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Primitives, variables, operators, arrays, strings, control flow, main method, and the JVM/JDK/JRE ecosystem.

Classes, inheritance, polymorphism, abstraction, encapsulation, interfaces, abstract classes, and enums.

List, Set, Map, Queue, ArrayList, LinkedList, HashMap, TreeMap, generics, and iterators.

try/catch/finally, checked and unchecked exceptions, custom exceptions, try-with-resources, and file I/O.

Threads, Runnable, synchronized, volatile, ExecutorService, Future, CompletableFuture, and thread safety.

Lambdas, streams, Optional, functional interfaces, method references, default methods, records, and sealed classes.