Iterable.kt

class A : Iterable<String> {
    override operator fun iterator(): Iterator<String> = MyIterator()
}

class MyIterator: Iterator<String> {
    override operator fun hasNext(): Boolean = Math.random() < 0.5
    override operator fun next(): String = "a"
}

// A().forEach({ print(it)})

for in だけなら、Iterable、Iterator への準拠を宣言しなくてもできる

class A {
    operator fun iterator() = MyIterator()
}

class MyIterator {
    operator fun hasNext(): Boolean = Math.random() < 0.5
    operator fun next(): String = "a"
}

// A().forEach({ print(it)}) ❌
// for (item in A()) { print(item) } ⭕️