Object-oriented design is the backbone of modern software engineering interviews and production codebases alike. Every large system, from payment processors to game engines, is built from classes that encapsulate state, expose behavior through interfaces, and collaborate through well-defined patterns. Yet most developers learn OOD backwards: they memorize pattern names without understanding when and why to apply them. This course fixes that by teaching design thinking first, pattern mechanics second, and real-world application throughout.

The course is structured as a progressive journey. You start with the raw materials (classes, objects, inheritance, polymorphism) and learn why encapsulation matters more than inheritance in practice. Next, SOLID principles give you a framework for evaluating whether a design is good or bad, and why. Then you learn creational, structural, and behavioral patterns, not as abstract catalog entries, but as solutions to concrete problems you will implement in code. Every lesson includes 2-3 CodeExecutor problems where you design classes, implement methods, and test against command sequences.

The final three sections apply everything to classic OOD interview problems: parking lots, elevator systems, chess games, and more. These are the problems that appear in interviews at top companies, and they require you to synthesize multiple patterns into a coherent design.

By the end of this course, you will think in objects naturally. You will look at a problem and see the classes, their responsibilities, and the patterns that connect them, not because you memorized a catalog, but because you understand the design forces that make one structure better than another.