generics - Scala，高级泛型扩展

Question

我正在尝试重写 https://gist.github.com/319827到斯卡拉。但我无法编译它。什么是正确的语法？

我总是收到错误:

class type required but java.util.Comparator[_ >: java.lang.Comparable[java.lang.Object]] found

来源:

package v6ak.util

import java.util.Comparator

object NaturalComparator extends Comparator[_ >: Comparable[Object]]{

    override def compare(o1:Comparable[Object], o2:Comparable[Object]) = {
        if( o1==null || o2==null ){
            throw new NullPointerException("Comparing null values is not supported!");
        }
        o1.compareTo(o2);
    }

}

Answer 1

A extends B写的是 A<:B在 Scala 中不是 A>:B
顺便说一句，scala 类型系统足够强大，可以避免在代码中使用 Object(scala 中的 AnyRef)

package v6ak.util

import java.util.Comparator

class NaturalComparator[T <: Comparable[T]] extends Comparator[T] {
  override def compare(o1: T, o2: T) = {
    if (o1 == null || o2 == null) {
      throw new NullPointerException("Comparing null values is not supported!");
    }
    o1.compareTo(o2);
  }
}

object StringComparator extends NaturalComparator[String]

object Examples {
  StringComparator.compare("a", "b")
  StringComparator.compare(2, "b") // error
}