scala继承问题：val与def

// 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)
  }
}

编译器抛出以下跟踪：

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)

所以编译器认为内容仍为null,但我在UnifiedContainer中定义了它！

当我用def替换val并且evrth工作完美时,事情变得更加奇怪！

你能解释一下这种行为吗？