Comparator

1. What is Comparator?

Comparator is an interface used to define custom sorting logic for objects, separately from the class itself. Unlike Comparable, you can create multiple different Comparator implementations to sort the same class in different ways.

2. Why is it used?

Sometimes you need to sort the same type of object in more than one way — like sorting students by marks in one place, and by name in another. Comparator allows this flexibility without changing the original class.

3. Real-Life Example

Think of a bookshelf that can be reorganized differently depending on the situation — sometimes by title, sometimes by author, sometimes by publication year — without changing anything about the books themselves. Comparator provides this kind of flexible, external sorting rule.

4. Syntax

java
Comparator<DataType> comparatorName = new Comparator<DataType>() {
    public int compare(DataType a, DataType b) {
        // comparison logic
    }
};

5. Example Program

java
import java.util.*;

class Student {
    String name;
    int marks;

    Student(String name, int marks) {
        this.name = name;
        this.marks = marks;
    }
}

public class ComparatorDemo {
    public static void main(String[] args) {
        List<Student> students = new ArrayList<>();
        students.add(new Student("Riya", 85));
        students.add(new Student("Aman", 70));

        Comparator<Student> byName = (a, b) -> a.name.compareTo(b.name);
        Collections.sort(students, byName);

        for (Student s : students) {
            System.out.println(s.name + ": " + s.marks);
        }
    }
}

Output:

Aman: 70
Riya: 85

6. Key Points to Remember

• Comparator is defined outside the class being sorted, allowing multiple different sorting strategies.
• It's commonly written using a lambda expression for brevity, as shown above.
• Use Comparable for a single default sort order; use Comparator when multiple sort orders are needed.