How is an abstract class enforced to provide implementations for its methods?

• A. It cannot contain any methods
• B. All its subclasses must implement its methods
• C. It inherits methods automatically from the superclass
• D. It can only be subclassed by other abstract classes

Answer: (B)

An abstract class serves as a blueprint for other classes, and one of its key features is that it can define abstract methods—methods without an implementation. When a class inherits from an abstract class, it is required to provide implementations for these abstract methods, ensuring that the concrete subclass fulfills the intended design of the abstract class.

Choosing the option that states all subclasses must implement its methods is correct because this requirement enforces a contract between the abstract class and its subclasses. By requiring the methods to be implemented, it ensures that any subclass retains the behaviors outlined by the abstract class, making the code more robust and predictable.

This mechanism also enhances polymorphism in Java, allowing developers to work with abstract classes and their subclasses interchangeably while relying on the implementation details to be properly defined in those subclasses. Thus, the enforcement of method implementation in subclasses is a fundamental aspect of how abstract classes promote the principles of object-oriented design.