迭代器函数不适用于内部变量

Question

I'm completing Kotlin Koans's For loop exercise and am wondering why:

class DateRange(val start: MyDate, val end: MyDate) : Iterable<MyDate> {
    override fun iterator(): Iterator<MyDate> = DateIterator(this)
}

class DateIterator(val daterange: DateRange) : Iterator<MyDate> {
    var currentDay: MyDate = daterange.start
    override fun hasNext(): Boolean = currentDay.next() <= daterange.end

    override fun next(): MyDate {
        val result = currentDay
        currentDay = currentDay.nextDay()
        return result

    }
}

I'm receiving an Unresolved reference: next on currentDay() .

I'm aware the answer to this question is the exact same code without next() , but am wondering why it doesn't work - considering currentDay is a MyDate object and next() returns a MyDate .

Any idea why?

Answer 1

considering currentDay is a MyDate object and next() returns a MyDate

But there is no method next() on the MyDate class (which is what the error message tries to say). So currentDay.next() will not compile.

There is only next() on the DateIterator class (but of course you don't want to call that as it would have the side-effect of advancing the iterator).

You must have meant to call currentDay.nextDay() instead.