What is a marker interface?

• A. An interface with no methods that enforces a mechanism.
• B. An interface that specifies a set of methods to implement.
• C. An interface that contains default method implementations.
• D. An interface that is always abstract.

Answer: (A)

A marker interface is defined as an interface that contains no methods or fields, but is used to signal or mark a class with a specific property or behavior. The primary function of a marker interface is to convey metadata about the class to the Java runtime or compiler, enabling certain mechanisms without requiring additional method definitions.

For example, the Serializable interface is a well-known marker interface in Java. If a class implements this interface, it indicates that instances of the class can be serialized—meaning their state can be converted into a byte stream. The presence of this interface allows serialization mechanisms to recognize classes that support serialization, without needing to implement any specific methods.

The other choices incorrectly describe the nature and purpose of interfaces in Java. One suggests that a marker interface specifies a set of methods to implement, which goes against the very definition of a marker interface. Another choice mentions default method implementations, which contradicts the concept since a marker interface does not include any methods at all. The final option describes an interface as always being abstract, which is true for all interfaces in Java, but it does not specifically pertain to marker interfaces. Marker interfaces are uniquely recognized by their lack of methods, which is what distinguishes them from standard interfaces.