Kotlin secondary constructor

问题:

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) { ... }
}

回答1:

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

---

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")

回答2:

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.

回答3:

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

回答4:

You can define multiple constructors in Kotlin with constructor but you need to skip default constructor class AuthLog(_data: String)

class AuthLog {

    constructor(_data: String): this(_data, -1)

    constructor(_numberOfData: Int): this("From count ", _numberOfData)

    private constructor(_data: String, _numberOfData: Int)

}

For more details see here

回答5:

I just saw this question and I think there may be another technique which sounds even better than those proposed by Andrey.

class C(a: Int) {
    class object {
        fun invoke(name: String) = C(name.length)
    }        
}

That you can either write something like val c:C = C(3) or val c:C = C("abc"), because the invoke methods work kind of the same way the apply methods work in Scala.

Update

As of now, secondary constructors are already part of the language spec so this workaround shouldn't be used.

回答6:

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
    }
    ...
}

回答7:

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

you can try this.

回答8:

The code snippet below should work

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

回答9:

kotlin Secondary constructor example

class Person(name: String){
    var name=""
    var age=0

    constructor(age :Int,name : String)  : this(name){
        this.age=age
        this.name=name
    }
    fun display(){
        print("Kotlin Secondary constructor $name  , $age")
    }
}

main function

fun main(args : Array<String>){

    var objd=Person(25,"Deven")
    objd.display()
}