Single Responsibility Principle

This is part one of a five part series about SOLID class design principles by Robert C. Martin. The SOLID principles focus on achieving code that is maintainable, robust, and reusable. In this post, I will discuss the Single Responsibility Principle.

The Single Responsibility Principle (SRP): A class should have one, and only one, reason to change.

The SRP is roughly the same as having "high cohesion". A class is said to have high cohesion if it's behaviours are highly related, and strongly focused. The SRP states that a class should be cohesive to the point that it has a single responsibility, where a responsibility is defined as "a reason for change."

The Benefits

• The class is easier to understand
  When the class only does "one thing", its interface usually has a small number of methods that are fairly self explanatory. It should also have a small number of member variables (less than seven-ish).
• The class is easier to maintain
  Changes are isolated, reducing the chance of breaking other unrelated areas of the software. As programming errors are inversely proportional to complexity, being easier to understand makes the code less prone to bugs.
• The class is more reusable
  If a class has multiple responsibilities, and only one of those is needed in another area of the software, then the other unnecessary responsibilities hinder reusability. Having a single responsibility means the class should be reusable without modification.