Kotlin: Function declaration must have a name

Question

Aim of the code: Class Pair can print out Product name and quantity, Product name stored in Class Product

class Pair<T, U>(var product: Product, var quantity: Int) {
    for ( (product,quantity) in productAndQuantityList) {
        println("Name: ${product.productName}")
        println("Quantity: $quantity")
    }
}

Above Error:(2, 9) Kotlin: Expecting member declaration Error:(2, 57) Kotlin: Function declaration must have a name

class ShoppingCart{
    private val productAndQuantityList = mutableListOf<Pair<Product,Int> >()
...
}

open class Product(
    val productName: String,
    var basePrice: Double,
    open val salesPrice: Double,
    val description: String) {
...}

---

1. may i know how to change my code?
2. after class Pair was suggested by Compiler, but should i fill in anything?
3. Which topic should i work for, to avoid the same errors again?

Thanks!

Answer 1

If you want to run the for loop when the object is instantiated, then you should use an initializer. You can't simply put statements inside class definitions directly.

class Pair<T, U>(var product: Product, var quantity: Int) {
  init {
    for ( (product,quantity) in productAndQuantityList) {
        println("Name: ${product.productName}")
        println("Quantity: $quantity")
    }
  }
}

However, this code is wrong because Pair does not have access to productAndQuantityList, although ShoppingCart does. As Mathias Henze suggested, you should make a function in ShoppingCart and move the for loop into that, like this:

fun printProducts() {
  for ( (product,quantity) in productAndQuantityList) {
    println("Name: ${product.productName}")
    println("Quantity: $quantity")
  }
}

As for your Pair class, the type parameters T and U are unnecessary, as you don't use them anywhere, and the class itself is provided by the standard library (The header looks something like data class Pair<out A, out B>(val first: A, val second: B).

If you're determined to use your own Pair class, be sure to make it a data class, so it can be destructured, and change the type of productAndQuantityList to mutableListOf<Pair> (without the type parameters Pair<Product, Int>).

Update

Please read the answer by Mathias Henze, which is correct. My answer, was originally completely wrong, but I have now corrected it.

Answer 2

The productAndQuantityList is just for storing the data. The Pair class is a provided class by Kotlin. You don't need to add anything to it in your usecase.

The ability to print the product and the quantity should be a function of the ShoppingCart, so just have:

class ShoppingCart{
    private val productAndQuantityList = mutableListOf<Pair<Product,Int> >()
    // ...
    fun printContents() {
        for ( (product,quantity) in productAndQuantityList) {
            println("Name: ${product.productName}")
            println("Quantity: $quantity")
        }            
    }
}