SOLID Principles

SOLID is a mnemonic for five object-oriented design principles introduced by Robert C. Martin in his 2003 book Agile Software Development, Principles, Patterns, and Practices. The acronym itself was coined by Michael Feathers around 2004.

• S — Single Responsibility Principle: a class should have only one reason to change.
• O — Open/Closed Principle: classes should be open for extension, but closed for modification.
• L — Liskov Substitution Principle: objects of a derived class must be usable in place of the base class without breaking the program.
• I — Interface Segregation Principle: clients should not be forced to depend on interfaces they do not use.
• D — Dependency Inversion Principle: depend upon abstractions, not concretes.

The principles aim to make software more understandable, flexible, and maintainable. They are guidelines, not laws — applying them dogmatically can lead to over-engineering, but ignoring them tends to produce rigid, fragile code.