What defines a functional interface in Java?

• A. It must contain multiple abstract methods
• B. It can contain any number of default methods
• C. It contains exactly one abstract method
• D. It cannot extend other interfaces

Answer: (C)

A functional interface in Java is defined as an interface that contains exactly one abstract method. This is a key aspect because functional interfaces are intended to be utilized primarily for implementing lambda expressions or method references, which are a powerful feature introduced in Java 8 to promote functional programming.

By having a single abstract method, the functional interface provides a clear target for the lambda expression. When you implement a functional interface using a lambda expression, that expression corresponds directly to the single abstract method of the interface, making the code more concise and readable.

The ability to contain any number of default methods is an interesting feature of functional interfaces, but these default methods do not count towards the "exactly one abstract method" requirement. They are non-abstract methods that can provide concrete implementations and serve as a way to add behavior to interfaces without breaking existing implementations.

In contrast, a functional interface can indeed extend other interfaces, including those that also have default methods or static methods, as long as it still adheres to the rule of having only one abstract method. This flexibility allows for the creation of more complex functional interfaces without violating their definition.