Probably a newbie question? I have 2 enums that c...

# getting-started

[elye]

Probably a newbie question?

I have 2 enums that cross-reference each other

enum class School(val zone: String, val captain: Student) {
    Metricon("New York", Student.David),
    Carlisle("London", Student.Paul)
}

enum class Student(val lastName: String, val school: School) {
    David("Samuel", School.Metricon),
    Solomon("Handsome", School.Metricon),
    Saul("Black", School.Metricon),
    Paul("Lewis", School.Carlisle),
    Joseph("Hardy", School.Carlisle),
    John("Baptise", School.Carlisle),
}

When I print it out

val student = Student.David
        val school = School.Carlisle

        println(student.school)
        println(school.captain)
        println(student.lastName)
        println(school.zone)

It resulted in

Metricon
null
Samuel
London

Notice the null there. The school.captain is missing and became null.
How can I solve this problem?

[elye]

Ask this in StackOverflow too, and feel free to answer there https://stackoverflow.com/questions/73331371/enum-value-became-null-when-cross-referencing-how-to-solve-this-issue Thanks 🙏

[Roukanken]

I think this is the same problem as:

class A() {
    val x: String = initX()
    val y: String = "value"

    fun initX(): String = y
}

A().x     // is null

eg, at the time x was "calculated", the y was not yet initialized, and was therefore null as all objects within JVM before initialization, and therefore is null even though types don't allow that

In your case: you first accessed Student.David, which started initializing Student enum, but to init the first David, it needed to access School.Metricon so the School enum started initializing - but at that point, every other Student than David is still null

[Michael de Kaste]

The problem is a constructor referenced cycle where like @Roukanken explained.

You kinda have to make sure that getting the value in one of the classes is done only when the value is called like so:

enum class School(val zone: String) {
    Metricon("New York") {
        override val captain: Student
            get() = Student.David
    },
    Carlisle("London") {
        override val captain: Student
            get() = Student.Paul
    };

    abstract val captain: Student
}

[Joffrey]

That said, it would be interesting to know why the student class is an enum. What is your real world use case?