如何在Scala上使用Trait创建类？

Question

Trait GenericLinkedList , case class Cons and case object Nil were created like below. The question is I want to use this genericLinkedList however as you know when we write this code var list = new GenericLinkedList , it will not cause Traits cannot create any object , Right? I want to create a class which extends GenericLinkedList but I cannot. How can I fix it ?

trait GenericLinkedList [+T] {
def prepend[TT >: T](x: TT): GenericLinkedList[TT] = this match {
    case _ => Cons(x,this) 
  }
}
case class Cons[+T](head: T,tail: GenericLinkedList[T]) extends GenericLinkedList[T]
case object Nil extends GenericLinkedList[Nothing]

Answer 1

Your issue seems to be unable of doing

val list = new GenericLinkedList

Is your goal creating an empty list?

You can do

val list = new GenericLinkedList[Int] { }

since the trait is not abstract, but it's not pretty. You can alternatively define a companion object for your trait

object GenericLinkedList {
  def apply[T](): GenericLinkedList[T] = Nil
}

and use it to initialize an empty list this way

scala> val x = GenericLinkedList[Int]()
// x: GenericLinkedList[Int] = Nil

scala> x.prepend(42)
// res0: GenericLinkedList[Int] = Cons(42,Nil)

---

By the way, the universal match in the prepend implementation is useless. You can just do

 def prepend[TT >: T](x: TT): GenericLinkedList[TT] = Cons(x, this)