Get maximum value of nested class property - Kotlin

Issue

I have a class that has the following format:

  class Student(var name: String) {
   var name: String? = null
   var Math: Course? = null
   var English: Course? = null
   var Spanish: Course? = null

  
   }

  class Course {
    var id: Int
    var courseGrade: Int? = null
   }

And I have a variable that is of type Student

    var studentData = Student("test")

I need to iterate through all courses and get the grade of the highest course that's in class Student, to assign it to a "highestScore" variable. How can I iterate through the properties that have values in other class types?

Solution

The Student class is not designed properly for such a feature (you would need reflection to do what you want, which is really not advised here).

If you need to consider courses as a collection of things (to be able to iterate on them or perform operations on them), you should represent it as such in your class. For instance, you can use List<Course>:

class Student(
    val name: String? = null,
    val courses: List<Course> = emptyList(),
)

class Course(
    val id: Int,
    val name: String,
    val courseGrade: Int? = null
)

Then you can easily get the information you need:

val maxGrade = studentData.courses
                          .filter { it.courseGrade != null }
                          .maxOfOrNull { it.courseGrade!! }

But probably it would be better to not have a nullable grade here. Is it expected to have some course instances without a grade? If not, making it non-nullable makes the code even simpler:

val maxGrade = studentData.courses.maxOfOrNull { it.courseGrade }

---

If you really have to stick with this class structure, then I see mainly 2 solutions to get the list of courses (and then apply the above):

1. list properties by hand
2. find properties via reflection

For #1:

val courses = listOfNotNull(student.Math, student.English, student.Spanish)

Or to make it less verbose if you have many courses properties:

val courses = with(student) { listOfNotNull(Math, English, Spanish) }

For #2 (requires kotlin-reflect and honestly I really don't advise this):

private fun KProperty1<Student, *>.isNullableCourseType() = returnType.isSubtypeOf(Course::class.createType(nullable = true))

And then:

val courses = Student::class.memberProperties
                            .filter { it.isNullableCourseType() }
                            .mapNotNull { it.get(studentData) }

Answered By - Joffrey