Which of the following interfaces is NOT a functional interface in Java?

The Iterable interface is not a functional interface in Java because it has more than one abstract method. A functional interface is defined as an interface that has exactly one abstract method, making it suitable for use with lambda expressions and method references. The Iterable interface, on the other hand, is a key part of the Java Collections Framework and is used to represent collections that can be iterated over, but it does not qualify as a functional interface.

Functional interfaces are crucial in Java’s functional programming paradigm because they provide the foundation for lambda expressions and method references. By limiting an interface to a single abstract method, Java ensures that lambda expressions can be used consistently and predictably across different parts of the language.

While the Iterable interface is not a functional interface, it plays an essential role in Java’s collection framework, allowing developers to traverse collections in a standard way. Understanding the distinction between functional and non-functional interfaces is key to mastering Java’s functional programming capabilities.