SOLID Design Principles are five popular guidelines in object-oriented programming that help developers design complex software that can grow organically. These five principles aim to write code and develop software applications that will facilitate change as the application grows and develops. These design principles help software engineers avoid long-term growth issues and help them to build software than can adapt to future needs.
The Five SOLID Design Principles
Single Responsibility Principle
The Single Responsibility Principle states that each class has a singular purpose and “should have one, and only one, reason to change.” Another way to put it is that a class should have one and only one job, task, or responsibility. This principle can prevent unanticipated side effects of future changes in classes, software components, and microservices.
The primary benefits of this principle are that you write applications that are easier to maintain, easier to understand, change less frequently, and are thoroughly testable.
Open-Closed Principle
The Open-Closed Principle ensures that the behavior of a class can be extended, but the source code cannot be changed through abstractions. In other words, a class should always be open for extension but closed for modification. The “closed” aspect of the principle states that once a class has been developed and tested, the code should only be changed to correct bugs. Additionally, the “open” aspect of the principle states that you should be able to extend the existing class to introduce new functionalities.
The primary benefits of this principle are that you write applications that minimize the introduction of bugs since you are not allowing modifications. Also, dependent classes will not need to adapt.
Liskov Substitution Principle
The Liskov Substitution Principle ensures that derived classes extend the base class without changing behavior. This means that objects of a parent class should be replaceable with objects of its child classes without breaking the code. Another way to state this is that derived classes must be able to substitute any object for their base classes. This principle prioritizes behavior over structure.
The primary benefits of this principle are that callers do not get unexpected behavior when substitutions occur. Also, bugs that result from conflicting behavior between inheritance are avoided. As with all of the SOLID principles, the Liskov Substitution Principle facilitates maintainability and upgradability.
Interface Segregation Principle
The Interface Segregation Principle promotes the design of granular interfaces specific to the client instead of having a massive generic single interface. As a result, the clients of your class should not be forced to depend on methods they do not use. This granulated approach aims to build loosely coupled applications whose components are not tightly related.
The primary benefits of this principle are that it facilitates maintainability and upgradability. Also, splitting the code into multiple independent parts means that changing one of the components reduces the side effects or has little to no impact on other components.
Dependency Inversion Principle
The Dependency Inversion Principle focuses on decoupling software modules. This principle states that programmers should depend on a high-level, conceptual description of some functionality and not an instance of that abstraction. In other words, high-level modules should not depend on low-level modules. They should rely on contracts, meaning abstract classes or interfaces, rather than concrete implementations. This principle splits the dependency between the high-level and low-level modules by introducing abstraction.