Java 8 – Functional Interface

 

What is Functional Interface ?

• An interface which contains only one abstract method is called as Functional interface

• In short, it is called as Single Abstract Method i.e.; SAM
• In addition to one abstract method, we can have any number of static or default methods, another feature added in Java 1.8 version
• An optional annotation @FunctionalInterface can be annotated for Functional Interface to make sure that not more than one abstract method is declared
• If more than one abstract method declared and also annotated with @FunctionalInterface, then compiler throws error
• Note: To invoke Lambda Expression we need Functional Interface and it is required

To multiply any two Integer

@FunctionalInterface interface DemoInterface {     public int multiply(int i, int j); }   public class TestLambdaExpression {       // main method     public static void main(String[] args) {           // Lambda Expression         DemoInterface d = (i, j) -> i*j;           // how to invoke Lambda Expression -> Functional Interface         System.out.println(d.multiply(7, 21));     } }

Output:

147