Scala:“类需要是抽象的，因为方法没有定义"错误 [英] Scala: "class needs to be abstract, since method is not defined" error

问题描述

我熟悉此错误消息，但在这种情况下我不太确定:

I'm familiar with this error message but I'm not quite sure about it in this case:

class Foo extends A {
  // getting error here
}

trait A extends B {
   def something(num:Int):Boolean = {
      num == 1
   }
}

trait B {
  def something[S](num:S):Boolean
}

但这编译得很好:

    class Foo extends A ...

    trait A extends B[Int] {
       def something(num:Int):Boolean = {
          num == 1
       }
    }

    trait B[S] {
      def something(num:S):Boolean
    }

完全错误:class Foo 需要是抽象的，因为未定义类型 [S](num: S)Boolean 的 trait A 中的方法

第一个不应该编译吗?我在这里做错了什么，我该如何解决?

Shouldn't the first compile? What am I doing wrong here and how can I fix it?

推荐答案

def something(num:Int):Boolean 和 def something[S](num:S):Boolean 有不同的方法签名.第一个的参数类型是Int.第二个的参数类型是泛型 S.

def something(num:Int):Boolean and def something[S](num:S):Boolean have different method signatures. The argument type of the first is Int. The argument type of the second is the generic S.

class Foo extends A {
  // You have to implement this abstract method
  // that is inherited from B via A.
  def something[S](num:S):Boolean = ???
}

trait A extends B {
  def something(num:Int):Boolean = {
    num == 1
  }
}

trait B {
  def something[S](num:S):Boolean
}

想象一下试图调用从 B 继承的 something[S](num:S) 方法:

Imagine trying to call the something[S](num:S) method inherited from B:

new Foo().something[String]("x")

如果您可以不实施它，您希望会发生什么?

If you could get away with not implementing it, what would you expect to happen?

在第二个例子中，由于A继承了B[Int]，类泛型S绑定到Int，所以A继承的方法是something(num:Int)，与实现的方法有相同的签名.

In the second example, since A inherits B[Int], the class generic S is bound to Int, so the method that A inherits is something(num:Int), which does have the same signature as the implemented method.

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