Kotlin Interface Property: Only require public getter without public setter

Question

I am trying to create an Interface that forces its deriving classes to have a certain property, but only with a public getter. Since I don't want to allow changes from outside its private scope, I don't want it to implement a public set.

interface Transaction{
   var transferDate: Date get //I only require a get, no set
}

class MoneyTransaction(){
   override var transferDate: Date get private set 
/*private set does not work*/
}

Answer 1

A property is an accessor to some data. You'll have a getter and if the property is mutable a setter as well. Therefore you can override any declared val property with a var property:

interface Transaction {
   val transferDate: Date
}

class MoneyTransaction: Transaction {
   override lateinit var transferDate: Date
       private set 
}

Note that you don't need to make the property a lateinit var if you initialize it with the object; I just added it to have your example compile properly.

Answer 2

In addition to @tynn's helpful answer, in my case I simply needed an accessor for data coming from another source. For this scenario, given that a value is being initialized with a getter, val could be used in place of lateinit var and there was no need to indicate private set.

As an example in line with the others:

interface Transaction {
   val transferDate: Date
}

class MostRecentTransaction(private val repo: AccountRepository) : Transaction {
   override val transferDate: Date
      get() = repo.transactions.first.transferDate
}