Kotlin initialize a variable before its declaration?

Question

Here is my test code:

class Test {
    init {
        a = 1
    }

    constructor() {
        a = 2
    }

    private var a: Int

    init {
        a = 3
    }
}

If I remove the secondary constructor:

class Test {
    init {
        a = 1 // Error: Variable cannot be initialized before declaration
    }

//    constructor() {
//        a = 2
//    }

    private var a: Int

    init {
        a = 3
    }
}

I know that

During an instance initialization, the initializer blocks are executed in the same order as they appear in the class body.

But why can I initialize the variable before its declaration if there is a secondary constructor?

Update:

And I found an interesting thing:

class Test {
    init {
        a = log(1)
    }

    constructor() {
        a = log(2)
    }

    private var a: Int = log(0)

    init {
        a = log(3)
    }
}

fun log(i: Int): Int {
    println(i)
    return i
}

fun main(args: Array<String>) {
    Test()
}

The output is: 1 0 3 2, this is same as Java, declaration and initialization are two different step, but that is weird for Kotlin's primary constructor, Er...

It’s important to know that any init effectively becomes part of the primary constructor, which is being executed before secondary — s1m0nw1
– s1m0nw1, Commented Jan 11, 2018 at 9:42
The second part of your question (regarding the sequence 1 0 3 2) is explained in the documentation (go to the section named Secondary Constructors). — Michael
– Michael, Commented Jan 11, 2018 at 10:30
constructor() is a weird thing since it clashes with the primary constructor signature. Shouldn't it be a error? — voddan
– voddan, Commented Jan 11, 2018 at 11:15

K F · Accepted Answer · 2018-07-13 19:08:21Z

3

This is expanding from Michael's comment which refers to the Kotlin documentation:

Note that code in initializer blocks effectively becomes part of the primary constructor. Delegation to the primary constructor happens as the first statement of a secondary constructor, so the code in all initializer blocks is executed before the secondary constructor body. Even if the class has no primary constructor, the delegation still happens implicitly, and the initializer blocks are still executed.

In other words, the init() blocks get associated to the (implicit) primary constructor, which is executed as the first line of your secondary constructor. -Kf

edited Jul 13, 2018 at 19:08

answered May 25, 2018 at 6:50

K F

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smt · Accepted Answer · 2018-02-05 17:35:35Z

1

From my point of view it relates not to kotlin but to JVM bytecode which does not actually has "initialization" of variables it just fills them up in constructor, you can inspect it with some decompiler.

answered Feb 5, 2018 at 17:35

smt

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Comments

Mikhail Glukhikh · Accepted Answer · 2018-01-16 14:57:40Z

0

Your example looks like minor inconsistency. It's provoked by a fact the first example has no primary constructor, but the second has one. However, https://youtrack.jetbrains.com/issue/KT-22317 was created.

answered Jan 16, 2018 at 14:57

Mikhail Glukhikh

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