Difference Between Comparable and Comparator

Comparable and Comparator are both interfaces used to sort objects in Java. Comparable defines the natural order of a class, written inside the class itself. Comparator defines other orders, written outside the class. In this lesson, you will learn all the differences between them through one example that uses both.

Example: Using Both Together

Let's see an example of a Student class that is naturally sorted by roll number but can also be sorted by name and by marks.

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
 
class Student implements Comparable<Student> {
    int roll;
    String name;
    int marks;
 
    Student(int roll, String name, int marks) {
        this.roll = roll;
        this.name = name;
        this.marks = marks;
    }
 
    @Override
    public int compareTo(Student other) {
        return Integer.compare(roll, other.roll);
    }
 
    @Override
    public String toString() {
        return roll + ":" + name + ":" + marks;
    }
}
 
public class BothOrders {
    public static void main(String[] args) {
        List<Student> list = new ArrayList<>(List.of(
            new Student(3, "Navika", 91),
            new Student(1, "Srihitha", 78),
            new Student(2, "Advitha", 85)));
 
        Collections.sort(list);
        System.out.println("By roll (Comparable): " + list);
 
        list.sort(Comparator.comparing(s -> s.name));
        System.out.println("By name (Comparator): " + list);
 
        list.sort(Comparator.comparingInt((Student s) -> s.marks).reversed());
        System.out.println("By marks (Comparator): " + list);
    }
}

By roll (Comparable): [1:Srihitha:78, 2:Advitha:85, 3:Navika:91]
By name (Comparator): [2:Advitha:85, 3:Navika:91, 1:Srihitha:78]
By marks (Comparator): [3:Navika:91, 2:Advitha:85, 1:Srihitha:78]

Explanation: Collections.sort(list) with no comparator uses compareTo, the natural order by roll number. The two comparators sort the same list by name and by marks, without changing the Student class. Note the (Student s) type in the last line: reversed() needs it because Java can't infer the type through a chained call.

Differences Between Comparable and Comparator

When to Use Each

Common Mistakes

Implementing Comparable for an order that isn't obvious, such as students by marks. Readers can't guess it; use a named comparator.

Calling Collections.sort(list) on a class without Comparable. It doesn't compile; pass a comparator.

Interview Corner

Practice

1. Easy: Make an Employee class Comparable by ID, then sort a list by salary with a comparator.
2. Harder: Create a TreeSet<Employee> using a comparator by name. Explain why adding two employees with the same name keeps only one.

Key Takeaways