5 patterns
Creational Patterns
Five patterns about the one thing every program does constantly and badly: making objects. They take the new out of your business logic so what gets built can change without the code that uses it changing.
01SingletonOne object, one door to reach it — and the surprising cost of that convenience.02Factory MethodLet a subclass decide which class to instantiate.03Abstract FactoryMake whole families of objects that are guaranteed to match.04BuilderAssemble a complicated object step by step instead of in one huge constructor.05PrototypeCopy an existing object instead of paying to build a new one.
7 patterns
Structural Patterns
Seven patterns about composition — how to glue objects and classes into bigger structures without the glue hardening. Adapters, wrappers, trees and stand-ins.
06AdapterMake an interface you have fit the interface you need.07BridgeSplit an abstraction from its implementation so both can vary.08CompositeTreat a tree of objects and a single object exactly the same way.09DecoratorAdd behaviour by wrapping, not by subclassing.10FacadePut one simple door in front of a complicated subsystem.11FlyweightShare what's identical so a million objects fit in memory.12ProxyA stand-in that controls access to the real object.
11 patterns
Behavioural Patterns
Eleven patterns about who talks to whom. They assign responsibility between objects so that a change in one behaviour doesn't ripple through every class that touches it.
13Chain of ResponsibilityPass a request along a chain until something handles it.14CommandTurn a request into an object you can queue, log and undo.15InterpreterGive a little language its own grammar and evaluator.16IteratorWalk a collection without exposing how it stores things.17MediatorStop objects talking directly and put a switchboard in the middle.18MementoSnapshot an object's state so you can roll it back later.19ObserverPublish a change once and let every subscriber react.20StateLet an object change its class when its state changes.21StrategySwap an algorithm at runtime without touching the caller.22Template MethodFix the shape of an algorithm and let subclasses fill in the blanks.23VisitorAdd a new operation to a class hierarchy without editing it.
Certificate of completion
Work through every pattern — drive its simulator, answer its quick check — and this unlocks into a shareable card with your name on it. Progress is saved in your browser, so there is nothing to sign up for.
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Frequently asked questions
How many design patterns are there?
The original Design Patterns book by the "Gang of Four" (Gamma, Helm, Johnson and Vlissides, 1994) catalogued 23 patterns, split into 5 creational, 7 structural and 11 behavioural. Many more have been named since — MVC, Repository, Dependency Injection, CQRS — but these 23 are the shared vocabulary most interviews and code reviews assume you know.
Which design patterns should I learn first?
Start with the five you will genuinely meet in the first month of any codebase: Strategy, Factory Method, Observer, Decorator and Adapter. Singleton is worth learning early too, mostly so you understand why experienced engineers are wary of it. Interpreter and Visitor can wait — they're real, but rare.
Are design patterns still relevant?
The vocabulary is more relevant than ever; some of the implementations are not. Languages absorbed several patterns directly — first-class functions make most of Strategy and Command a one-liner, Python and Go generators are Iterator, and dependency-injection containers do the job Abstract Factory used to. What survives is the ability to say "this is a Decorator" in a code review and have everyone immediately understand the shape of the change.
Do I need to know every pattern by heart?
No. Recognising them when you read code matters far more than reciting them. The useful test is whether you can spot the problem a pattern solves — duplicated conditionals, a constructor that keeps growing, a class that changes for five different reasons — because the pattern is only ever the answer to a question your code is already asking.
Is this course really free?
Yes. All 23 patterns, every simulator, every language and the completion certificate are free, with no signup and no paywall. Your progress is stored in your own browser.
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