feat(ThreadSafetyInHashMap): add demo showing non-thread-safe behavior of HashMap

What
- Added ThreadSafetyInHashMap class.
- Created a shared HashMap<Integer,String>.
- Spawned two threads:
  • thread1 inserts keys 0–999 with value "Thread1".
  • thread2 inserts keys 1000–1999 with value "Thread2".
- Joined both threads to ensure completion.
- Printed final size of the HashMap.

Why
- To demonstrate that HashMap is not thread-safe under concurrent modifications.
- Highlights that concurrent put() calls can cause:
  • Lost updates
  • Unexpected final sizes
  • Possible data corruption in extreme cases
- Sets the stage for introducing thread-safe alternatives like ConcurrentHashMap.

How
- Shared mutable HashMap passed implicitly to both threads.
- Each thread performs 1000 put operations.
- After both complete, program prints map.size().
- Expected size = 2000, but due to race conditions may differ on some runs.

Logic
- Inputs:
  - Keys 0–1999 spread across two threads.
  - Values "Thread1" and "Thread2".
- Outputs:
  - Final map size (should be 2000 in sequential scenario).
  - Actual size may be < 2000 under race conditions.
- Flow:
  1. Initialize HashMap.
  2. Start two writer threads.
  3. Wait for completion using join().
  4. Print map size.
- Constraints:
  - No synchronization used (deliberate).
  - Demonstrates unsafe concurrent access.
- Complexity:
  - Each thread: O(n) inserts (n=1000).
  - Overall O(2000) operations.

Real-life applications
- Serves as cautionary example for multi-threaded programs using HashMap.
- Encourages using:
  • Collections.synchronizedMap(new HashMap<>())
  • ConcurrentHashMap
  • Or explicit synchronization
- Useful in teaching thread-safety fundamentals in Java collections.

Notes
- HashMap is designed for single-threaded use.
- Concurrent modifications without synchronization can lead to data loss.
- Thread-safe alternatives should be preferred in concurrent environments.

Signed-off-by: https://github.com/Someshdiwan <[email protected]>

Author: Someshdiwan

Section25CollectionFramework/src/HashMapDemo/ThreadSafetyInHashMap.java renamed to Section 25 Collections Frameworks/Map Interface/HashMap/src/ThreadSafetyInHashMap.java

@@ -1,5 +1,3 @@
-package HashMapDemo;
-
 import java.util.HashMap;
 
 public class ThreadSafetyInHashMap {
@@ -29,7 +27,6 @@ public static void main(String[] args) {
         } catch (InterruptedException e) {
             e.printStackTrace();
         }
-
         System.out.println("Final size of HashMap: " + map.size());
     }
-}
+}