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SOLID (software design principles)

Contributor(s): Matthew Haughn

SOLID is a mnemonic acronym for a set of design principles created for software development in object-oriented languages.

The principles in SOLID are intended to foster simpler, more robust and updatable code from software developers. Each letter in SOLID corresponds to a principle for development:

  • Single responsibility
  • Open/closed
  • Liskov substitution
  • Interface segregation
  • Dependency inversion

The Single responsibility principle, created by Robert C. Martin, states that a class should have one, and only one, reason to change. This principle ensures that any class should have only one function, to help ease updating and limit the possible complications by future changes. The principle can be applied to software components or microservices tools.

The Open/closed principle ensures that software entities be open to extension but closed for modification. Bertrand Meyer is mainly credited with the creation of this principle, which focuses on making the abilities of a class easy to enhance by extension but prevents possible complications that could be brought about by modifying the entity and affecting dependencies of other functions that rely on it.

The Liskov principle stipulates that objects of the same type be replaceable with others from this same category without altering function of the program. Created by Barabara Liskov, this principle ensures that more effective means of performing a task can be switched to without unduly affecting the program or requiring substantial code updates.

The Interface segregation principle stipulates that the application’s interfaces should always be kept smaller and separate from one another. This principle ensures that clients need only familiarize themselves with functionality of methods or interfaces that are used, so they can respectively be updated without one complicating the other. This separation made by layers of abstraction provides space to document functionality while avoiding the coupling of dependencies in code.

The Dependency inversion principle specifies a form of decoupling dependencies in code. In dependency inversion, the dependencies in high-level code that set policies are inverted to the low-level dependency modules. This separates the high-level code from the details of low-level modules.

SOLID software design principles

While ideas for the SOLID principles were developed in 1995 by Robert C. Martin, co-author of the Agile Manifesto, the acronym was coined by Michael Feathers in the early 2000s as a way to remember the concepts. Other popular software development methodologies include Agile, KISS principle, GRASP (General Responsibility Assignment Software Principles) and the DRY principle.

This was last updated in October 2018

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