Kotlin secondary constructor

Question

How do I declare a secondary constructor in Kotlin?

Is there any documentation about that?

Following does not compile...

class C(a : Int) {
  // Secondary constructor
  this(s : String) : this(s.length) { ... }
}

Answer 1

Update: Since M11 (0.11.*) Kotlin supports secondary constructors.

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For now Kotlin supports only primary constructors (secondary constructors may be supported later).

Most use cases for secondary constructors are solved by one of the techniques below:

Technique 1. (solves your case) Define a factory method next to your class

fun C(s: String) = C(s.length)
class C(a: Int) { ... }

usage:

val c1 = C(1) // constructor
val c2 = C("str") // factory method

Technique 2. (may also be useful) Define default values for parameters

class C(name: String? = null) {...}

usage:

val c1 = C("foo") // parameter passed explicitly
val c2 = C() // default value used

Note that default values work for any function, not only for constructors

Technique 3. (when you need encapsulation) Use a factory method defined in a companion object

Sometimes you want your constructor private and only a factory method available to clients. For now this is only possible with a factory method defined in a companion object:

class C private (s: Int) {
    companion object {
        fun new(s: String) = C(s.length)
    }
}

usage:

val c = C.new("foo")