class User(val userName: String,val age: Int) extends Comparable[User] {
  override def compareTo(o: User): Int = this.age - o.age
}


object OrderingDemo extends App {

  /**
    * 類型界定，上界
    *
    * @param u1
    * @param u2
    * @tparam T T必須是Comparable[T]的子類
    * @return
    */
  def compare[T 
 
<: Comparable[T]](u1: T, u2: T): Int = { 

    if (u1.compareTo(u2) == 0) 0 else if (u1.compareTo(u2) > 0) 1 else -1

  }


  val u1 = new User("u1", 18)
  val u2 = new User("u2", 19)

  //  compare(u1,u2) //class User(userName: String, age: Int)的話，編譯不通過

  println(compare(u1,u2)) //class User(val userName: String,val age: Int) extends Comparable[User]
 



}

Scala 類型界定