1. Open/Closed Principle (OCP): The Open/Closed Principle states that software entities (classes, modules, etc.) should be open for extension but closed for modification. In other words, we should be able to add new functionality to our code without modifying existing code.

For example, if we want to add a new type of report to our application, we should be able to do so without changing the code that generates the existing reports. This can be achieved by using interfaces, abstract classes, or other design patterns that allow us to add new functionality through inheritance or composition.

By following the Open/Closed Principle, we create code that is more modular and easier to extend, reducing the risk of introducing new bugs when adding new features.

2. Liskov Substitution Principle (LSP): The Liskov Substitution Principle states that objects of a superclass should be able to be replaced with objects of a subclass without affecting the correctness of the program. In other words, the subclass should behave in the same way as the superclass, and any assumptions made about the superclass should also hold for the subclass.

For example, if we have a class that handles database connections, we should be able to replace it with a subclass that provides a different implementation of the same functionality, such as a mock object for testing. The behavior of the mock object should be consistent with the behavior of the original class, so that our application functions correctly.

By following the Liskov Substitution Principle, we create code that is more robust and easier to test, as we can substitute objects in our code without breaking its functionality.

3. Interface Segregation Principle (ISP): The Interface Segregation Principle states that clients should not be forced to depend on interfaces they do not use. In other words, we should design interfaces that are specific to the needs of their clients, rather than creating monolithic interfaces that contain all possible methods.

For example, if we have a class that generates reports, we should define an interface that includes only the methods needed by the classes that use the reports, rather than including all possible methods for generating reports. This makes the code more modular and easier to maintain, as changes to the interface do not affect unrelated classes.

By following the Interface Segregation Principle, we create code that is more flexible and easier to test, as we can define interfaces that are specific to the needs of their clients, rather than forcing clients to depend on interfaces they do not use.

4. Dependency Inversion Principle (DIP): The Dependency Inversion Principle states that high-level modules should not depend on low-level modules, but both should depend on abstractions. In other words, we should design our code to depend on abstractions, rather than concrete implementations.

For example, if we have a class that needs to access a database, we should define an abstract interface that represents the database connection, rather than directly depending on a specific implementation of the database connection. This makes the code more modular and easier to test, as changes to the implementation of the database connection do not affect the rest of the code.

By following the Dependency Inversion Principle, we create code that is more flexible and easier to test, as we can change the implementation of a dependency without affecting the rest of the code.

Thank you for reading this.