The Core Java Roadmap

What a program is, what Java, the JDK and the JVM are, which Java version to use, how to run your first program, and what really happens when it runs.

How Java stores values: variables, the eight primitive types, overflow, decimals, casting, every operator, Strings, var, formatted output, input and constants.

Make Java programs decide and repeat: if/else, the ternary operator, switch statements and expressions, every loop, break, continue and labels.

Split programs into named methods, learn exactly how Java passes values, use recursion and arrays, and see where your data lives on the stack and heap.

Design your own types: classes, objects, constructors, this, encapsulation, static, packages, equals/hashCode, records, enums and immutability.

extends, super, overriding, dynamic dispatch, abstract classes, interfaces, sealed types, final, composition and nested classes, down to the bytecode.

Master Java text: the string pool, == vs equals, StringBuilder, every everyday method, text blocks, formatting, regular expressions and Unicode.

Handle failure like a professional: exceptions, try/catch/finally, checked vs unchecked, custom exceptions, try-with-resources and helpful NPEs.

Type-safe reusable code: why generics exist, generic classes and methods, bounds, wildcards and PECS, and what type erasure really does.

Lists, sets, maps, queues and heaps from the inside: ArrayList growth, HashMap buckets and treeify, TreeMap, PriorityQueue, iterators and Big-O.

Pass behaviour as values: lambdas, functional interfaces, method references, streams, collectors, Optional and parallel streams, down to invokedynamic.

Pattern matching for instanceof and switch, record patterns, unnamed variables, compact source files, and a map of every release from Java 8 to 25.

Byte and character streams, buffering, Path and Files (NIO.2), why Java serialization is dangerous, and correct dates and times with java.time.

Threads, synchronized, volatile and the Java Memory Model, locks, executors, CompletableFuture, concurrent collections, deadlocks and virtual threads.

Class loading, the JVM's memory areas, garbage collectors (G1, ZGC, Shenandoah), the JIT compiler, jcmd/jstack/JFR, and everyday performance pitfalls.

Annotations, reflection, the module system, Maven and Gradle, JUnit 5 testing and logging done right: the tools every professional Java codebase is built on.

A Core Java interview bank, OOP and design questions, tricky output puzzles verified on a real JDK, and a library management system built end to end.