How can I use a variable in my inner class?

Question

I have this code using Kotlin:

class A{
    var myVariable:String?=null

    constructor(myVariable:String){
        this.myVariable = myVariable
    }

    init {
        println("Success !")
    }

    inner class B{
        init {
            println(myVariable)
        }
    }
}

fun main(args:Array<String>){
    var b = A("test").B // this does not work
}

And unfortunately it does not work I get:

Error:(20, 23) Kotlin: Nested class 'B' accessed via instance reference
Error:(20, 23) Kotlin: Classifier 'B' does not have a companion object, and thus must be initialized here

How can I solve my problem?

Zoe · Accepted Answer · 2019-03-04T19:41:54.413

When you do:

A("test").B

You're saying "Fetch something from an instance of A" - in this case, it's a class. However, this isn't the syntax you're looking for in your case. You can get mostly anything, but getting a reference is a separate issue. If you want to get a function, the syntax is different from getting a field or calling a function. Although this isn't very important, bit might still be worth keeping that in mind.

Since B is an inner class, you're so far entirely correct that you need an instance of A first. But you also need an instance of B. When you initialize B, it's still "connected" to the parent class, which is why you can access outer variables without any problems. However, it's still an initializable class - and you need to initialize it.

So you need to initialize B as well. The Kotlin syntax for this is pretty nice (where as the syntax in Java is slightly horrible) - all you need to add is () at the end.

So you'll end up with this:

val b = A("test").B()
// ...

This is because it's an inner class. If you had a static inner class (in Kotlin, that's a class within a class without the inner keyword), the initialization would've been A.B() - A isn't initialized in those cases.

You can also split up the initialization:

val a = A("test")
val b = a.B();

Once you have the variable, it's exactly like every other variable - the only difference here is the initialization.

Sergio · Answer 2 · 2019-03-04T19:25:16.190

You should create an instance (call constructor) of class B before accessing its members:

fun main(args:Array<String>) {
    val b = A("test").B()
    b.someFunction() 
}

class A {
    var myVariable:String? = null

    constructor(myVariable: String) {
        this.myVariable = myVariable
    }

    init {
        println("Success !")
    } 

    inner class B {
        init {
            println(myVariable)
        }

        fun someFunction() {
            myVariable = "set new value to the variable"
        }
    }
}

score 0 · Answer 3 · answered Mar 04 '19 at 19:19

Try this:

class A{
    var myVariable:String?=null

    constructor(myVariable:String){
        this.myVariable = myVariable
    }

    init {
        println("Success !")
    }

    inner class B{
        init {
            println(myVariable)
        }
    }
}

fun main(args:Array<String>){
    var b = A("test").B() // You have to call the constructor here
}

How can I use a variable in my inner class?

3 Answers3