SOLID is an acronym for the first five object-oriented design (OOD) principles by Robert C. Martin (also known as Uncle Bob).
There should never be more than one reason for a class/code unit to change. Every class should have only one responsibility. It will enable to keep specific functionality to be present in specific classes.
Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification. Existing functions in the class should be used and they should not be changed or modified to enable new functionality. New changes should be added in such a way such that little to minimum change needs to be done in existing code.
Extend properties of parent not narrow it down. If child class is not able to provide the result or type of result as that of parent class, it indicates violation of principle.
Interface should be such a class do not need to implement unecessary functions. This is similar to single responsibility principle. This enables to add specific interfaces for specific functionalities. This will reduce implementation of methods which will be unused and promotes modularity and reusability.
Rely on abstractions, not on concrete implementations. High level modules should not depend on low level modules. Both should depend on abstraction. Abstaraction should not depend on details . Details should depend upon abstraction. This is done to reduce tight coupling and it allow us to create extensible classes.
