " + result) // result = Some(TestObject(Some(yyy),Some(34.0)))
}
def getObject[A](nodeSeq: NodeSeq, obj: Option[A]): Option[A] = {
if ((nodeSeq.isEmpty) || (!obj.isDefined)) None else {
for (field <- obj.get.getClass.getDeclaredFields) {
field.setAccessible(true)
val newValue = field.getGenericType match {
case t: ParametrizedType if (t.getActualType == classOf[Option[_]]) =>
val paramType = t.getActualTypeArguments()(0)
if (paramType == classOf[java.lang.Double]) // since Double can't be a generic type parameter on JVM
// from your original code, but it probably needs to be modified
// to handle cases where field isn't in nodeSeq or not a number
Some((nodeSeq \ field.getName).text.trim.toDouble)
else if (paramType == classOf[String])
Some((nodeSeq \ field.getName).text.trim)
else None
case _ => None
}
if (newValue.isDefined)
field.set(obj.get, newValue)
}
obj
}
}
}
Version creating a new object instead (might need modification for your requirements):
def getObject[A](nodeSeq: NodeSeq, objClass: Class[A]): Option[A] = {
if (nodeSeq.isEmpty) None else {
try {
val fieldValues = objClass.getDeclaredFields.flatMap { field =>
field.setAccessible(true)
field.getGenericType match {
case t: ParametrizedType if (t.getActualType == classOf[Option[_]]) =>
val paramType = t.getActualTypeArguments()(0)
if (paramType == classOf[java.lang.Double])
Some(Some((nodeSeq \ field.getName).text.trim.toDouble))
else if (paramType == classOf[String])
Some(Some((nodeSeq \ field.getName).text.trim))
else None
case _ => None
}
}
// assumes only one constructor, otherwise get the desired one
val ctor = objClass.getConstructors()(0)
Some(ctor.newInstance(fieldValues))
} catch {
case _ => None
}
}
}