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SOLID

The acronym 𝗦𝗢𝗟𝗜𝗗 stands for:

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

The principles

  1. 𝗦𝗶𝗻𝗴𝗹𝗲 𝗥𝗲𝘀𝗽𝗼𝗻𝘀𝗶𝗯𝗶𝗹𝗶𝘁𝘆 𝗣𝗿𝗶𝗻𝗰𝗶𝗽𝗹𝗲 (𝗦𝗥𝗣)

A class should have only one reason to change. In other words, a class should have a single, well-defined responsibility. That class should entirely encapsulate responsibility.

  1. 𝗢𝗽𝗲𝗻/𝗖𝗹𝗼𝘀𝗲𝗱 𝗣𝗿𝗶𝗻𝗰𝗶𝗽𝗹𝗲 (𝗢𝗖𝗣)

Software entities (classes, functions, etc.) should be open for extension but closed for modification. You should be able to add new functionality to a class without changing its existing code, but you should not need to modify the class itself to do so.

  1. 𝗟𝗶𝘀𝗸𝗼𝘃 𝗦𝘂𝗯𝘀𝘁𝗶𝘁𝘂𝘁𝗶𝗼𝗻 𝗣𝗿𝗶𝗻𝗰𝗶𝗽𝗹𝗲 (𝗟𝗦𝗣)

Subtypes must be substitutable for their base types. In other words, if a class is derived from another class, you should be able to use the derived class in the same way as the base class without any issues.

  1. 𝗜𝗻𝘁𝗲𝗿𝗳𝗮𝗰𝗲 𝗦𝗲𝗴𝗿𝗲𝗴𝗮𝘁𝗶𝗼𝗻 𝗣𝗿𝗶𝗻𝗰𝗶𝗽𝗹𝗲 (𝗜𝗦𝗣)

Clients should not depend on interfaces they do not use. This means you should design your interfaces as specific and focused as possible.

  1. 𝗗𝗲𝗽𝗲𝗻𝗱𝗲𝗻𝗰𝘆 𝗜𝗻𝘃𝗲𝗿𝘀𝗶𝗼𝗻 𝗣𝗿𝗶𝗻𝗰𝗶𝗽𝗹𝗲 (𝗗𝗜𝗣)

High-level modules should not depend on low-level modules. Both should depend on abstractions. Design your software so high-level modules depend on abstractions rather than concrete implementations.