Comparator.comparing 中的多项检查

Question

I have a list of objects, 'Students' - each 'Student' storing Name & Student ID. I want to sort the list in ascending order of their ID and also make sure that no two students have the same ID number. If there are multiple students with same ID, throw an exception, else do something.

For sorting part, I am using a Comparator -

students.sort((Comparator.comparing(Student::getStudentID).reversed()));

For the second part, I can add another for loop, iterate through students, and do the check.

I want to avoid the second loop and check for multiple IDs while I am sorting itself. Is there an efficient way to do this ?

[Sorry if the question is too basic, I am trying to look for more efficient way of coding]

Answer 1

You could write your own Comparator - ie:

class StudentsComparator implements Comparator<Student> {
    @Override
    public int compare(Student student1, Student student2) {
        if (student1.getStudentId().equals(student2.getStudentId())) {
            throw new IllegalStateException("Same ids");
        }

        return student2.getStudentId().compareTo(student1.getStudentId());
    }
}

then you could just do: students.sort(new StudentsComparator()); to get the proper result.

However, searching for the duplicates first and then sorting in another pass would probably be a cleaner solution, decoupling two different requirements from each other. Unless you really really care about performance, it's a better choice to do 2 runs - it is a cleaner code.

UPDATE

As for finding duplicates, you could go in various directions. One being:

private boolean hasDuplicates(List<Student> students) {
    return students.size() != students.stream().map(Student::getStudentId).distinct().count();
}

Here, for every student, you're getting his id (in the map method you're replacing student with his id), and then you're getting only distinct values. If the number of distinct values is different from the size of the original list, then there are duplicates in the list.