用val重写def的Scala抛出NPE

The uniform access principle is just one aspect where Scala treats
fields and methods more uniformly than Java. Another difference is
that in Scala,fields and methods belong to the same namespace. This
makes it possible for a field to override a parameterless method. For
instance,you could change the implementation of contents in class
ArrayElement from a method to a field without having to modify the
abstract method definition of contents in class Element,as shown in
Listing 10.4:

我的代码基于这个例子

用def

abstract class Element {
  def contents: Array[String]

  val height = contents.length

  val width = if (height == 0) 0 else contents(0).length
}


class ArrayElement(contnts: Array[String]) extends Element {
  def contents: Array[String] = contnts
}

// --
val ae = new ArrayElement(Array("hello","world"))
ae.height
ae.width

我明白了

ae: ArrayElement = ArrayElement@7cd3ba8e
res0: Int = 2
res1: Int = 5

将def覆盖为ArrayElement中的val

abstract class Element {
  def contents: Array[String]

  val height = contents.length

  val width = if (height == 0) 0 else contents(0).length
}


class ArrayElement(contnts: Array[String]) extends Element {
  val contents: Array[String] = contnts
}

// --
val ae = new ArrayElement(Array("hello","world"))
ae.height
ae.width

我得到了NPE

java.lang.NullPointerException
    at #worksheet#.Element.<init>(scratch.scala:4)
    at #worksheet#.ArrayElement.<init>(scratch.scala:10)
    at #worksheet#.ae$lzycompute(scratch.scala:15)
    at #worksheet#.ae(scratch.scala:15)
    at #worksheet#.#worksheet#(scratch.scala:14)

我错过了什么？