NPE in using scala Array.fill - scala

Question

I am reading Programming in Scala, Third Edition (Also present in Fourth Edition), by Lex Spoon; Bill Venners; Martin Odersky, and trying out examples along the way.
Following example form the book | Run in ScalaFiddle
abstract class Element {
def contents: Array[String]
val height = contents.length
val width = if (height == 0) 0 else contents(0).length
}
class UniformElement(
ch: Char,
override val width: Int,
override val height: Int
) extends Element {
private val line = ch.toString * width
def contents = Array.fill(height)(line)
}
val e: Element = new UniformElement('x', 2, 3)
gives java.lang.NullPointerException, when tried in REPL, or in Eclipse worksheet.
If I change
private val line = ch.toString * width
to
private def line = ch.toString * width
no error occurs.
Can someone explain, please?
I am using scala 2.11.8

Answer 1

The problem here is that contents is still not defined in the constructor, when you define line. If line is a val, it does not pick the overridden width, instead it uses the abstract one which in turn uses contents, which is still undefined and you get the NPE. You can see this by looking at the stacktrace and noting that the NPE is thrown by the definition of width in the abstract class.
When line is defined as a method, it does not execute until you call it and by that time contents will be fully defined, because it can call line (another method) which will be fully defined.
Run on ScalaFiddle
abstract class Element {
def contents: Array[String]
val height = contents.length
val width = if (height == 0) 0 else contents(0).length
}
class UniformElement(
ch: Char,
override val width: Int,
override val height: Int
) extends Element {
private def line: String = ch.toString * width
def contents = Array.fill(height)(line)
}
val e3: Element = new UniformElement('x', 2, 3)
Bottom line: you have a kind of "circular dependency" between line and contents.