NullPointerException when accessing overriden val in abstract constructor

Question

Consider the following (simplified) example:

abstract class Bar[T] {
    val f: PartialFunction[T, T]
    val default: PartialFunction[T, T] = { case x => x }
    val chained = f orElse default
}

class Foo extends Bar[Int] {
    val f: PartialFunction[Int, Int] = { case 1 => 2 }
}

And watch it crash:

scala> val foo = new Foo
java.lang.NullPointerException
        at Bar.<init>(<console>:8)
        at Foo.<init>(<console>:6)
        at .<init>(<console>:7)
        at .<clinit>(<console>)
        at RequestResult$.<init>(<console>:9)
        at RequestResult$.<clinit>(<console>)
        at RequestResult$scala_repl_result(<console>)
        ....

However, if we put chained in the concrete class:

abstract class Bar[T] {
    val f: PartialFunction[T, T]
    val default: PartialFunction[T, T] = { case x => x }
}

class Foo extends Bar[Int] {
    val f: PartialFunction[Int, Int] = { case 1 => 2 }
    val chained = f orElse default
}

It works as expected:

scala> val foo = new Foo
foo: Foo = Foo@16132c4

I have to admit I have absolutely no idea what is happening here. Bug? (This is on Scala 2.8.1.)

See this question for a solution/workaround: http://stackoverflow.com/questions/4712468/in-scala-what-is-an-early-initializer — Raphael, Jan 18 '11 at 09:30

score 5 · Accepted Answer · answered Jan 15 '11 at 20:20

5

"Why is my abstract or overriden val null?" https://github.com/paulp/scala-faq/wiki/Initialization-Order

answered Jan 15 '11 at 20:20

retronym

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1

A: Link is now 404, B: some people want to avoid lazy vals – samthebest Nov 05 '14 at 17:39
2

Link is now [here](http://docs.scala-lang.org/tutorials/FAQ/initialization-order.html) and it's poor practice to answer with links. – nedim Nov 26 '15 at 14:48

score 0 · Answer 2 · answered Jan 15 '11 at 19:57

My guess: This is an optimization done at compile time. The compiler recognizes that you override f in Bar. Therefore, the "constructor" of Bar executes first the stuff from Foo -- sans f! -- and then the stuff in the definition of Bar. Therefore, f is not defined when chained is constructed.

I call bug.

NullPointerException when accessing overriden val in abstract constructor

2 Answers2

Linked