Behavioral design patterns describe how objects interact and distribute responsibility in a well-designed system. They are the patterns most often discussed in code reviews, system design interviews, and architecture decisions — and the ones that most clearly separate a junior engineer from a senior one.

What you'll learn

• The intent, structure, and trade-offs of each Gang of Four behavioral pattern
• Observer — event-driven communication and pub/sub
• Strategy — interchangeable algorithms and policy objects
• Command — encapsulating requests, undo/redo, and queuing
• State — clean state machines without sprawling conditionals
• Iterator, Mediator, Memento, Visitor, Chain of Responsibility, Template Method, and Interpreter
• When not to use a pattern, and how to spot over-engineering
• How modern languages (lambdas, generics, reactive streams) change classic implementations

How it works

Each pattern is broken down into focused topics covering its basics, implementation, and real-world application. Concepts are reinforced with carefully curated multiple-choice questions that test design judgement — not memorization. You'll be asked to pick the right pattern for a scenario, spot misuse, and reason about trade-offs.

Who this is for

Working developers preparing for system design interviews, computer science students taking a software design course, and self-taught engineers who want to level up from "code that works" to "code that scales."