Design Patterns — Introduction
What is Design Patterns?
Design Patterns are models of code that solve classic problems. They are solutions to software design problems that you can find in a real-world application. A Design Pattern is not a code that is ready to be used in your application, but it is a model that you can use to solve a problem.
Design patterns are language neutral, so they can be applied to any language that supports object-orientation.
Why and when should we use Design Patterns?
Design Patterns exists to help us, developers. They enable us to implement tried and tested solutions (that is also used by other developers) to known problems, this way saving time and effort during the implementation of the code. They also define common words to talk about specific problems, making it easier to communicate, for example, you could say “you can use a factory in this case”, and other developers will know what they need to do.
Sometimes you do not need a design pattern, and this is good to keep in mind, we should just use it when it’s necessary. Each design pattern can be used for a specific situation, and that’s one of the reasons why it’s important to know about them, because you can have a problem that is a known problem in the software development world and already exists a good solution to solve it, by using a design pattern. This way you do not need to ‘reinvent the wheel’, you can just use something that already exists and works well. So the best way to know when you should use it or not, it’s to learn them and understand which problem each design pattern solve.
Gang of Four (GoF)
In the year of 1995, four developers, Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides, also known as ‘Gang of Four’ (GoF), published the book ‘Design Patterns: Elements of Reusable Object-Oriented Software’, and they introduced the concept of patterns, introducing 23 patterns of design. Those patterns from Gang of Four (GoF), are generally considered the foundation for all other patterns, and they are categorized into three groups:
- Creational — Concern with the process of creation (construction) of objects.
- Structural — Deals with the composition of classes and objects.
- Behavioral — Characterize the ways in which classes or objects interact and distribute responsibility.
The elements of a Design Pattern
The book ‘Design Patterns’, says that a pattern has four essential elements:
- A name — that is used to describe a design problem, the solution and the consequences.
- The problem — that describes when we need to apply the pattern, explaining the problem and its context.
- The solution — that describes the elements that make up the design, the relationships, the responsibilities and collaborations. The solutions do not describe a concrete implementation, because a pattern is like a template which can be applied in many different situations, but provides an abstract description of a design problem and how a general arrangement of elements (classes and objects) solves it.
- The consequences — that are the results of applying the pattern.
Here there is a brief explanation about each of the 23 patterns. Those definitions are also from the book ‘Design Patterns’ from the GoF:
- Abstract Factory — Provide an interface for creating families of related or dependent objects without specifying their concrete classes.
- Build — Separate the construction of a complex object from its representation so that the same construction process can create different representations.
- Factory Method — Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.
- Prototype — Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.
- Singleton — Ensure a class only has one instance, and provide a global point of access to it.
- Adapter — Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn’t otherwise because of incompatible interfaces.
- Bridge — Decouple an abstraction from its implementation so that the two can vary independently.
- Composite — Compose objects into tree structures to represent part-whole
hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
- Decorator — Attach additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.
- Facade — Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.
- Flyweight — Use sharing to support large numbers of fine-grained objects
- Proxy — Provide a surrogate or placeholder for another object to control access to it.
- Chain of Responsibility — Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it.
- Command — Encapsulate a request as an object, thereby letting you parameterized clients with different requests, queue or log requests, and support undoable operations.
- Interpreter — Given a language, define a represention for its grammar along with an interpreter that uses the representation to interpret sentences in the language.
- Iterator — Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
- Mediator — Define an object that encapsulates how a set of objects interact. Mediator promotes loose coupling by keeping objects from referring to each other explicitly, and it lets you vary their interaction independently.
- Memento — Without violating encapsulation, capture and externalize an object’s internal state so that the object can be restored to this state later.
- Observer — Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
- State — Allow an object to alter its behavior when its internal state changes. The object will appear to change its class.
- Strategy — Define a family of algorithms, encapsulate each one, and make them interchangeable. Strategy lets the algorithm vary independently from clients that use it.
- Template Method — Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm’s structure.
- Visitor — Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.
Design Patterns exists to help us, developers. Of course that also exists an initial learning curve to know how to work with them, but once that you learned it, they can make your life easier. In the next articles, I will show examples of how we can use some of those patterns.
Thanks for reading!
Design Patterns: Elements of Reusable Object-Oriented Software — Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides, Grady Booch