How to sort list alphabetically in each group in Java

Question

I have a class Student with name and department field.

class Student{
    String name;
    String department;
    public Student(String name, String department){
        this.name = name;
        this.department = department;
    }
}

Also I have these sample data in a list-

    List<Student>students = new ArrayList<>();
    students.add(new Student("A", "Science"));
    students.add(new Student("B", "Arts"));
    students.add(new Student("L", "Science"));
    students.add(new Student("C", "Science"));
    students.add(new Student("D", "Science"));
    students.add(new Student("A", "Arts"));
    students.add(new Student("X", "Arts"));
    students.add(new Student("C", "Arts"));

I want them to sort alphabetically in each group so the output will be looks like this-

A, Science
C, Science
D, Science
L, Science
A, Arts
B, Arts
C, Arts
X, Arts

I used comparator and tried to sort the list but it doesn't work. Seems like other solved it using java 8 stream. Could anyone solve it using Stream api.

"I used comparator and tried to sort the list but it doesn't work." What didn't work? What code and what was the problem? Is a solution with streams acceptable? — Turtle, Oct 12 '17 at 09:11

score 2 · Answer 1 · answered Oct 12 '17 at 09:24

Using streams would effectively need a comparator as well. Depending on the exact ordering you need (departments first or names first). Should probably make it null safe as well.

List<Student> sortedStudents = students.stream().sorted((o1, o2) -> {
    int depDiff = o1.department.compareTo(o2.department);
    if (depDiff == 0) {
        return o1.name.compareTo(o2.name);
    }
    return depDiff;
}).collect(Collectors.toList());

score 0 · Answer 2 · answered Oct 12 '17 at 09:33

If I understood correctly, you first need to group by department and keep that sorted in descending order; while the values in each group should be sorted in ascending order.

After the grouping, you need to recreate the List in the order that you specified.

students.stream()
            .collect(Collectors.groupingBy(
                    Student::getDepartment,
                    () -> new TreeMap<>(Comparator.<String> reverseOrder()),
                    Collectors.toCollection(() -> new TreeSet<>(Comparator.comparing(Student::getName)))))
            .entrySet()
            .stream()
            .flatMap(e -> e.getValue().stream().map(x -> new Student(x.getName(), e.getKey())))
            .collect(Collectors.toList())

How to sort list alphabetically in each group in Java

2 Answers2