scala inheritance issue: val vs. def

Question

Writing a simple example from Odersky's book resulted in the following problem:

// AbstractElement.scala
abstract class AbstractElement {
  val contents: Array[String]
  val height: Int = contents.length // line 3
}

class UnifiedElement(ch: Char, _width: Int, _height: Int) extends AbstractElement { // line 6
  val contents = Array.fill(_height)(ch.toString() * _width)
}

object AbstractElement {
  def create(ch: Char): AbstractElement = {
    new UnifiedElement(ch, 1, 1) // line 12
  }
}

,

// ElementApp.scala
import AbstractElement.create

object ElementApp {

  def main(args: Array[String]): Unit = {
  val e1 = create(' ') // line 6
    println(e1.height)
  }
}

The compiler throws the following trace:

Exception in thread "main" java.lang.NullPointerException
    at AbstractElement.<init>(AbstractElement.scala:3)
    at UnifiedElement.<init>(AbstractElement.scala:6)
    at AbstractElement$.create(AbstractElement.scala:12)
    at ElementApp$.main(ElementApp.scala:6)
    at ElementApp.main(ElementApp.scala)

So the compiler thinks that contents is still null, but I defined it in UnifiedContainer!

Things get even more weird when I replace val with def and evrth works perfect!

Could you please xplain this behaviour?

Answer 1

Here is a great article by Paul P that explains the initialization order intricacies in Scala. As a rule of thumb, you should never use abstract vals. Always use abstract defs and lazy vals.