Explain the difference between Comparable and Comparator in Java

💡 Concept: Comparable vs Comparator

Comparable and Comparator are Java interfaces used for sorting objects, but they differ in implementation and usage.

📘 Quick Intro

Comparable defines natural ordering by implementing compareTo(), while Comparator allows custom ordering through compare().

🧠 Analogy

Think of Comparable as a default sorting rule built into an object, while Comparator is a separate rulebook you can apply whenever needed.

🔧 Technical Explanation

Comparable interface requires implementing compareTo() method.
Comparator interface requires implementing compare() method.
Comparable sorts based on natural order inside the class.
Comparator provides external sorting logic.
Comparator can be used to sort by multiple fields.
Comparator allows multiple different sorting sequences.

🎯 Use Cases

✅ Use Comparable to define default sort order.
✅ Use Comparator to define alternative sorting criteria.
✅ Useful in sorting collections and arrays.

💻 Code Example: Comparable vs Comparator


import java.util.*;

class Person implements Comparable {
    String name;
    int age;

    Person(String name, int age) {
        this.name = name;
        this.age = age;
    }

    // Natural ordering by name
    public int compareTo(Person other) {
        return this.name.compareTo(other.name);
    }
}

class AgeComparator implements Comparator {
    public int compare(Person p1, Person p2) {
        return Integer.compare(p1.age, p2.age);
    }
}

public class CompareExample {
    public static void main(String[] args) {
        List people = new ArrayList<>();
        people.add(new Person("Alice", 30));
        people.add(new Person("Bob", 25));
        people.add(new Person("Charlie", 35));

        // Sort by natural order (name)
        Collections.sort(people);

        // Sort by age using Comparator
        Collections.sort(people, new AgeComparator());
    }
}

❓ Interview Q&A

Q1: What is Comparable?
A: Interface for natural ordering with compareTo().

Q2: What is Comparator?
A: Interface for custom ordering with compare().

Q3: Can a class implement both?
A: Yes, but uncommon.

Q4: Which method does Comparable require?
A: compareTo().

Q5: Which method does Comparator require?
A: compare().

Q6: Can Comparator be used to sort by multiple fields?
A: Yes.

Q7: What is natural ordering?
A: Default sorting order defined by Comparable.

Q8: Can Comparator be anonymous class?
A: Yes.

Q9: Does Comparable modify the class?
A: Yes, by implementing interface.

Q10: Is Comparator reusable?
A: Yes, can be used with different classes.

📝 MCQs

Q1. What does Comparable interface do?

Defines natural ordering
Defines custom ordering
No ordering
None

Q2. What does Comparator interface do?

Defines natural ordering
Defines custom ordering
No ordering
None

Q3. Which method is in Comparable?

compare()
compareTo()
equals()
hashCode()

Q4. Which method is in Comparator?

compare()
compareTo()
equals()
hashCode()

Q5. Can a class implement both Comparable and Comparator?

Yes
No
Sometimes
Never

Q6. What is natural ordering?

Default sort order
Custom sort order
Random order
None

Q7. Can Comparator be anonymous class?

Yes
No
Sometimes
Never

Q8. Does Comparable modify the class?

Yes
No
Sometimes
Never

Q9. Is Comparator reusable?

Yes
No
Sometimes
Never

Q10. Can Comparator sort by multiple fields?

Yes
No
Sometimes
Never

💡 Bonus Insight

Understanding Comparable and Comparator helps write flexible and reusable sorting logic in Java applications.

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