Scala, Extend object with a generic trait

问题:

I'm using Scala and I want to extend a (singleton) object with a trait, which delivers a data structure and some methods, like this:

trait Tray[T] {
  val tray = ListBuffer.empty[T]

  def add[T] (t: T) = tray += t
  def get[T]: List[T] = tray.toList
}

And then I'll would like to mix-in the trait into an object, like this:

object Test with Tray[Int]

But there are type mismatches in add and get:

Test.add(1)
// ...

How can I'll get this to work? Or what is my mistake?

回答1:

The problem is that you're shadowing the trait's type parameter with the T on the add and get methods. See my answer here for more detail about the problem.

Here's the correct code:

trait Tray[T] {
  val tray = ListBuffer.empty[T]

  def add (t: T) = tray += t      // add[T] --> add
  def get: List[T] = tray.toList  // get[T] --> add
}

object Test extends Tray[Int]

Note the use of extends in the object definition—see section 5.4 of the spec for an explanation of why with alone doesn't work here.