如何正确覆盖 Scala 中的方法？

Question

我有以下“播放器”的实现。

// interface for a player
trait Player {
  def jump(): Unit
  def swim(): Unit
}

// a beginner player can't jump nor swim
class Beginner extends Player {
  def jump(): Unit = throw Error()
  def swim(): Unit = throw Error()
}

// movement decorator abstraction
abstract class MovementDecorator(my_player: Player) extends Player {
}

// concrete decorator
class Swim(player_x: Player) extends MovementDecorator(player_x) {
  def jump(): Unit = throw new Error("I can't jump")
  override def swim(): Unit = println("I swam!")
}

// concrete decorator
class Jump(player_x: Player) extends MovementDecorator(player_x) {
  override def jump(): Unit = println("I jumped!")
  def swim(): Unit = throw new Error("I can't swim")
}

具体的Player （即Beginner ）不能有jump()或swim()等方法。 但是我可以用Swim和/或Jump类来装饰对象，也就是包装对象，从而为Beginner提供这种能力。

但是，当我运行 main 方法时，我得到了带有注释的输出。

@main
def main(): Unit = {
  val swimmer = Swim(Beginner())
  swimmer.swim() // "I swam!"

  val jumper = Jump(Beginner())
  jumper.jump() // "I jumped!"

  val swimmer_jumper = Jump(Swim(Beginner()))
  swimmer_jumper.jump() // "I jumped!"
  swimmer_jumper.swim() // Exception at Jump.swim: I can't swim
}

我想保留以下内容来创建我的对象

val swimmer_jumper = Jump(Swim(Beginner()))

是否有一种特定的方法可以覆盖我缺少的方法？ 我认为当您重写一个方法时，它会重写该对象的任何其他方法——并且它成为首选方法。

Answer 1

您需要修改MovementDecorator以将实现委托给Player的装饰实例：

// movement decorator abstraction
abstract class MovementDecorator(my_player: Player) extends Player {
  override def swim(): Unit = my_player.swim()
  override def jump(): Unit = my_player.jump()
}

然后在MovementDecorator继承者中只覆盖相应的方法：

// concrete decorator
class Swim(player_x: Player) extends MovementDecorator(player_x) {
  override def swim(): Unit = println("I swam!")
}

// concrete decorator
class Jump(player_x: Player) extends MovementDecorator(player_x) {
  override def jump(): Unit = println("I jumped!")
}

演示。