Is there something wrong with an abstract value us

问题:

I have

trait Invoker {
  val method: Method
}

Intellij IDEA code inspection is warning me that "Abstract value used in trait". Everything compiles fine. Is there something wrong with having an abstract value in a trait? If so, how should I specify that all extenders of the trait must define a method property?

回答1:

What is meant by this is the following weirdness:

trait A {
  val i: String
  def j: String
}

class C extends A {
  println ("val i = " + i)
  println ("def j = " + j)

  val i = "i"
  def j = "j"
}

val c = new C
// prints
// val i = null
// def j = j

So, as you can see i is initialised to it default value (null for AnyRef) before it is finally overridden by the constructor in C. (def declarations are re-referenced immediately.)

To avoid this one would have to put the val initialisations to the beginning of the constructor, if possible.

---

Additional weirdness (and how to solve it) in the following case

Consider

trait A {
  val i: String
  def j: String
}

abstract class D extends A {
  println ("val i = " + i)
  println ("def j = " + j)
}

class C extends D {
  val i = "i"
  def j = "j"
}
val c = new C
// prints
// val i = null
// def j = null

Now we seem to be out of luck; it looks as if there is no chance for us to initialise val i and def j before our superclass D tries to print them. In order to solve this problem, we must use Early definitions (§5.1.6 Scala reference):

class C extends {
  val i = "i"
  def j = "j"
} with D

val c = new C
// prints
// val i = i
// def j = j

And it works!

回答2:

There are plenty of good reasons to use an abstract val in a trait. Unfortunately, IntelliJ IDEA does not distinguish those and simply warns against their use universally. In practice, this means that IntelliJ IDEA warnings are ignored.