GoF design patterns are classified into three main categories:

• Creational patterns: These cover how to construct objects in various ways. Since object-oriented programming brings together data and behavior, and a class may inherit the structure and behavior of an ancestor class, there are some complexities involved when building a large application. Creational patterns help standardize object creation methods in various situations.
• Structural patterns: These cover how objects can be extended or composed to make bigger thing. The purpose of these patterns is to allow software components to be reused or replaced more easily.
• Behavioral patterns: This cover how objects can be designed to perform separate tasks and communicate with each other. A large application can be decomposed into independent components and the code becomes easier to maintain. The object-oriented programming paradigm requires solid interaction between objects. The purpose of these patterns is to make software components more flexible and more convenient for collaboration with each other. 

One school of thought is that design patterns are created to address limitations in their respective programming language. Two years after the GoF book was published, Peter Norvig published research showing that 16 of the 23 design patterns are either unnecessary or can be simplified in a dynamic programming language such as Lisp.

This is not an unimportant observation. In the context of object-oriented programming, additional abstraction from a class hierarchy requires the software designer to think about how objects are instantiated and interact with each other. In a strong, statically typed language such as Java, it is even more necessary to reason about the behavior and interaction of objects. In Chapter 11, Traditional Object-Oriented Patterns, we will circle back to this topic and discuss how Julia works differently compared to object-oriented programming.

For now, we will start with the basics and review some software design principles. These principles are like the North star, guiding us as we build applications.