Scala : Why can't we do super.val?

问题:

I am practicing this code from JavaTpoint for learning inheritance in Scala. But I cannot access the member Bike from the class Vehicle who's value is initialized to zero. I tried by super type reference but it still shows the overridden value. Why does it not allow to access the super class field and directs to the overridden sub class field (speed) . here is the code and the output. Thanking in advance.

class Vehicle {
  val speed = 0
  println("In vehicle constructor " +speed)
  def run() {
    println(s"vehicle is running at $speed")
  }
}

class Bike extends Vehicle {
  override val speed = 100
  override def run() {
    super.run()
    println(s"Bike is running at $speed km/hr")
  }
}

object MainObject3 {
  def main(args:Array[String]) {
    var b = new Bike()
    b.run()
    var v = new Vehicle()
    v.run()
    var ve:Vehicle=new Bike()
    println("SuperType reference" + ve.speed)
    ve.run()
  }
}

回答1:

As we know, after Scala compile, Scala will be transfered to Java bytecode, it's for compatible with JVM.

And for the class Vehicle variable val speed, after compile it's visible for it's subclass Bike(the protected variable), we can view the bytecode of Vehicle:

  public Vehicle();
    Code:
       0: aload_0
       1: invokespecial #63                 // Method java/lang/Object."<init>":()V
       4: aload_0
       5: bipush        10
       7: putfield      #13                 // Field speed:I
      10: return

As we can see, it's init the speed's value 10 in the Vehicle constructor method.

and we also can find the init action in Bike constructor method:

  public Bike();
    Code:
       0: aload_0
       1: invokespecial #67                 // Method Vehicle."<init>":()V
       4: aload_0
       5: bipush        100
       7: putfield      #13                 // Field speed:I
      10: return

it's setting 100 for speed in constructor method.

so when init the Bike object, the speed field's value has been updated to 100 in the superclass Vehicle. so super.val will not make sense in there.

and there is another thing need to call out: when you use the super.speed directly in your subclass Bike, the compiler will throw:

super may not be used on value speed

so this compiler error thrown is also caused by the above reason.

回答2:

Answers to similar questions here overriding-vals-in-scala, or here cannot-use-super-when-overriding-values say: Scala compiler does not allow to use super on a val

Why is that? Discussion in last link above points to: SI-899. And the first comment there says following: it was changed so that traits could override vals to be more uniform with classes