For a class in Java that implements an interface, what must be true about its method implementations?

• A. It can ignore some methods
• B. It must implement all methods defined in the interface
• C. It can use the default implementation
• D. It may choose which methods to implement

Answer: (B)

When a class in Java implements an interface, it is required to provide concrete implementations for all abstract methods defined in that interface unless the class itself is declared as abstract. This foundational rule is essential for ensuring that any class that claims to adhere to an interface's contract fulfills that promise by implementing the necessary functionality.

The reason for this requirement lies in the role of interfaces in Java. Interfaces are designed to define a contract of behavior that implementing classes must adhere to. This ensures that any instance of the implementing class can be treated as an instance of the interface, allowing for polymorphism. If a class were allowed to ignore some methods, it could lead to instances where the class does not behave as expected when interacted with via the interface, violating the very purpose of interfaces.

While a class can utilize "default methods" in an interface (introduced in Java 8), allowing it to inherit implementations of methods without overriding them, this only applies to methods specifically designated as default. In cases where a method is abstract (i.e., it does not have a default implementation), the implementing class must provide an implementation.

The requirement for all abstract methods to be implemented promotes overall code reliability and consistency in object-oriented design.