A functional interface is a concept in programming, specifically in languages that support functional programming paradigms, such as Java 8 and above. In these languages, a functional interface is an interface that has only one abstract method, known as the functional method.


The term "functional" in functional interface refers to the fact that the interface represents a function or a behavior. Since the interface has only one abstract method, it can be implemented using lambda expressions or method references, providing a concise way to represent functions as values.


Functional interfaces are the basis for working with functional programming constructs, such as higher-order functions, lambdas, and streams. They allow developers to write more expressive and concise code by treating functions as first-class citizens.


In Java, functional interfaces are identified using the @FunctionalInterface annotation, which is optional but recommended for clarity. The annotation ensures that the interface has only one abstract method. If a developer accidentally adds more than one abstract method to an interface marked with @FunctionalInterface, the compiler will report an error.


Functional interfaces can be used in various contexts, such as implementing callbacks, defining predicates, specifying comparators, or working with Java 8's stream API. They enable a more functional style of programming, promoting code reusability and flexibility.