n Core Java, strong foundational knowledge of Object-Oriented Programming (OOP) is essential — this includes concepts like abstraction, encapsulation, inheritance, and polymorphism. Java provides a robust Collections Framework with key differences between List, Set, and Map. Understanding how HashMap works internally, especially with hashing and bucket mechanisms, is important. Java also supports exception handling, where developers should know the difference between checked and unchecked exceptions, and how to use try-catch-finally effectively. In terms of concurrency, Java offers tools like the synchronized keyword, volatile variables, wait() and notify(), and higher-level APIs like ExecutorService, Callable, and Future. With Java 8, functional programming became more powerful using lambdas, streams, and Optional for null safety.