Kotlin primary constructor calling secondary constructor - kotlin

Question

Why does this not compile?
class test
{
constructor() {
var a = Date().day
this(a)
}
constructor(a:Int) {
}
}
error is:
Expression 'this' of type 'test' cannot be invoked as a function. The function 'invoke()' is not found.
The suggested fix is to add this:
private operator fun invoke(i: Int) {}
Why?

Answer 1

First, both of these constructors are secondary constructors. A primary constructor is one which is located outside of the body of the class.
Second, as described in the documentation, the correct syntax to call another constructor is as follows:
class Test {
constructor() : this(1) { }
constructor(a: Int) { }
}

Answer 2

class test constructor(){ // primary constructor (The primary constructor is part of the class header: it goes after the class name (and optional type parameters))
constructor(a: Int) : this() { // secondary constructor
}
}
If you class have define primary constructor, secondary constructor needs to delegate to the primary constructor. See here.
I think primary constructor can not be called from secondary constructor.
You can think like this: secondary calls primary and primary calls secondary => endless loop => not possible
In your case, there are 2 secondary constructor, so you can do like
class test {
constructor() : this(Date().day) // I see it quite like Java here https://stackoverflow.com/questions/1168345/why-do-this-and-super-have-to-be-the-first-statement-in-a-constructor
constructor(a: Int) {
}
}

Answer 3

Couple of things are wrong here:
Classes should always use camel-case for their names (test -> Test)
You cannot call another constructor as you tried to (calling this(1) inside of the other constructors body)
I think what you actually want is a being a property and alternatively initialize it with a default value. You could do it like this
class Test(val a: Int) {
constructor() : this(1) // notice how you can omit an empty body
}
or even better, like this:
class Test(val a: Int = 1) // again an empty body can be omitted.
Edit:
If you need to do some calculations, as asked in the comment below Yole's answer:
class Test(val day: Int) {
// you can use any expression for initialization
constructor(millis: Long) : this(Date(millis).day)
}
or if things get more complicated:
class Test(var day: Int) {
// pass something (i.e. 1) to the primary constructor and set it properly in the body
constructor(millis: Long) : this(1) {
// some code
day = // initialize day
}
}