Is it permissible to define a class inside an interface?

• A. No, it is not permissible
• B. Yes, it is permissible
• C. Only if the class is abstract
• D. Only if it's a static class

Answer: (B)

Defining a class inside an interface is permissible in Java, as interfaces can contain nested classes. When a class is defined within an interface, it is implicitly treated as a static member of that interface. This means that the nested class can be instantiated without the need for an instance of the interface itself.

The nested class can access the interface's constants and methods, provided they are static, but it cannot access instance methods or variables of an implementing class without an object. This feature can be useful for organizing related classes and for keeping the code more cohesive.

Having nested classes within an interface can enhance encapsulation and can also serve as an alternative to free-standing classes, allowing for better structure and organization in the codebase. Overall, the ability to nest classes within interfaces provides flexibility in design and implementation.