Explaining the Gang of Four design patterns

The Gang of Four (GoF) design patterns are 23 patterns that are classified as creational patterns, structural patterns, and behavioral patterns. The creational patterns control the creation and initialization of the object and class selection; the structural patterns define the relationship between classes and objects, and the behavioral patterns control the communication and interaction between objects. As well as this, the GoF design patterns have two types of scope which define the focus of solutions. These scopes are object scope, which resolves problems about object relations, and class scope, which resolves problems about class relations. 

The object scope works with composition and the behavior changes are done in a runtime. Thus, the object can have a dynamic behavior. The class scope works with inheritance and its behavior is static-fixed at compile-time way. Then, to change the behavior of a class-scope pattern, we need to change the class and recompile.

Patterns classified as class scope solve problems about the relationship between classes and are static (fixed at compile time and cannot be changed once compiled). However, patterns classified under the object scope solve problems about the relationship between objects and can be changed at runtime.

The following diagram shows us the three classifications, as well as their patterns and scope:

In the preceding diagram, we can see the Factory Method pattern on the Class section and the Abstract Factory pattern on the Object section. This occurs because the Factory Method works with inheritance and the abstract method pattern works with composition. Then, the Factory Method is static-fixed at compile time and cannot be changed after compilation. However, the Abstract Factory is dynamic and can be changed at runtime.

GoF design patterns are generally described using a graphical notation such as a use case diagram, and an example of the implementation's code. The used notation must be able to describe the classes and objects as well as the relationship between these classes and objects. 

The pattern's name is an important part of the design patterns. This is because it is what the developer uses to quickly identify the problem related to the pattern and to understand how the pattern will solve it. The name of the pattern must be brief and refer to the problem and its solution.

A design pattern is a great tool for designing software development, but its use needs to be analyzed to determine if the design pattern is really required in order to solve the problem.