Kotlin - Property must be initialized or be abstract even if there is an init() function

Question

I have converted this code from Java to Kotlin using Android Studio 3.0

internal var background: Drawable
internal var xMark: Drawable

private fun init() {
    background = ColorDrawable(Color.RED)
    xMark = ContextCompat.getDrawable(this@Subscriptions_main, R.drawable.delete)
}

On line 1 and 2 I'm getting the error:

Property must be initialized or be abstract

even though it is going to be initialized in the init function.

Is writing:

internal var background: Drawable? = null
internal var xMark: Drawable? = null

a viable and efficient solution? Is there any other better way?

Solution 1

Before using lateinit, you must understand what that means.

Your variables are not initialized properly. Various ways to solve that problem:

• Initialize the variables inside the constructor or a proper init block (not a private function as you declared it)
• Initialize the variables at the same time as you declare them
• Initialize the variables later and let Kotlin know that (this is the lateinit keyword)

Those 3 options are not equivalent, and depending on your code, the two first ones may be more appropriate than the third one.

lateinit will make your app crash if you access the variables before they are actually initialized.

Solution 2

The init blocks are not functions, just remove the private fun part and the parentheses:

internal var background: Drawable
internal var xMark: Drawable

init {
    background = ColorDrawable(Color.RED)
    xMark = ContextCompat.getDrawable(this@Subscriptions_main, R.drawable.delete)
}

Comments

[Héctor]

You can use lateinit keyword

[Daniele]

Thanks this worked, write an answer argomenting on that if you want. It's gonna be useful for other users too

[Héctor]

Great, glad to help you ;)

[Vincent Mimoun-Prat]

Don't use lateinit blindly, understand what it does and why it may not be what you want.

[BakaWaii]

init() is only a function, you cannot guarantee it is called before the property being accessed.

[Vincent Mimoun-Prat]

This solves the problem ... until the code path changes and variables are accessed before being initialized. We do not have enough code to know if lateinit is appropriate or if initializing in constructor / declaration is better.

[Héctor]

Well, in this case, it works for the OP... but ok, you're right.

[Dale Wilson]

OMG, OMG, Never use FEATURE XYZZY because it has A PITFALL is seldom helpful (there are a few cases of language features that should not actually be used, but they are rare.) lateinit is in the language for a reason. As long as the answer mentions the pitfall (or in this case says "read the docs") it is helpful.