Basic Syntax and Concepts: Variables, data types, control structures (loops, if-else statements), methods, classes, and objects.
Object-Oriented Programming (OOP): Inheritance, polymorphism, encapsulation, and abstraction.
Collections Framework: Lists, sets, maps, and how to use them.
Exception Handling: Try-catch blocks, custom exceptions, and exception hierarchy.
Concurrency: Threads, synchronization, and concurrent collections.
Java 8 Features: Lambda expressions, Streams API, Optional class, and default methods in interfaces.
Java Virtual Machine (JVM): Garbage collection, JVM tuning, and performance optimization.
Frameworks: Spring, Hibernate, and other popular libraries and frameworks.
APIs: Working with RESTful APIs, HTTP requests, and JSON handling.
Let me know what you're specifically interested in!