Strategy
When one task has several valid ways to run, Strategy keeps the workflow stable while the decision-making rule can change independently.
Book 01 · Second Edition
Head First Design Patterns
A practical guide to reusable object-oriented design. Everyday as Code uses it as a source, then adds original simulations, trade-offs, and real-world examples.
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Original collection
10 systems to run.
Each page explains one pattern with a new example and includes the condition where the abstraction becomes a bad trade.
Observer
Observer turns a state change into a broadcast so multiple dependents can react without the source knowing their concrete implementations.
Decorator
Decorator wraps an object with optional layers, allowing capabilities to be combined at runtime without creating a subclass for every combination.
Factory Method
Factory Method separates object creation from the workflow that uses the object, allowing a subtype or configuration to choose the concrete product.
Singleton
Singleton guarantees one globally reachable instance, but the convenience behaves like shared global state and must be treated as a trade-off.
Command
Command packages a request as an object so actions can be queued, logged, retried, composed, or reversed independently from the caller.
Adapter
Adapter converts an existing interface into the interface a client expects, containing incompatibility at one explicit system boundary.
Facade
Facade offers a small task-oriented interface over a complicated subsystem, reducing the knowledge each caller needs for common workflows.
Template Method
Template Method defines a stable algorithm sequence while allowing selected steps to vary through subclass hooks or overrides.
Iterator
Iterator provides a standard way to traverse a collection while hiding whether the underlying structure is an array, tree, stream, or remote page.