How to sort a list by a private field?

Question

My entity class looks like this:

public class Student {

   private int grade;

   // other fields and methods
 }

and I use it like that:

List<Student> students = ...;

How can I sort students by grade, taking into account that it is a private field?

Answer 1

You have these options:

1. make grade visible
2. define a getter method for grade
3. define a Comparator inside Student
4. make Student implement Comparable
5. use reflection (in my opinion this is not a solution, it is a workaround/hack)

---

Example for solution 3:

public class Student {
    private int grade;

    public static Comparator<Student> byGrade = Comparator.comparing(s -> s.grade);
}

and use it like this:

List<Student> students = Arrays.asList(student2, student3, student1);
students.sort(Student.byGrade);
System.out.println(students);

This is my favorite solution because:

• You can easily define several Comparators
• It is not much code
• Your field stays private and encapsulated

---

Example of solution 4:

public class Student implements Comparable {
    private int grade;

    @Override
    public int compareTo(Object other) {
        if (other instanceof Student) {
            return Integer.compare(this.grade, ((Student) other).grade);
        }
        return -1;
    }
}

You can sort everywhere like this:

List<Student> students = Arrays.asList(student2, student3, student1);
Collections.sort(students);
System.out.println(students);

Aspects of this solution:

• This defines, that sorting by grade represents the natural order of students
• Some preexisting methods will automatically sort (like TreeMap)