Design patterns are reusable solutions to problems that recur again and again in software design. Rather than being finished code that can be dropped into a project, patterns are templates or blueprints that describe how to solve a particular kind of problem, leaving the developer to adapt them to the surrounding context. The concept was popularized by the so-called Gang of Four (GoF) — Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides — in their influential 1994 book. Because patterns capture wisdom that has been tested across many projects, they offer several practical benefits: they give developers a shared vocabulary for talking about design, they promote proven approaches over reinvented solutions, and they tend to produce code that is easier to maintain and more flexible when requirements change.
Design patterns are usually grouped into three broad categories based on what kind of problem they address. Creational patterns deal with object creation mechanisms, controlling how objects are instantiated in ways that fit the situation. Structural patterns deal with object composition and relationships, showing how classes and objects can be combined to form larger structures. Behavioral patterns deal with communication between objects, defining how responsibilities are distributed and how objects interact. Familiar representatives include Singleton and Factory Method for creational concerns, Adapter and Decorator for structural concerns, and Observer and Strategy for behavioral concerns.
Patterns are best understood as conceptual tools, not goals in themselves. The aim is to recognize recurring forces in a design and apply the pattern whose intent matches those forces. A good way to start is by asking what varies in the system and how that variation can be encapsulated. When in doubt, simpler code without explicit patterns is often the better starting point, with patterns introduced through refactoring as complexity grows.