Implementing scala trait with property that is class object

Question

I have a simple trait

trait SomeTrait {
  val sourceData: SourceData
}

SourceData class has constructor parameter p: Array[String] . Now, when I extend this trait in Object, we must provide implementation for sourceData.

object SomeObject extends SomeTrait {
  override val sourceData: SourceData = ???
  def main(sysArgs: Array[String]){...}
}

But what if class SourceData needs sysArgs from main method, how can I override sourceData in main method, not in body of SomeObject. Something like this:

object SomeObject extends SomeTrait {
  def main(sysArgs: Array[String]){
    override val sourceData: SourceData = new SourceData(sysArgs)
  }
}

I do not want to use var, as val immutability is preferred. And also I want to have trait with no implementation in order to force all sub classes to implement sourceData. What other solution I have for this?

Answer 1

You can't avoid mutability in this situation. sourceData must have a value before main is called, and main must be able to change that value, so the value must be mutable.

One option is to make sourceData a def (which is a good idea anyway) and have it access a private var in SomeObject :

trait SomeTrait {
  def sourceData: SourceData
}

object SomeObject extends SomeTrait {
  private var mySource: SourceData = ???
  def sourceData = mySource

  def main(sysArgs: Array[String]) = {
    mySource = new SourceData(sysArgs)
  }
}

---

The root problem here is having a top-level object that needs run-time initialisation. This is required because SomeObject is a top-level object that is accessed directly from other parts of the code.

The solution to this is dependency injection rather than a global object .

trait SomeTrait {
  def sourceData: SourceData
}

object SomeObject {
  case class SomeData(sourceData: SourceData) extends SomeTrait

  def main(sysArgs: Array[String]) = {
    val theSource = SomeData(SourceData(sysArgs))

    // Pass this instance to the rest of the code that needs it
    restOfTheProgram(theSource)
  }
}

The rest of the code uses the instance of SomeTrait that is passed to it rather than using SomeObject directly.

Answer 2

There is no way to do that in Scala. You need to have a class inherit SomeTrait and instantiate it from the main method.