The Art of Designing Object-Oriented Software
They are an elegant solution to a repeating problem in software design.
How to structure our classes - and how these classes should talk to each other.
Design Patterns
Elements of Reusable Object-Oriented Software (1994)
by the Gang-of-Four
Erich Gamma, Richard Helm, John Vlissides, Ralph Johnson
This book contains 23 popular design patterns, which are formerly known as Gang-of-Four patterns. These patterns are divided into three categories.
- Creational
Different ways to create objects - Structural
Relationships between objects - Behavioural
Interaction & communication between objects
These three categories or those Gang-of-Four patterns don't represent all the design patterns in the world. There are other undocumented or less popular design patterns.
These design patterns help us communicate with other developers in a more abstract level. Sometimes we can just use the name of the pattern to communicate an idea or an improvement.
Understanding design patterns helps us create reusable, extensible and maintainable software, no matter what programming language we use or what kind of application we build.
These design patterns are extensively used in frameworks and libraries. So, even though they might have different syntax, the patterns are just the same. They make it easier to understand new concepts of programming.
Coupling in OOP refers to, how much a class is dependent on another class. Think of a time, where we have a class named X. And we instantiate an X object in another class called Y. So, the Y class depends on X class. If we make a change in X, both X and Y might have to be changed, and re-compiled.
For an example, think of a car. If we get a flat tire, we would only have to
change the tire that has a flat. We don't need to change all the other tires, or
any other parts of the car.
So, the car is a loosely-coupled system.
Our applications should be the same way. Interfaces can be used to build loosely-coupled classes.
An interface is a contract that specifies the capabilities that a class should provide.