How are interface methods treated by default in Java?

• A. All methods are private
• B. All methods are public by default
• C. All methods must be static
• D. All methods must be final

Answer: (B)

In Java, all methods declared in an interface are public by default. This means that when you define a method in an interface, it is implicitly treated as public, allowing any class that implements the interface to access those methods and provide their own implementations. This design choice emphasizes the principle of interfaces being contracts; any class that implements the interface must adhere to the defined method signatures, ensuring that they are accessible when the interface is used.

The rationale behind this design is to promote the idea of abstraction in object-oriented programming, where the implementation details of the methods are hidden, and only the method signatures (or contracts) are exposed to the implementing classes. This allows for greater flexibility and interoperability among different classes that conform to the same interface.

Unlike in other contexts, methods in an interface cannot be private or static. Furthermore, they cannot be marked as final since that would contradict the very purpose of an interface, which is to allow for overriding and providing specific functionality in implementing classes. Thus, stating that all methods in an interface are public by default accurately reflects this fundamental aspect of Java's interface design.