Kotlin: Pass object in function as reference and change its instance

Question

I cannot find how to pass the reference of the objects so that I can let them point to a new location in the memory. How to do this in Kotlin?

Code:

class Parent { } class Child : Parent { } class GlobalDataHolder { var globalChildVriable: Child? = null // imagine this is a static variable and can be accessed anywhere } class Activity() { var variable1 = Child() Helper.init(variable1, GlobalDataHolder.globalChildVriable) // inside onCreate() } class Helper { initStuff(variable1: Parent, globalVariable: Parent?) { if (globalVariable == null) { globalVariable = variable1 // error saying val cannot be re-assigned } else { variable1 = globalVariable!! } } }

I want variable1 and globalVariable to be modifiable. So the original owners of them will have the latest values.

Kotlin does not support mutable method parameters. discuss.kotlinlang.org/t/… — TheTechWolf
– TheTechWolf, Commented Oct 19, 2017 at 8:45
@guenhter is there a policy to post compiling code? we can post pseudo code right? — iadcialim24
– iadcialim24, Commented Oct 19, 2017 at 9:19
Hi, yes, but when you already using kotlin, than it's quite useful to also make the code compiling (because in your sample, just a few keywords were missing to have compiling and valid kotlin code). This makes it easier for anyone to help you. — guenhter
– guenhter, Commented Oct 19, 2017 at 9:38

RobCo · Accepted Answer · 2017-10-19 09:47:34Z

Kotlin (and Java) do not natively support reference types like C/C++ do. However, kotlin does have property and functional types.
A property is basically just a combination of 2 functions (a getter and a setter). You can pass properties as arguments and, with reflection, call their getter setter individually from a different context.

Updated (compilable) code:

open class Parent { } open class Child : Parent() { } var globalVariable: Child? = null class Activity() { var localVariable = Child() init { Helper.initStuff(this::localVariable, ::globalVariable) } } object Helper { fun initStuff(localProperty: KMutableProperty0<out Parent>, globalProperty: KMutableProperty0<out Parent?>) { val globalVariable = globalProperty.call() val variable1 = localProperty.call() if (globalVariable == null) { globalProperty.setter.call(variable1) } else { variableProperty.setter.call(globalVariable) } } }

hi thanks for this. But i got a Type Mistmatch error for the line: Helper.initStuff. I just followed ur example by passing the localVariable
Good alternative however it will require the kotlin-reflection jar

guenhter · Accepted Answer · 2017-10-19 08:52:48Z

Such problems can "always" be solved by introducing one more level of abstraction. You can for example add a new interface VarHolder which is responsible for getting/setting the contained variable.

interface VarHandle<E> { var variable: E } class Activity() { var variable1 = Child() fun onCreate() { Helper.initStuff(object : VarHandle<Child> { override var variable: Child get() = variable1 set(value) { variable1 = value } }, object : VarHandle<Child?> { override var variable: Child? get() = GlobalDataHolder.globalChildVariable set(value) { GlobalDataHolder.globalChildVariable = value } }) } } object Helper { fun initStuff(localVarHolder: VarHandle<Child>, globalVarHolder: VarHandle<Child?>) { val globalVar = globalVarHolder.variable val localVar = localVarHolder.variable if (globalVar == null) { globalVarHolder.variable = localVar } else { localVarHolder.variable = globalVar } } }

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