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50 Java Collection Framework interview questions

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Section 1: Java Collections – Basics & Core Concepts (Beginner)

  1. What is the Java Collection Framework (JCF)?
  2. Difference between Collection and Collections.
  3. Explain the collection hierarchy in Java.
  4. Difference between List, Set, and Map.
  5. What are the main advantages of using collections over arrays?
  6. Difference between ArrayList and LinkedList.
  7. Difference between HashSet and TreeSet.
  8. Difference between HashMap and Hashtable.
  9. What is the default initial capacity and load factor of HashMap?
  10. What is meant by ordering and sorting in collections?

Section 2: List Implementations & Internals (ArrayList, LinkedList, Vector)

  1. How does ArrayList work internally?
  2. How does ArrayList grow dynamically?
  3. Time complexity of common ArrayList operations (add, get, remove).
  4. Why is LinkedList preferred for frequent insertions/deletions?
  5. Difference between ArrayList and Vector.
  6. Is Vector thread-safe? Why is it considered obsolete?
  7. What is CopyOnWriteArrayList and when should it be used?

Section 3: Set Implementations (HashSet, TreeSet, EnumSet)

  1. How does HashSet work internally?
  2. Why does HashSet not allow duplicate elements?
  3. How does TreeSet maintain sorted order?
  4. Difference between Comparable and Comparator.
  5. What is EnumSet and why is it faster than other Set implementations?

Section 4: Map Implementations & Advanced Internals

  1. How does HashMap work internally? (bucket, hash, collision)
  2. What happens when two keys have the same hash code?
  3. What is rehashing in HashMap?
  4. Difference between HashMap, LinkedHashMap, and TreeMap.
  5. Can we use a mutable object as a key in HashMap?
  6. What are WeakHashMap and IdentityHashMap?
  7. How does LinkedHashMap help in implementing LRU cache?

Section 5: Iterators, Fail-Fast & Fail-Safe Behavior

  1. What is an Iterator?
  2. Difference between Iterator and ListIterator.
  3. What is fail-fast iterator?
  4. What is fail-safe iterator?
  5. Why does ConcurrentModificationException occur?
  6. How can you avoid ConcurrentModificationException?

Section 6: Concurrency & Thread-Safe Collections (Very Important)

  1. Difference between synchronized collections and concurrent collections.
  2. How is ConcurrentHashMap different from HashMap?
  3. How did ConcurrentHashMap change from Java 7 to Java 8?
  4. What is CopyOnWriteArrayList vs synchronized ArrayList?
  5. How do you make a collection thread-safe?

Section 7: Queue & Deque Implementations

  1. Difference between Queue and Deque.
  2. How does PriorityQueue work internally?
  3. What is ArrayDeque and why is it preferred over Stack?
  4. Explain BlockingQueue and its use cases.
  5. Difference between BlockingQueue and normal Queue.

Section 8: Java 8 Enhancements & Streams with Collections

  1. Difference between Collections and Streams.
  2. How do you sort a collection using Java 8 lambda expressions?
  3. What is Collectors.groupingBy() and partitioningBy()?
  4. What is a Spliterator?
  5. Difference between sequential streams and parallel streams.

Section 9: Real-World & Design-Based Questions (High-Pay MNC Focus)

  1. Which collection would you use for fast lookup and why?
  2. Which collection is best for maintaining insertion order?
  3. How would you design an LRU cache using Java collections?
  4. Which collection is best for read-heavy multi-threaded applications?
  5. How do you choose the right collection for performance-critical systems?