In what cases is it recommended to use interfaces in system design?

Interfaces are recommended to be used in the following cases:

• When you need to define a contract (a set of methods) that different classes should implement, ensuring polymorphism.
• To separate abstraction from implementation, which facilitates component replacement and testing.
• When designing modular and extensible systems, where different implementations can be used interchangeably.
• To simplify writing tests using mock objects that implement interfaces.
• When you need to ensure loose coupling between components.

For example, if you have different data sources (file, database, network), you can define a DataSource interface with a readData() method, and specific classes implement this interface in their own way.