Design Patterns and Related Principles

Nowadays, learning and applying design patterns is an important aspect of software engineering. Design patterns are like water – you can't live without them. Don't believe me? Just ask hiring managers, and you will find that many of them have design patterns in their job postings as well as related questions in job interviews. It is a common belief that design patterns are important ingredients for software development and everyone should know them.

In this chapter, we will provide some context about why design patterns are useful and how they have served us well in the past few decades. By understanding the motivation behind design patterns, we will be able to set forth a set of guiding principles for developing software. The following topics will be discussed in this chapter:

• The history of design patterns 
• Software...

The history of design patterns

Design patterns is not a new concept to computer programmers. Since personal computers became more affordable and popular in the 1980s, the programming profession flourished and a lot of code was written for a variety of applications.

I remember that, when I was 14 years old, learning the GOTO statement for a BASIC program was one of the coolest things. It literally allowed me to take a control flow to a different part of the code at any time. Perhaps not too surprisingly, when I learned about structured programming and the Pascal language in college, I started to realize how GOTO statements produce messy spaghetti code. Using GOTO for branching purposes is a pattern. It's just a bad one because it makes code difficult to understand, follow, and debug. In today's lingua franca, we call them anti-patterns. When it comes to...

Software design principles

While this book does not cover object-oriented programming, some object-oriented design principles are universal and could be applied to any programming language and paradigm. Here, we will take a look at some of the most well-known design principles. In particular, we will cover the following:

• SOLID: Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, Dependency Inversion
• DRY: Don't Repeat Yourself
• KISS: Keep It Simple, Stupid!
• POLA: Principle of Least Astonishment
• YAGNI: You Aren't Gonna Need It
• POLP: Principle of Least Privilege

Let's start with SOLID.

SOLID

The SOLID principle consists of the following:

• S: Single Responsibility Principle
• O: Open...

Software quality objectives

Everyone likes beautiful design. I do, too. But, the use of design patterns is not just to make something look good. Everything we do should have a purpose.

The GoF classified object-oriented design patterns as creational, structural, and behavioral. For Julia, let's take a different perspective and classify our patterns by their respective software quality objectives as follows:

• Reusability
• Performance
• Maintenance
• Safety

Let's understand each of these in the following sections.

Reusability

People often talk about top-down and bottom-up approaches when designing software.

The top-down approach starts with a large problem and breaks it down into a set of smaller problems. Then, if the...

Summary

In this chapter, we started by going back in time and reviewing the history of design patterns. We discussed why design patterns can be useful for software professionals and how we would like to organize design patterns in this book given what we have learned in the past. 

We went over several key software design principles that can be applied universally in any programming language, as it is important that we keep them in mind when developing code and applying design patterns in Julia. We covered SOLID, DRY, KISS, POLA, YAGNI, and POLP. These design principles are well known and well received by the object-oriented programming community.

Finally, we discussed some software quality objectives that we want to achieve by using design patterns. In this book, we have decided to focus on reusability, maintainability, performance, and safety objectives. We also appreciated...

Questions

Review the following questions to reinforce your understanding of the subjects in this chapter. Answers are provided at the back of the book:

1. What are the benefits of using design patterns?
2. Name some key design principles.
3. What problem does the Open/Closed Principle solve?
4. Why is interface segregation important for software reusability?
5. What are the simplest ways to keep an application maintainable?
6. What is a good practice for avoiding over-engineered and bloated software?
7. How does memory usage affect system performance?