kotlinlang #getting-started

class MyClass2(private val constructor_parameter: String) {
    private val initialized_by_class: String

    init {
        initialized_by_class = "asdasdasd"
    }
}

richodemus

10/24/2016, 6:35 AM

but that would mean that I'd have to define fields in two places? some in the primary constructor-place and some in the class body?

teedee

10/24/2016, 6:50 AM

@richodemus secondary constructors always have to call the primary constructor first. Any code in the constructor block will only be executed after the primary constructor has finished running. What you want to do is not possible unless you pass SomeObjectThatNeedsCodeToInitialize to the secondary constructor. I think the best solution here is to use a factory function in the class Companion Object.

richodemus

10/24/2016, 6:53 AM

That sounds pretty weird, initializing fields in the constructor is a pretty normal use case I'd say, why can't we do that in a nice way in kotlin, or am I misunderstanding something?

teedee

10/24/2016, 6:57 AM

I think I understood your question wrong. Your conclusion seems right, fields that you want passed in as arguments are defined in the class header, other fields are declared in the class body.

teedee

10/24/2016, 6:58 AM

and you can then use the init blocks to initialize those fields or just directly assign a value to them without using an init block

richodemus

10/24/2016, 6:59 AM

Yeah, but atleast to me it seems kind of weird that I'd have to define my fields in different places, like constructor_parameter and intitialized_by_class in my above example. I'm used to having all my class fields in the same place 🙂

elect

10/24/2016, 7:03 AM

@richodemus, you can skip completly the primary constructor, define your member variables (with

var

and

val

), use only secondary constructors and write there the code you want to write in the constructor

elect

10/24/2016, 7:04 AM

be sure every member variable gets initialized by each constructor

elect

10/24/2016, 7:04 AM

Copy code

class MyClass {   
    val myObject: SomeObjectThatNeedsCodeToInitialize
    constructor(whatever: Any) {
        //do what you need and init myObject
    }
    constructor(whatever2: Any2) {
        //do what you need and init myObject
    }
}

teedee

10/24/2016, 7:05 AM

I think the reasoning behind doing it this way was to avoid boilerplate. Often in Java the only thing you do with constructor params is assign them to a class field so the Kotlin team wanted to avoid all that boilerplate and just gave you a way to declare a constructor param as a class property in the constructor itself. For small classes properties will still be located together since usually the first thing in your class body are the class properties and constants.

👍 1

teedee

10/24/2016, 7:07 AM

It's also possible to not declare any properties in the primary constructor and define those properties alongside the other properties in the class body. They can then be assigned just like the other properties.

richodemus

10/24/2016, 7:08 AM

it seems like you can't assign anything to val-fields in the secondary-constructor

Copy code

class MyClass2() {
    private val constructor_parameter: String
    private val initialized_by_class: String
    
    constructor(param:String) : this() {
        this.constructor_parameter = param
        this.initialized_by_class = "asdasd"
        
    }
}

here it complains that both fields can't be reassigned

teedee

10/24/2016, 7:09 AM

All val fields have to be assigned in the primary constructor. Imagine you would call the primary constructor of this class.

teedee

10/24/2016, 7:09 AM

You would have 2 unassigned val fields which Kotlin doesn't allow

elect

10/24/2016, 7:10 AM

Copy code

class MyClass2 {
    private val constructor_parameter: String
    private val initialized_by_class: String
    constructor(asd:String)  {
        this.constructor_parameter = asd
        this.initialized_by_class = "asdasd"
    }
}

elect

10/24/2016, 7:10 AM

the moment you write

MyClass2()

()

stands for primary constructor

richodemus

10/24/2016, 7:11 AM

ah!

elect

10/24/2016, 7:11 AM

😄

richodemus

10/24/2016, 7:11 AM

now I got it working

richodemus

10/24/2016, 7:11 AM

sneaky paranthesis

elect

10/24/2016, 7:12 AM

use primary especially for dummy-classes, to avoid boilerplate code, as @teedee wrote before

richodemus

10/24/2016, 7:13 AM

but it still means you can do it two different ways, either without a primary constructor and do everything in a secondary constructor or read parameters in the primary and to other initialization in an init-block just a matter of taste I guess?

teedee

10/24/2016, 7:13 AM

I think the preferred way would be to use the init block. Secondary constructors are meant to extend the primary constructor + init blocks, not replace it.

richodemus

10/24/2016, 7:14 AM

init-blocks feels kind of nice, a bit more explicit I think

elect

10/24/2016, 7:14 AM

if you have primary constructor, you can insert init code in

init{}

, if you have secondary constructor, you do it naturally there

elect

10/24/2016, 7:14 AM

and if you have primary and secondary, every secondary must call the primary in turn

richodemus

10/24/2016, 7:15 AM

my main gripe with primary+init is that then the fields will get declared in two different places

elect

10/24/2016, 7:15 AM

if you have primary, it's like forcing everything passing through that, one way or the other

teedee

10/24/2016, 7:16 AM

You don't have to declare fields in the primary constructor, you can just pass normal params and declare all fields in your class body. The params can then just be assigned to those fields

👍 1