Private getter and public setter for a Kotlin property

How to make a property in Kotlin that has a private getter (or just do not have it) but has a public setter?

var status
private get

doesn't work with an error: Getter visibility must be the same as property visibility

In my case, the reason is for Java interop: I want my Java code to be able to call setStatus but not getStatus.

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"
}

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.