Remove element in the set while iterating-CodePudding

I have the following scenario: In the details variable, I need to remove a course from the mutable set of courses, for each student based on a condition.

If I remove as I have done below, I get concurrent modification exception, because I am modifying the variable while iterating through it.

How can I iterate the map in reverse so that it doesn't throw the deadlock, or is there a better way to handle this scenario ?

val details = MutableMap<Student, MutableSet<CourseDetail>>

details?.forEach { student, courses ->
    courses?.forEach { course ->
       if (b?.contains(course) == false) {
           details[student]?.remove(course)
    }
  }
}

CodePudding user response：

It sounds like you just want

for (courses in details.values) {
  courses.retainAll(b)
}

(Notably, you should also avoid null collections so you don't have to scatter ?. everywhere.)

CodePudding user response：

You can use ConcurrentHashMap and ConcurrentHashSet it provide thread safety or

simply use iterator to remove element while iterating

details?.forEach { student, courses ->
    Iterator<CourseDetail> itr = courses.iterator();
    while (itr.hasNext())
        if (b?.contains(itr.next()) == false) itr.remove();
 }