Private getter and public setter for a Kotlin property
JavaKotlinKotlin InteropJava Problem Overview
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
.
Java Solutions
Solution 1 - Java
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
Solution 2 - Java
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"
}
Solution 3 - Java
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
-
The main use case are obviously APIs that allow properties to be written, but not read:
user.password = "secret" val pw = user.password // forbidden
-
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
-
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 withby
.