Scala: Trait initialisation code using hard-coded values

Question

The following code:

trait A{
  val s: String = "A"
  println(s"A's initialiser run, s= $s")
}

object O1 extends A {
  override val s = "O1"
  println(s"Object's initialiser run, s= $s")

  def foo: Unit = println("I am just being called to instantiate the object! :| ")
}

object O2 extends AnyRef with A {
  override val s = "O2"
  println(s"Object's initialiser run, s= $s")

  def foo: Unit = println("I am just being called to instantiate the object! :| ")
}

println("////with inheritance:")
O1.foo
println("////with mix-in:")
O2.foo

prints:

////with inheritance:
A's initialiser run, s= null
Object's initialiser run, s= O1
I am just being called to instantiate the object! :| 
////with mix-in:
A's initialiser run, s= null
Object's initialiser run, s= O2
I am just being called to instantiate the object! :|

I find the behaviour very strange. I would have expected one of the following behaviours :

"...s = O1", "...s = O1"
"...s = A ", "...s = O1"

but neither happens.

I can understand a similar behaviour when s is left an abstract val, as opposed to a def, but I find it weird that the value is simply ignored. My questions are:

What is happening under the surface that leads to this behaviour ?
how similar/different does the compiler see the cases of running the initialisation block in a trait with hardcoded value vs a trait with abstract values.
Why has Scala decided to do this as opposed to the two expected scenarios detailed above.
Shouldn't the compiler warn us about having anything other than value setting in the body of traits?

chengpohi · Accepted Answer · 2018-02-23 16:43:06Z

2

This is caused by s is override by the subclass, Scala compiler will translate it to the abstract val, like:

public abstract java.lang.String s();

the s will initialize after all superclass has finished initialization(A) and it will only initialize once(in O1 or O2) after superclass finished initialization, so when A's initialize block is trying to print s will get null.

and There is a reference is helpful:

https://docs.scala-lang.org/tutorials/FAQ/initialization-order.html

answered Feb 23, 2018 at 16:43

chengpohi

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