Question

我基本上想做这样的事情:

class Shape

class CoordSystem
class C3D(val x: Double, y: Double, z: Double) extends CoordSystem
class C2D(val x: Double, y: Double) extends CoordSystem

abstract class Shape {
  def getCoords[C <: CoordSystem]: List[C]
} 

class Pyramid extends Shape {
  def getCoords: List[C3D] = 
    List(new C3D(1,2,1), new C3D(1,1,1), new C3D(2,2,1), new C3D(2,1,1), new C3D(1.5,1.5,3))
}   
>> error: class Pyramid needs to be abstract, since method getCoords in class Shape of type [C <: CoordSystem]List[C] is not defined

我在上看到了几个不同的想法,但其中没有一个意见对本案来说是正确的,因为它们似乎不让我在其他地方撰写提及myShape.Coords。如在<代码>Shape下分类中正确定义,将物体清单从<代码>下分类。 CoordSystem。

我还发现 ,在Schala Lang电子邮件名单上就通用s 进行了令人感兴趣的讨论,但不能将其与我的情况相重。

感激之情!

Answer 1

如何做这样的事情:

class CoordSystem
class C3D(val x: Double, y: Double, z: Double) extends CoordSystem
class C2D(val x: Double, y: Double) extends CoordSystem

trait ShapeLike[+C <: CoordSystem] {
  def getCoords: List[C]
}

abstract class Shape extends ShapeLike[CoordSystem]

class Pyramid extends Shape with ShapeLike[C3D] {
  def getCoords: List[C3D] =
    List(new C3D(1, 2, 1), new C3D(1, 1, 1), new C3D(2, 2, 1), new C3D(2, 1, 1), new C3D(1.5, 1.5, 3))
}

当然,你没有义务宣布额外类别ShapeLike;其目的是允许你在没有额外类型参数的情况下使用<代码>Shape。

So, actually the answer to your question as stated in the title is: you may “tighten up” the type bound of a type parameter in a subclass if it is defined as a covariant type parameter in the superclass; conversely, you may “loosen” the type bound of a contravariant type parameter.

Answer 2

一类参数为一类参数,超级合同要求你不要收紧合同——使其不那么严格——不会违反合同,尽管我不认为允许这样做。

但是,你可以做其他事情:将这种类型放在该类。

class Shape

class CoordSystem
class C3D(val x: Double, y: Double, z: Double) extends CoordSystem
class C2D(val x: Double, y: Double) extends CoordSystem

abstract class Shape {
  type C <: CoordSystem
  def getCoords: List[C]
} 

class Pyramid extends Shape {
  override type C = C3D
  def getCoords: List[C3D] = 
    List(new C3D(1,2,1), new C3D(1,1,1), new C3D(2,2,1), new C3D(2,1,1), new C3D(1.5,1.5,3))
}

友情链接