What is the difference between List and Set in Java?

List allows duplicates and maintains order; Set does not allow duplicates and may or may not preserve order.

When should you use Set over List?

Use Set when you need to store unique elements and duplicates are not allowed.

Difference between List and Set in Java | FullStackPrep.Dev

💡 Concept: List vs Set

List and Set are two key interfaces in Java's Collection Framework with different characteristics: Lists allow duplicates and preserve insertion order, while Sets do not allow duplicates and may not preserve order.

📘 Quick Intro

Use List when order and duplicates matter. Use Set when uniqueness is required. Examples: ArrayList vs HashSet.

🧠 Analogy

A List is like a queue of people — order matters and people can appear more than once. A Set is like a guest list — each name is unique, and order is not always guaranteed.

🔧 Technical Explanation

List allows duplicates; Set does not.
List maintains insertion order; Set does not (except LinkedHashSet).
List supports positional access; Set does not.
Set is generally faster for search due to hashed structure.
Common implementations: ArrayList, LinkedList, HashSet, TreeSet.

🎯 Use Cases

✅ Use List when duplicates and order are important (e.g., tasks, playlists).
✅ Use Set to maintain a unique collection (e.g., unique usernames).
✅ Use TreeSet for sorted data without duplicates.

💻 Example: List vs Set


import java.util.*;

public class ListSetExample {
    public static void main(String[] args) {
        List<String> list = new ArrayList<>();
        list.add("A"); list.add("B"); list.add("A");
        System.out.println("List: " + list); // Allows duplicates

        Set<String> set = new HashSet<>();
        set.add("A"); set.add("B"); set.add("A");
        System.out.println("Set: " + set); // Removes duplicates
    }
}

❓ Interview Q&A

Q1: Can List have duplicates?
A: Yes.

Q2: Does Set allow duplicates?
A: No.

Q3: Which maintains insertion order?
A: List and LinkedHashSet.

Q4: Is positional access supported in Set?
A: No.

Q5: Which is faster for contains()?
A: Set (especially HashSet).

Q6: Can we sort a Set?
A: Yes, with TreeSet.

Q7: Which is better for storing tasks in order?
A: List.

Q8: Which should be used for unique IDs?
A: Set.

Q9: Does List guarantee uniqueness?
A: No.

Q10: Can we iterate both using for-each loop?
A: Yes.

📝 MCQs

Q1. Which allows duplicates?

Set
List
TreeSet
LinkedHashSet

Q2. Which does not maintain order?

ArrayList
LinkedList
HashSet
LinkedHashSet

Q3. Which supports positional access?

Set
List
HashSet
TreeSet

Q4. Which ensures unique values?

List
Set
ArrayList
Vector

Q5. Which is typically faster for lookup?

ArrayList
HashSet
TreeMap
Stack

Q6. Which class maintains sorted order?

List
HashSet
TreeSet
ArrayList

Q7. Which is ideal for ordered duplicates?

Set
TreeSet
List
Map

Q8. Does HashSet allow null?

Yes
No
Only once
Never

Q9. Which maintains insertion order?

HashSet
LinkedHashSet
TreeSet
HashMap

Q10. Which does not allow duplicates?

List
ArrayList
Set
Vector

💡 Bonus Insight

While both List and Set are collections, choosing between them impacts performance, memory usage, and behavior significantly depending on your application needs.

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Difference between List and Set in Java