What is a Set in Java and its implementations
💡 Concept: Java Set and Implementations
A Set is a Collection that contains no duplicate elements. It models the mathematical set abstraction.
📘 Quick Intro
Common Set implementations include HashSet, TreeSet, and LinkedHashSet, each with unique properties.
🧠 Analogy
Think of a Set like a group of unique attendees at an event — no duplicates allowed.
🔧 Technical Explanation
- HashSet: Uses hashing, allows null, does not maintain order.
- TreeSet: Implements SortedSet, stores elements in sorted order.
- LinkedHashSet: Maintains insertion order.
- Set does not allow duplicates — adding a duplicate element has no effect.
- Useful for ensuring uniqueness in collections.
🎯 Use Cases
- ✅ Remove duplicates from collections.
- ✅ Store unique elements without any particular order or with sorted order.
- ✅ Perform set operations like union, intersection.
💻 Example: Using HashSet
import java.util.HashSet;
import java.util.Set;
public class SetExample {
public static void main(String[] args) {
Set set = new HashSet<>();
set.add("Java");
set.add("Java"); // Duplicate ignored
set.add("Collections");
System.out.println(set);
}
}
❓ Interview Q&A
Q1: What is a Set in Java?
A: A collection with no duplicate elements.
Q2: Name common Set implementations.
A: HashSet, TreeSet, LinkedHashSet.
Q3: Can Sets contain null?
A: HashSet can contain one null element.
Q4: Which Set maintains insertion order?
A: LinkedHashSet.
Q5: What interface does TreeSet implement?
A: SortedSet.
Q6: Can Sets have duplicate elements?
A: No.
Q7: Is HashSet synchronized?
A: No.
Q8: How to synchronize Sets?
A: Using Collections.synchronizedSet().
Q9: Difference between HashSet and TreeSet?
A: HashSet is unordered; TreeSet is sorted.
Q10: Can TreeSet contain null?
A: No, throws NullPointerException.
📝 MCQs
Q1. What is a Set in Java?
- Allows duplicates
- No duplicates
- Sorted list
- Queue
Q2. Name common Set implementations
- ArrayList, LinkedList
- HashSet, TreeSet, LinkedHashSet
- Map, Queue
- List, Map
Q3. Can Sets contain null?
- No
- Yes, one null in HashSet
- Multiple nulls
- Never
Q4. Which Set maintains insertion order?
- HashSet
- TreeSet
- LinkedHashSet
- ArrayList
Q5. What interface does TreeSet implement?
- Set
- SortedSet
- List
- Queue
Q6. Can Sets have duplicate elements?
- Yes
- No
- Sometimes
- Only nulls
Q7. Is HashSet synchronized?
- Yes
- No
- Sometimes
- Always
Q8. How to synchronize Sets?
- Using synchronized keyword
- Using Collections.synchronizedSet()
- Using ReentrantLock
- No way
Q9. Difference between HashSet and TreeSet?
- Both sorted
- Both unordered
- HashSet unordered; TreeSet sorted
- None
Q10. Can TreeSet contain null?
- Yes
- No
- Sometimes
- Throws exception
💡 Bonus Insight
Understanding Set implementations helps you choose the right collection for unique element storage and order guarantees in Java.
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