Scala：構建器模式的返回運行時對象類型

Question

所以我試圖爲Scala實現Builder pattern的一個版本，我遇到了一些返回類型的問題。這裏是我的問題：

abstract class Car() { 
    protected var fuelConsumption = 10.0 

    def setFuelConsumption(con: Double): Car = { 
    fuelConsumption = con 
    this 
    } 
} 

trait HasHorn extends Car { 
    protected var hornSound = "Toot!" 

    def setHornSound(sound: String): HasHorn = { 
    hornSound = sound 
    this 
    } 

} 

class ModelT extends Car with HasHorn 

// I can do this: 
val aCar = new ModelT().setHornSound("Beep!").setFuelConsumption(5.0) 
// But not this, because setFuelConsumption returns a Car 
val bCar = new ModelT().setFuelConsumption(12.0).setHornSound("Beep!") 

所以我的問題是：我怎樣才能返回對象的運行時類型，因此像BCAR的聲明是可能的嗎？

Answer 1

基本上，你只需要說Car.setFuelConsumption不返回不僅僅是一輛汽車，而是返回自身的類型，就像這樣：

def setFuelConsumption(con: Double): this.type = { 
    fuelConsumption = con 
    this 
}