Private getter and public setter for a Kotlin property

Write-only properties with compile-time errors can be achieved since Kotlin 1.0, using a workaround based on @Deprecated.

Implementation

Kotlin allows to mark functions deprecated with level ERROR, which leads to a compile-time error when called. Annotating the get accessor of a property as error-deprecated, combined with a backing field (so that private reads are still possible), achieves the desired behavior:

class WriteOnly {
    private var backing: Int = 0

    var property: Int
        @Deprecated("Property can only be written.", level = DeprecationLevel.ERROR)
        get() = throw NotImplementedError()
        set(value) { backing = value }

    val exposed get() = backing // public API
}

Usage:

val wo = WriteOnly()
wo.property = 20         // write: OK

val i: Int = wo.property // read: compile error
val j: Int = wo.exposed  // read value through other property

The compile error is quite helpful, too:

Using 'getter for property: Int' is an error. Property can only be written.

---

Use cases

1. The main use case are obviously APIs that allow properties to be written, but not read:

   user.password = "secret"
   val pw = user.password // forbidden
   

2. Another scenario is a property which modifies the internal state, but is not stored itself as a field. (Could be done more elegantly using different design).

   body.thrust_force = velocity
   body.gravity_force = Vector(0, 0, 9.8)
   // only total force accessible, component vectors are lost
   val f = body.forces
   

3. This pattern is also useful for DSLs of the following kind:

   server {
       port = 80
       host = "www.example.com"
   }
   

   In such cases, values are simply used as one-time settings, and the write-only mechanism described here can prevent accidentally reading a property (which might not be initialized yet).

---

Limitations

Since this feature was not designed for this use case, it comes with certain limitations:

• If accessed using a property reference, the compile-time error turns into a runtime error:

  val ref = wo::property
  val x = ref.get() // throws NotImplementedError
  

• The same is true for reflection.

• This functionality cannot be outsourced into a delegate, because an error-deprecated getValue() method cannot be used with by.

It's impossible at the moment in Kotlin to have a property with a setter that is more visible than the property. There's a language design issue in the issue tracker on this, feel free to watch/vote for it or share your use cases: https://youtrack.jetbrains.com/issue/KT-3110

In current Kotlin version (1.0.3) the only option is to have separate setter method like so:

class Test {
    private var name: String = "name"

    fun setName(name: String) {
        this.name = name
    }
}

If you wish to restrict external libraries from accessing the getter you can use internal visibility modifier allowing you to still use property syntax within the library:

class Test {
    internal var name: String = "name"
    fun setName(name: String) { this.name = name }
}

fun usage(){
    val t = Test()
    t.name = "New"
}