What are closed classes and what are their advantages and disadvantages?

In the context of Android, "closed classes" usually refer to classes with limited visibility, such as those declared with the access modifier private or internal (in Kotlin). Such classes are not accessible outside a specific module or file.

Advantages:

• Encapsulation: hiding implementation details from other parts of the application.
• Improved code security, preventing unwanted access and modifications.
• Simplified maintenance and refactoring, as changes in the closed class do not affect external code.

Disadvantages:

• Less flexibility for code reuse.
• Possible difficulties in testing if the class is not accessible from outside.

Example in Kotlin:

private class Helper {
    fun doWork() { /*...*/ }
}

This class will only be accessible within the file where it is declared.