How to sort an attribute of an object using Collections

Question

Good day!

I have an object student with the following attributes:

class Student
    String name
    Date birthday

I used arrayList to store the Student Objects My problem is, how can I sort the StudentList by birthday using the collecitons sort?.

List <Student> studentList = new ArrayList<Student>();

How can I code it?

Collections.sort(????);

Thank you

Answer 1

You can pass a Comparator to Collections.sort() to handle the sorting by birthday:

Collections.sort(studentList, new Comparator<Student>() {
    public int compare(Student s1, Student s2) {
        return s1.getBirthday().compareTo(s2.getBirthday());
    }
});

You'll need to add getBirthday() to your Student class if you don't have it already.

Answer 2

In Java 8 you can sort the list with a one-liner using Lambda expressions and Comparator.comparing:

Collections.sort(studentList, Comparator.comparing(s -> s.getBirthday()));

Alternatively you can use the method reference:

Collections.sort(studentList, Comparator.comparing(Student::getBirthday));