grade2D

public class StudentUtil {

    public static double[] calculateGPA(int[] studentIdList, char[][] studentsGrades) {
        double[] grades = new double[studentIdList.length];
        int i = 0;

        for (char[] studentGrade : studentsGrades) {
            double sumGrades = 0.0;

            for (char grade : studentGrade) {
                if (grade == 'A') {
                    sumGrades += 4;
                } else if (grade == 'B') {
                    sumGrades += 3;
                } else {
                    sumGrades += 2;
                }
            }
            grades[i++] = sumGrades / studentGrade.length;
        }

        return grades;
    }

    public static int[] getStudentsByGPA(double lower, double higher, int[] studentIdList, char[][] studentsGrades) {
        // perform parameter validation. Return null if passed parameters are not valid
        if (lower < 0.0) {
            return null;
        }

        if (higher > 4.0) {
            return null;
        }

        if (lower > higher) {
            return null;
        }

        if (studentIdList.length != studentsGrades.length) {
            return null;
        }
        // invoke calculateGPA
        double[] gpas = calculateGPA(studentIdList, studentsGrades);
        ArrayList<Integer> ids = new ArrayList<>();

        for (int i = 0; i < studentIdList.length; i++) {
            if (gpas[i] >= lower && gpas[i] <= higher) {
                ids.add(studentIdList[i]);
            }
        }
        // construct the result array and return it. You would need an extra for loop to compute the size of the resulting array
        int[] ret = new int[ids.size()];

        for (int i = 0; i < ret.length; i++) {
            ret[i] = ids.get(i);
        }

        return ret;
    }

}