如何按字段按降序对列表进行排序？

Question

I am trying to achieve a sort for object's field value stored in List.

I found the following solution to compare the string but How I can compare the String value and sort accordingly?

I am looking to sort the "Y" status values first then "N"

class Student {
    int rollno;
    String name, status;

    // Constructor
    public Student(int rollno, String name,
        String status) {
        this.rollno = rollno;
        this.name = name;
        this.status = status;
    }

    public String getStatus() {
        return status;
    }

}
ArrayList < Student > ar = new ArrayList < Student > ();
ar.add(new Student(111, "bbbb", "Y"));
ar.add(new Student(131, "aaaa", "N"));
ar.add(new Student(121, "cccc", "Y"));

Collections.sort(ar, (a, b) - > a.getStatus().compareTo(b.getStatus()));

Answer 1

If I understood correctly you need refence to method getStatus() its natural order sort

ar.sort(Comparator.comparing(Student::getStatus));

if need reverse order

ar.sort(Comparator.comparing(Student::getStatus).reversed());

Answer 2

The string "Y" comes lexicographically after "N", so you need to reverse the default ordering.

There are some ways to do it, one is negating the result of the comparison function:

Collections.sort(ar, (a, b) - > -a.getStatus().compareTo(b.getStatus());

Another is changing the order of the operands:

Collections.sort(ar, (a, b) - > b.getStatus().compareTo(a.getStatus());

Answer 3

For this, you need a comparator class, that will compare the two students, in that case, if you want that when a student have "Y" in status, it's sorted before "N", you will need something like that:

    public static class StudentComparator implements Comparator<Student> {

    @Override
    public int compare(Student o1, Student o2) {
        // Compare the students, return -1 if o1 is "greater" than o2. Return 1 if o2 is "greater" than o1
        if (o1.status.equals("Y")) return -1;
        if (o2.status.equals("Y")) return 1;
        return 0;
    }
}

And then you can compare like that:

    List<Student> ar = new ArrayList<Student>();
    ar.add(new Student(111, "bbbb", "Y"));
    ar.add(new Student(131, "aaaa", "N"));
    ar.add(new Student(121, "cccc", "Y"));

    Collections.sort(ar, new StudentComparator());

Output:

[Student{rollno=121, name='cccc', status='Y'}, Student{rollno=111, name='bbbb', status='Y'}, Student{rollno=131, name='aaaa', status='N'}]