Java Interview Question: Comparator vs Comparable in Java (With Examples & Differences)

 

🔹 Introduction

Sorting objects is a common requirement in Java, and interviewers often ask:

👉 “What is the difference between Comparator and Comparable?”

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🔹 What is Comparable?

Comparable is used to define natural sorting.

👉 It is implemented inside the class itself

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🔹 Example (Comparable)

import java.util.*;

class Employee implements Comparable<Employee> {
    int salary;

    Employee(int salary) {
        this.salary = salary;
    }

    public int compareTo(Employee e) {
        return this.salary - e.salary;
    }
}

public class Test {
    public static void main(String[] args) {
        List<Employee> list = Arrays.asList(
            new Employee(50000),
            new Employee(30000),
            new Employee(70000)
        );

        Collections.sort(list);

        list.forEach(e -> System.out.println(e.salary));
    }
}

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🔹 What is Comparator?

Comparator is used to define custom sorting.

👉 It is implemented outside the class

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🔹 Example (Comparator)

import java.util.*;

class Employee {
    int salary;

    Employee(int salary) {
        this.salary = salary;
    }
}

public class Test {
    public static void main(String[] args) {
        List<Employee> list = Arrays.asList(
            new Employee(50000),
            new Employee(30000),
            new Employee(70000)
        );

        list.sort((a, b) -> b.salary - a.salary);

        list.forEach(e -> System.out.println(e.salary));
    }
}

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🔍 Key Differences

Feature	Comparable	Comparator
Package	java.lang	java.util
Method	compareTo()	compare()
Sorting Type	Natural	Custom
Location	Inside class	Outside class

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🔹 When to Use What?

✔ Use Comparable

• Default sorting
• Only one logic needed

✔ Use Comparator

• Multiple sorting logic
• External control

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🔹 Time Complexity

• O(n log n)

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🔹 Key Takeaways

✔ Comparable → internal sorting
✔ Comparator → external/custom sorting
✔ Both are important for interviews

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🔹 Conclusion

Understanding Comparator and Comparable is essential for sorting objects efficiently in Java.