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 serve as the target types for lambda expressions and method references, enabling a more functional programming style in Java ¹⁴.

The introduction of the @FunctionalInterface annotation in Java 8 allows developers to explicitly declare that an interface is intended to be a functional interface. While this annotation is not mandatory, it helps the compiler enforce the rule of having only one abstract method. This prevents accidental modifications that could introduce additional abstract methods, which would violate the functional interface requirement ²³.

Functional interfaces are beneficial for simplifying code and enhancing readability. They can be used to represent single actions or functionalities, making it easier to implement behavior in a concise manner. Java’s standard library includes 43 built-in functional interfaces in the java.util.function package, categorized into six types, which developers can utilize instead of creating their own ²⁴.