In Java 8 and later, a functional interface is defined as an interface that contains exactly one abstract method, aside from the methods inherited from the Object class. This characteristic allows functional interfaces to be used as the target types for lambda expressions and method references, which are key features introduced in Java 8 ¹⁴.

To help developers, Java introduced the @FunctionalInterface annotation. While it’s not mandatory to use this annotation, it serves as a way to indicate that the interface is intended for functional programming. The compiler will enforce that the interface has only one abstract method, preventing accidental modifications that could break its functional nature ²³.

Functional interfaces can simplify code by allowing the use of lambda expressions, which provide a concise way to represent instances of these interfaces. Java’s standard library includes 43 built-in functional interfaces in the java.util.function package, categorized into six types, such as UnaryOperator and BinaryOperator ²⁴.