The Java Collections Framework is a fundamental topic in interviews. Here's a curated list of commonly asked Java Collections questions to help you crack your next technical round with confidence!
1️⃣ What is the Java Collections Framework?
It is a set of classes and interfaces that implement commonly reusable collection data structures like List, Set, Map, Queue, etc.
2️⃣ What are the key interfaces in the Collections Framework?
- Collection
- List
- Set
- Queue
- Map (not a part of Collection hierarchy)
3️⃣ What is the difference between List and Set?
- List: Allows duplicates, maintains insertion order.
- Set: Does not allow duplicates, no guaranteed order.
4️⃣ Explain HashSet vs TreeSet
- HashSet: Unordered, backed by HashMap.
- TreeSet: Sorted in natural order, backed by TreeMap.
5️⃣ Difference between ArrayList and LinkedList?
- ArrayList: Better for search, index-based operations.
- LinkedList: Better for frequent insertions/deletions.
6️⃣ What is the difference between HashMap and Hashtable?
- HashMap: Not synchronized, allows one null key and multiple null values.
- Hashtable: Synchronized, doesn’t allow null keys or values.
7️⃣ What is the difference between HashMap and TreeMap?
- HashMap: Unordered, faster.
- TreeMap: Sorted by keys, slower.
8️⃣ What is the difference between Collection and Collections?
- Collection: Interface.
- Collections: Utility class with static methods like
sort(),shuffle().
9️⃣ How does HashMap work internally?
HashMap uses an array of buckets. It uses the hashCode() and equals() methods to store and retrieve key-value pairs efficiently.
🔟 What is a ConcurrentHashMap?
Thread-safe version of HashMap. It allows concurrent reads and segmented locking for writes.
1️⃣1️⃣ What are fail-fast and fail-safe iterators?
- Fail-fast: Throws ConcurrentModificationException (e.g., ArrayList, HashMap).
- Fail-safe: Don’t throw exception (e.g., ConcurrentHashMap, CopyOnWriteArrayList).
1️⃣2️⃣ What is the difference between Iterator and ListIterator?
- Iterator: One-way traversal.
- ListIterator: Two-way traversal, can add/set elements.
1️⃣3️⃣ How can you synchronize a collection?
Using Collections.synchronizedList(list) or using concurrent classes.
1️⃣4️⃣ What is the default capacity of ArrayList?
Default capacity is 10.
1️⃣5️⃣ How do you sort a List of custom objects?
Using Collections.sort(list, comparator) or implementing Comparable.
🧠Pro Tip
Always remember internal implementations — like how HashMap handles collisions (chaining) or TreeMap's use of Red-Black Trees. These are high-value points in interviews.
📌 Conclusion
Understanding Java Collections is crucial for any Java developer. These interview questions will give you a strong edge in technical rounds. Practice writing code for each collection to gain real understanding.