Scala 伴生对象和应用方法

Question

I'm trying to make this statement work in my Scala code:

val dvd1 = Item(new Description("The Matrix DVD", 15.50, "DVD World"))

I have the following classes and companion object:

class Item(){
    private var id = 0
    def getId(): Int = this.id
}

object Item{
    def apply(description: String, price: Double, supplier: String): Description = {
        new Description(description, price, supplier)
    }

    def nextId: Int = {
        this.id += 1
    }
}

class Description(description: String, price: Double, supplier: String){
    def getDescription(): String = description
    def getPrice(): Double = price
    def getSupplier(): String = supplier
}

And I get the following error with my apply function and nextId:

error: not enough arguments for method apply: (description: String, price: Double, supplier: String)Description in object Item.
Unspecified value parameters price, supplier.
    val dvd1 = Item(new Description("The Matrix DVD", 15.50, "DVD World"))
                   ^
indus.scala:16: error: value id is not a member of object Item
        this.id += 1
             ^

It's not apparent to me what I'm doing wrong.

Question: What do I need to change with my apply function so that dvd1 will work as expected. Also, nextId should increment the Item's id when Item.nextId is called, what's wrong there?

Answer 1

1) you are trying to access class data from companion object which you can't.

scala> case class Order(id: String)
defined class Order

scala> object Order { println(id) }
<console>:11: error: not found: value id
       object Order { println(id) }
                              ^

The reverse works, once you import your companion object inside class.

2) you when define apply inside your companion, you you have two apply functions now, one with empty args and one with three args as you defined which you want to call. Your args is wrong.

Based on your comments below, you want to have Item has Description datastructure which can be done as below using immutable classes in scala called case class

import scala.util.Random

case class Description(description: String, price: Double, supplier: String)
case class Item(id: Int, description: Description)

object Item {
  def apply(description: String, price: Double, supplier: String): Item = {
    val itemId = Random.nextInt(100)
    new Item(itemId, Description(description, price, supplier))
  }
}


//client code
val dvd1 = Item("The Matrix DVD", 15.50, "DVD World")

assert(dvd1.isInstanceOf[Item])
assert(dvd1.description.description == "The Matrix DVD")
assert(dvd1.description.price == 15.50)
assert(dvd1.description.supplier == "DVD World")

see code here in online scala editor - https://scastie.scala-lang.org/prayagupd/P3eKKPLnQYqDU2faESWtfA/4

Also, read case class explanation