Scala特征：定义嵌套属性

Question

I'm very beginner Scala programmer who's coming from Java. I'm trying to build an understanding of Scala's traits, as a superior alternative to Java's interfaces. In this case, I want to create a trait which, when implemented, will require an object to have attributes, and one or more of those attributes will themselves be objects with required traits. The following code demonstrates what I want, but it doesn't currently work.

trait Person{
  def name: String
  def age: Int
  def address extends Address

}

trait Address{
  def streetName: String
  def streetNumber: Int
  def city: String
}

object aPerson extends Person {
  override val name = "John"
  override age = 25
  override address = object { //this doesn't work
     def streetName = "Main St."
     def streetNumber = 120
     def city = "Sometown"
  }
}

So I want the Person trait to require the object to have an Address attribute, which itself has some required attributes. The compiler doesn't like the above code defining the address in aPerson .

What's the right way to do this?

Bonus question : Let's say the Address trait is only used here. Is there a way to define the Address trait anonymously inside the Person trait so it won't clutter the file?

Answer 1

I think this is what you're trying to do.

trait Person{
  val name: String
  val age: Int
  val address: Address
}

trait Address{
  val streetName: String
  val streetNumber: Int
  val city: String
}

object aPerson extends Person {
  val name = "John"
  val age = 25
  val address: Address = new Address { //this now works
    val streetName = "Main St."
    val streetNumber = 120
    val city = "Sometown"
  }
}

The Address trait can be made anonymous, but then traits like Person can't reference it because it has no named type.

trait Person{
  val name: String
  val age: Int
//val address: ?type?
}

object aPerson extends Person {
  val name = "John"
  val age = 25
  val address = new { //this also works
    val streetName = "Main St."
    val streetNumber = 120
    val city = "Sometown"
  }
}

aPerson.address.city  //res0: String = Sometown

Answer 2

You can override a def with an object .

trait Person {
  def name: String
  def age: Int
  def address: Address
}

object aPerson extends Person {
  val name = "John"
  val age = 25
  object address extends Address {
    val streetName = "Main St."
    val streetNumber = 120
    val city = "Sometown"
  }
}