Java 8: Functional Interface

Definition: 

A function interface in java is an interface with a single abstract method. They are used to type lambda expressions.

Example: 

In the following example, 'Cat' is a functional interface.

 interface Cat {
    void meow();
 }

 public static void main(String... args) {
    Cat cat = () -> System.out.println("Meowwwwww");    
    cat.meow();
 }

It is good practice to annotate functional interfaces with the annotation @FunctionalInterface .