Why is it recommended to separate functionality implementation into separate APIs and internal module implementations?

Separating functionality into separate APIs and implementations within a module is recommended for improving architecture and code maintenance.

Advantages of this approach:

• Encapsulation: The API defines what the module provides, hiding implementation details.
• Simplified testing: APIs can be tested independently of specific implementations.
• Flexibility: Implementations can be changed without affecting API consumers.
• Reusability: Different implementations of the same API can be used in different conditions.
• Maintainability: Changes in implementation do not require changes in the code that uses the API.

For example, in an Android module, you can define an interface for data handling, and a specific implementation in a separate class. This makes it easier to replace the data source (e.g., from a local database to a remote server) without changing the rest of the code.