Kotlin secondary constructor

Question

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")

As the documentation points, you can use a secondary constructor this way

class GoogleMapsRestApiClient constructor(val baseUrl: String) {

    constructor() : this("https://api.whatever.com/")

}

Remember that you must extended the first constructor behavior.

for declaring a secondary constructor Kotlin just use the constructor keyword: like

this is a primary constructor:

class Person constructor(firstName: String) {

}

or

class Person(firstName: String) {

}

for the secondary constructor code like this:

class Person(val name: String) {
    constructor(name: String, parent: Person) : this(name) {
        parent.children.add(this)
    }
}

it is mandatory to call the primary constructor otherwise, the compiler will throw the following error

Primary constructor call expected

Constructors with init:

class PhoneWatcher : TextWatcher {

    private val editText: EditText
    private val mask: String

    private var variable1: Boolean = false
    private var variable2: Boolean = false

    init {
        variable1 = false
        variable2 = false
    }

    constructor(editText: EditText) : this(editText, "##-###-###-####")

    constructor(editText: EditText, mask: String) {
        this.editText = editText
        this.mask = mask
    }
    ...
}