Is it mandatory to mark a functional interface with @FunctionalInterface annotation in Java?

An interface defined with only one abstract method is known as Functional Interface. It's not mandatory to mark the functional interface with @FunctionalInterface annotation, the compiler doesn't throw any error. But it’s good practice to use @FunctionalInterface annotation to avoid the addition of extra methods accidentally. If an interface annotated with @FunctionalInterface annotation and more than one abstract method then it throws a compile-time error.


interface interface_name {
   // only one abstarct method declaration


interface Shape {
   void printArea(int x);
public class SquareTest {
   public static void main (String args[]) {
      Shape square = (x) -> {     // Lambda Expression
         System.out.println("The area of a square is: "+ x*x);


The area of a square is: 49

Updated on: 10-Jul-2020

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