Kotlin override generic variable

Question

I have the following classes, but myfield variable in both NumberField and TextField cannot compile with:

Var-property type is InputField<String?>, which is not a type of overridden public abstract var inputField: InputField<*> defined in [my project]


interface FieldComponent {
    var myfield: InputField<*>  // <-- what should this be
}

interface InputField<T> {
    fun collectInput(): T
}

class NumberField(): FieldComponent{
    override lateinit var myfield: InputField<Int> // won't compile

    fun doSomething(){
        val x: Int = myfield.collectInput()
    }
}

class TextField(): FieldComponent{
    override lateinit var myfield: InputField<String> // won't compile

    fun doSomething(){
        val x: String = myfield.collectInput()
    }
}

I don't really need to know about the type in FieldComponent, but I need to have access to myfield if I have an instance of FieldComponent Can we make this work? Thank you

myfield should be val in FieldComponent, otherwise you will not be able to use the subtypes when overriding the property — IR42
– IR42, Commented Nov 5, 2021 at 7:15
Why not use a type parameter like you did with the InputField interface? — Abby
– Abby, Commented Nov 5, 2021 at 7:25
Do you intend to let users of FieldComponent replace the value of myfield? I ask, because it was declared as var. — broot
– broot, Commented Nov 5, 2021 at 8:21
Changing myfield in FieldComponent to val solve the problem. Thank you all. — BabyishTank
– BabyishTank, Commented Nov 5, 2021 at 16:27
@IR42 consider converting your comment into an answer so that it can be accepted so that the question can be ”closed” and others can benefit from it and easily understand which could be a possible solution for similar questions ;) Thanks! — João Dias
– João Dias, Commented Nov 5, 2021 at 20:30

James Lan · Accepted Answer · 2021-11-06 05:53:16Z

3

Make FieldComponent generic.

interface FieldComponent<T> {
    var myfield: InputField<T>
}

interface InputField<T> {
    fun collectInput(): T
}

class NumberField(): FieldComponent<Int> {
    override lateinit var myfield: InputField<Int>
    fun doSomething(){
        val x: Int = myfield.collectInput()
    }
}

class TextField(): FieldComponent<String> {
    override lateinit var myfield: InputField<String>
    fun doSomething(){
        val x: String = myfield.collectInput()
    }
}

answered Nov 6, 2021 at 5:53

James Lan

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