``` class A val foo = hello class B a A init a foo bar null kotlinlang #announcements

05/15/2019, 3:19 PM

class A {
    val foo = "hello"
}
class B(a: A) {
    init {
        a.foo.bar // null pointer
    }
}
`

streetsofboston

05/15/2019, 3:20 PM

bar

? Where does

bar

05/15/2019, 3:20 PM

foo

isn't a string. It's just a method of the field

a.foo

null

streetsofboston

05/15/2019, 3:21 PM

Could you post a snippet of the actual code so that we can see what might wrong?

E.g. also how the constructor of the

class is called.

05/15/2019, 3:23 PM

hm yeah I can't reproduce it with this snippet. Will play a bit with a scratch and post if I can't figure it out

ok, so the problem is this

class B(a: A) {
    init {
        println(a.foo.length)
    }
}

class A {
    val b = B(this)
    val foo = "hello"
}

B(A())

And the solution is to move the line with

val foo = "hello"

above the one that's initializing

05/15/2019, 3:27 PM

circular dependencies are a b**ch

streetsofboston

05/15/2019, 3:28 PM

yes, this makes sense.

foo

has not yet been initialized when you access it in `B`’s constructor block

05/15/2019, 3:30 PM

Nothing unexpected here

streetsofboston

05/15/2019, 3:33 PM

Try something like this. It won’t run:

open class A(val b: B)
open class B(val a: A)

object objectA : A(objectB)
object objectB : B(objectA)

If you change it to this:

open class A(val b: B?)
open class B(val a: A?)

Then the value of

objectA.b

objectB.a

may or may not be null… 🙂

05/15/2019, 3:37 PM

yeah I understand the reason... bit hairy 😄

thanks for the help in any case!

Stephan Schroeder

05/16/2019, 8:54 AM

you probably did this already, if not, just exchange the lines in A

class B(a: A) {
    init {
        println(a.foo.length)
    }
}

class A {
    val foo = "hello"
    val b = B(this)
}

B(A())

05/16/2019, 6:58 PM

@Stephan Schroeder yes I did 😄