Design Patterns: Behavioral Patterns & Mastery
Master the behavioral design patterns every senior engineer is expected to know — Observer, Strategy, Command, State, and more.
What you'll learn
- Identify the intent and structure of each Gang of Four behavioral pattern
- Choose the right behavioral pattern for a given design problem
- Implement Observer, Strategy, Command, and State patterns cleanly in modern languages
- Refactor sprawling conditional logic using State and Strategy
- Design undo/redo and request queuing systems with the Command pattern
- Recognize when a pattern is being over-applied or misused
- Reason about trade-offs between coupling, flexibility, and complexity
- Discuss behavioral patterns confidently in code reviews and design interviews
Curriculum
- observer pattern
- subject and observer
- one to many dependency
- push vs pull model
- weak references in observer
- observer registration and removal
- publish subscribe
- decoupled notification system
- event driven systems
- observer pattern pitfalls
About this course
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."
What learners say
Best behavioral patterns refresher I've found online. The Command pattern questions around undo/redo and queuing finally made the pattern click for me.
I used this to prep for senior engineer interviews and it paid off. The Observer and State sections in particular helped me articulate trade-offs much more clearly on the whiteboard.
Finally a design patterns resource that doesn't just regurgitate the GoF book. The questions actually force you to choose between patterns — that's where the real learning happens.