JPA.kt

data class Course(val id: Long, val students: MutableSet<Student> = mutableSetOf())
data class Student(val id: Long, val courses: MutableSet<Course> = mutableSetOf())

fun main() {
    val english = Course(1L)
    val john = Student(1L)
    english.students.add(john)
    john.courses.add(english)

    println(john) // StackOverflowError
    println(english.students.contains(john)) // StackOverflowError
}