Java

1. Object-oriented programming concepts

   Objects, classes, inheritance, interfaces and packages as core OOP ideas.

2. Language basics

   Variables, primitive types, arrays, operators, expressions, statements and control flow.

3. Classes and objects

   Class declarations, constructors, access modifiers, static members, nested classes, enums, lambdas and method references.

4. Interfaces and inheritance

   Interfaces, default methods, subclassing, overriding, polymorphism and abstract classes.

5. Numbers and strings

   Wrapper classes, autoboxing, String, StringBuilder and formatting.

6. Packages

   Package declarations, imports, naming and the class path.

7. Generics

   Generic types and methods, bounded types, wildcards and type erasure.

8. Annotations

   Declaring and applying annotations, retention and repeating annotations.

9. Exceptions

   Checked and unchecked exceptions, try/catch/finally, try-with-resources and custom exceptions.

10. Basic I/O

    Streams, readers and writers, NIO.2 Path and Files, and serialization.

11. Concurrency

    Threads, synchronization, locks, atomic variables, executors, fork/join and concurrent collections.

12. Collections framework

    List, Set, Map, Queue, Deque, algorithms and implementations.

13. Aggregate operations and streams

    Stream pipelines, intermediate and terminal operations, reduction and parallelism.

14. Date-Time API

    java.time classes, instants, time zones, periods and durations.

15. Regular expressions

    Pattern and Matcher, character classes, quantifiers and groups.

16. Reflection

    Inspecting classes, members and arrays at runtime and dynamic proxies.

17. Records, sealed classes and pattern matching

    Records, sealed classes and interfaces, and pattern matching for instanceof and switch.

18. Virtual threads

    Lightweight threads, structured thread-per-request design, and pinning.

19. JDBC database access

    Connections, statements, result sets, transactions and prepared statements.

20. JVM garbage collection tuning

    Heap generations, collectors (G1, ZGC), pause times and GC tuning.

21. Java Language Specification

    Authoritative rules for types, conversions, scoping, initialization, the memory model and linking.

Each section links to the official page. Titles and summaries by Extra Credit; the docs belong to their authors.