Scala 泛型类型扩展了可比较的接口

Question

以下scala代码工作正常：

class A(val value : Int) extends Comparable[A]
{
    override def compareTo(o: A): Int = this.value.compareTo(o.value)
}
class B(value : Int) extends A(value)
class C[T <: Comparable[T]](val value : T) extends Comparable[C[T]]
{
    override def compareTo(o: C[T]): Int = value.compareTo(o.value)
}
val a1 = new A(1)
val a2 = new A(2)
println(a1.compareTo(a2))
val b1 = new B(1)
val b2 = new B(2)
println(b1.compareTo(b2))
val ac1 = new C[A](a1)
val ac2 = new C[A](a2)
println(ac1.compareTo(ac2))

但是下一个编译有错误：

val bc1 = new C[B](b1)
val bc2 = new C[B](b2)
println(bc1.compareTo(bc2))

错误：类型参数 [B] 不符合类 C 的类型参数边界 [T <：可比较 [T]]

实际上，B类也有一个比较成员。如何更改 C 类的定义以使其与 B 类兼容？谢谢！

Answer 1

考虑使用Ordering类型类的替代实现，如下所示

class A(val value: Int)
class B(value: Int) extends A(value)
class C[T](val value: T)
implicit val aOrdering = new Ordering[A] {
  def compare(a1: A, a2: A): Int = a1.value compare a2.value
}
implicit val bOrdering = new Ordering[B] {
  def compare(b1: B, b2: B): Int = b1.value compare b2.value
}
implicit class CompareToA[T <: A](`this`: T) {
  def compareTo(that: T)(implicit o: Ordering[T]): Int = o.compare(`this`, that)
}
implicit class CompareToC[T](`this`: C[T]) {
  def compareTo(that: C[T])(implicit o: Ordering[T]): Int = o.compare(`this`.value, that.value)
}

哪些输出

val b1 = new B(1)
val b2 = new B(2)
val bc1 = new C[B](b1)
val bc2 = new C[B](b2)
bc1.compareTo(bc2) // res3: Int = -1

Answer 2

因为B扩展Comparable[A]，而不是Comparable[B]。它如何与T <: Comparable[T]不匹配(因为B <: Comparable[B]不是真的(。

以下代码工作正常：

val bc1 = new C[A](b1)
val bc2 = new C[A](b2)
println(bc1.compareTo(bc2))