Chapter 10 - Classes

• Classes should be small

• Single Responsibility Principle (SRP)

  • Class or module should have one and only one reason to change
  • SRP is often overlooked as developers often focus on if code works and not if it is clean code

• Classes and Functions must have High Cohesion

  • Cohesion: A measure of how many methods an instance variable is read into
  • Cohesion relates to how well the elements of a module belong together

• Maintaining Cohesion results in many small classes

  • Breaking down process
    • We start with a very large method that we want to break down into smaller method
    • As we split the large method into smaller methods we create more instance variables
    • Even though we have broken our method into smaller methods, as we have many instance variables our class cohesion is very low
    • This results in creating smaller classes from our one large class with the appropriate methods in each class
    • High cohesion has finally been achieved

• Conclusion - Reducing the Risk of Change

  • Whenever we modify a class we introduce a risk of breaking another part of that class
  • Clean code means having small classes, small functions and high cohesion
  • It is very hard to break code of small classes or fuctions as there not as many factors to worry about when changing code.