Functional Interface Annotation in Java

Java introduced the FunctionalInterface annotation in Java 8 to designate an interface as a functional interface. Functional interfaces are interfaces in Java that have one abstract method and are essential for enabling lambda expressions and simplifying code. The @FunctionalInterface annotation acts as a marker interface and provides documentation to maintain the functional nature of an interface. It is important to use the @FunctionalInterface annotation when creating custom functional interfaces, to communicate intent clearly to other developers, and to encourage the use of lambda expressions.