Usages of Constants in Constructor - scala

Question

It´s most probably a stupid question, I saw an answer already here but seems does not work in my particular case. Here my code
object ProductAdded{
val PRODUCT_NAME:String="productName"
val PRODUCT_PRICE:String="productPrice"
}
class ProductAdded #JsonCreator()(#JsonProperty(PRODUCT_NAME) var productName: String,
#JsonProperty(PRODUCT_PRICE) var productPrice: String) extends Event {
#JsonProperty(PRODUCT_NAME) def getProductName: String = {
productName
}
#JsonProperty(PRODUCT_PRICE) def getProductPrice: String = {
productPrice
}
}
I´m trying to use those constants in the constructor, but when I compile show this error
Error:(12, 49) annotation argument needs to be a constant; found: event.ProductAdded.PRODUCT_NAME
class ProductAdded #JsonCreator()(#JsonProperty(PRODUCT_NAME) var productName: String,

Answer 1

This is a pretty unfortunate error message, I must say, since Scala doesn't actually have a concept named "constant", so how are you supposed to make something a "constant" if you don't even know what it is?
Scala does, however, have a concept of "constant expression" (bold emphasis mine):
6.24 Constant Expressions
Constant expressions are expressions that the Scala compiler can evaluate to a constant. The definition of "constant expression" depends on the platform, but they include at least the expressions of the following forms:
A literal of a value class, such as an integer
A string literal
A class constructed with Predef.classOf
An element of an enumeration from the underlying platform
A literal array, of the form Array(c1, …, cn), where all of the ci's are themselves constant expressions
An identifier defined by a constant value definition.
and "constant value definition" (bold emphasis mine):
A constant value definition is of the form
final val x = e
where e is a constant expression. The final modifier must be present and no type annotation may be given.
So, as you can see, your expression is not a constant value expression, because the definition is not a constant value definition.
In order to turn it into a constant value definition, you need to
remove the type annotation : String
add the final modifier explicitly: even though members of a singleton object are implicitly final, constant value definitions require an explicit annotation
The result would look something like this:
object ProductAdded {
final val PRODUCT_NAME = "productName"
final val PRODUCT_PRICE = "productPrice"
}
class ProductAdded #JsonCreator()(
#JsonProperty(PRODUCT_NAME) var productName: String,
#JsonProperty(PRODUCT_PRICE) var productPrice: String
) extends Event {
#JsonProperty(PRODUCT_NAME) def getProductName = productName
#JsonProperty(PRODUCT_PRICE) def getProductPrice = productPrice
}