What Is A Design Pattern?

"Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice" -- Christopher Alexander

A design pattern is a description or template for how to solve a problem that can be used in many different situations. It is a general repeatable solution to a commonly occurring problem in software design.

The Gang of Four describes four essential elements:

Pattern name. A word or two used to describe a design problem, its solutions and consequences.
Problem. When to apply the pattern. Explains the problem and its context.
Solution. The elements that make up the design, their relationships, responsibilities, and collaborations.
Consequences. Results and trade-offs of applying the pattern.

The Gang Of Four's Design Patterns.

The Gang of Four describes 23 classic design patterns, divided into 3 categories: creational, structural, and behavioral.

Creational Design Patterns

The creational design patterns are all about class instantiation, being divided into class-creation patterns and object-creation patterns.

Abstract Factory.
Builder.
Factory Method.
Prototype.
Singleton.

Structural Design Patterns

The structural design patterns are all about class and object composition, using inheritance to compose interfaces.

Adapter.
Bridge.
Composite.
Decorator.
Facade.
Flyweight.
Proxy.

Behavioral Design Patterns

The behavioral design patterns are all about classes' objects communication. It is concerned with communication between objects.

Chain of Responsibility.
Command.
Interpreter.
Iterator.
Mediator.
Memento.
Observer.
State.
Strategy.
Template Method.
Visitor.

The Design Patterns That Matter.

Although it is extremely advised to learn, understand and apply the patterns presented above, not all of them are entirely necessary to work on a N-Layer Architecture. Moreover, some modern patterns are not described by the Gang of Four's book.

Design patterns should be applied as needed, varying from application to application.