Structured notes + runnable examples for every topic in Shrayansh's JAVA from Basics to Advanced playlist. Each package has:

• NOTES.md — concise concepts, gotchas, and interview angles
• Demo.java (and supporting classes) — runnable examples

# compile everything
javac -d target/classes $(find src/main/java -name "*.java")

# run a single demo
java -cp target/classes revision._25_hashmap_internals.Demo

# run everything
bash run_all.sh

1. OOP — four pillars
2. JVM, JRE, JDK
3. Why only one public class per file
4. Variables — primitive types
5. IEEE 754 — float/double memory layout
6. Variables — reference types
7. Methods
8. Constructors
9. Memory management & GC

10. Nested / Abstract / Super-Sub classes
11. Generic classes
12. Enum, POJO, Final
13. Singleton & Immutable
14. Interfaces — Part 1
15. Interfaces — Part 2 (default/static/private, Java 8/9)
16. Functional Interfaces & Lambdas
17. Reflection
18. Annotations

19. Exception handling
20. Operators
21. Control flow

22. Collections framework overview
23. Comparator vs Comparable, PriorityQueue
24. Deque & List
25. HashMap internals
26. LinkedHashMap & TreeMap
27. Set
28. Streams (Java 8)

29. Threads, Process, Memory model
30. Thread lifecycle & inter-thread communication
31. Join, Daemon, Priority
32. Locks & Conditions (Reentrant, ReadWrite, Stamped, Semaphore)
33. Lock-free concurrency (CAS, Atomic, Volatile)
34. ThreadPoolExecutor framework
35. CompletableFuture / Future / Callable
36. ForkJoinPool & specialized pools
37. ScheduledExecutor, shutdown vs awaitTermination
38. Virtual Threads vs Platform Threads, ThreadLocal

39. Optional (Java 8-11)
40. Text blocks (Java 15)
41. Records (Java 16)
42. Pattern matching for instanceof (Java 16)
43. Sealed classes & interfaces (Java 17)
44. Switch expressions (Java 14)
45. Pattern matching for switch (Java 21)
46. Sequenced collections (Java 21)

47. File handling — Streams, Buffers, Readers, Writers
48. NIO Part 1 — advantages over traditional I/O
49. NIO Part 2 — ByteBuffer variants
50. NIO Part 3 — FileChannel