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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.
Syntax
@FunctionalInterface
interface interface_name {
// only one abstarct method declaration
}Example
@FunctionalInterface
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);
};
square.printArea(7);
}
}Output
The area of a square is: 49
Updated on: 10-Jul-2020
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