In Design principle, SOLID – the "O" in "SOLID" stands for the open/closed principle.
Open Closed principle is a design principle which says that a class, modules, and functions should be open for extension but closed for modification.
This principle states that the design and writing of the code should be done in a way that new functionality should be added with minimum changes in the existing code (tested code). The design should be done in a way to allow the adding of new functionality as new classes, keeping as much as possible existing code unchanged.
Benefits of Open Closed Design Principle
1) The application will be more robust because we are not changing already tested class.
2) Flexible because we can easily accommodate new requirements.
3) Easy to test and less error-prone.