Scala中的伴侣对象和类访问

Question

Say I have this class and object hierarchy:

class A extends BB
class B extends BB
object A extends BB

abstract class BB {
   def writer: Writer (<say Writer is a custom class>)

   def write: {
       writer.<dothing>
   }
}

So writer has to be implemented in class A and B but not object A right in order for this code to work. Correct?

new A.write
new B.write

So writer has to be implemented in object A in order for this code to work. Correct?

A.write

Answer 1

trait : is stateless, and class or object both can extend trait .

You define something as object when it does not have any states.

scala>     trait CanFly {
     |       def fly: String
     |     }
defined trait CanFly

scala>     class DuckCanFly extends CanFly {
     |       override def fly: String = "duck duck"
     |     }
defined class DuckCanFly

scala>     object EagleCanFly extends CanFly {
     |       override def fly: String = "eagle eagle"
     |     }
defined object EagleCanFly

scala> new DuckCanFly().fly
res0: String = duck duck

scala> EagleCanFly.fly
res1: String = eagle eagle

If you want companion object define it as below, you don't have to extend the trait again.

scala> :paste
// Entering paste mode (ctrl-D to finish)

    class DuckCanFly extends CanFly {
      override def fly: String = "duck duck"
    }

    object DuckCanFly {
      def apply(): DuckCanFly = new DuckCanFly()
    }

// Exiting paste mode, now interpreting.

defined class DuckCanFly
defined object DuckCanFly

scala> DuckCanFly().fly
res8: String = duck duck

In your example, you can define it as trait as I do not see any state, and you can define childs as object .

trait BB {
   def writer: Writer
   def write = writer.write()
}

object A extends BB {
   def writer: Writer = new AWriter()
}

object B extends BB {
   def writer: Writer = new BWriter()
}