Java Collection Framework

Short Summary:
Java Collection Framework provides dynamic data structures. It includes List, Set, Queue, Deque, and Map along with concurrent and fail-safe mechanisms.

1️⃣ Collection Framework Hierarchy

                 Iterable
                     |
                Collection
              /     |      \
           List     Set     Queue
                                   \
                                    Deque

                 Map (Separate Hierarchy)

2️⃣ Fail-Fast vs Fail-Safe

Short Answer:
Fail-Fast throws ConcurrentModificationException. Fail-Safe does not throw exception and works on copy.

Fail-Fast

• Works on original collection.
• Detects structural modification.
• Throws ConcurrentModificationException.
• Examples: ArrayList, HashMap

List list = new ArrayList<>();
list.add("Java");

for(String s : list) {
    list.add("Spring");   // Exception
}

Fail-Safe

• Works on clone/copy.
• No exception thrown.
• Safe in multi-threading.
• Examples: ConcurrentHashMap, CopyOnWriteArrayList

3️⃣ Concurrent Collections Deep Dive

Short: Thread-safe collections designed for high concurrency.

ConcurrentHashMap Internal Working

• Java 7 → Segmented locking
• Java 8 → CAS + synchronized on bucket
• No null key/value allowed
• Better performance than Hashtable

4️⃣ Map Hierarchy Explained

                  Map
                /     \
          HashMap    SortedMap
             |           |
       LinkedHashMap   TreeMap
             |
     ConcurrentHashMap

Map Implementations Comparison

5️⃣ Important Interview Concepts

• Fail-fast uses modCount mechanism.
• Fail-safe works on snapshot.
• Map is not part of Collection hierarchy.
• ConcurrentHashMap avoids full locking.
• Hashtable is legacy class.
• CopyOnWriteArrayList best for read-heavy scenarios.