ExecutorService, Thread Pools, Atomic Types & Concurrent Collections
Master ExecutorService thread pool management (`newFixedThreadPool`), AtomicInteger (lock-free CAS), ConcurrentHashMap, and Java 21 Virtual Threads (Project Loom).
ExecutorService & ConcurrentHashMap
### Modern Java Concurrency Utilities
1. **ExecutorService**: Reusable thread pool managing worker thread life cycles efficiently.
2. **ConcurrentHashMap**: Lock-striping hashtable allowing concurrent reads without locking and thread-safe writes per bucket node.
3. **Java 21 Virtual Threads (Loom)**: Ultra-lightweight JVM-managed threads enabling millions of concurrent threads with minimal memory footprint!
