Java 8 functional interface with example

• Java 8 introduced Lamda expressions and functional interfaces.
• An interface with one abstract method is known as functional interfaces.
• Functional interfaces in java 8 marked with @FunctionalInterface.
• These functional interfaces implementation will be provided by Lamda expressions.
• Default methods in java 8 interfaces are not abstract methods. But we can define  multiple default methods in functional interface.
• Java 8 functional interface example program

#1: Java Example program to create java 8 functional interface.

1. package com.instanceofjava.java8;
2. 
   
3. /**
4.  * @author www.Instanceofjava.com
5.  * @category interview programming questions
6.  * 
7.  * Description: java 8 functional interfaces
8.  *
9.  */
10. @FunctionalInterface
11. public interface FunctionalInterfaceExample {
12. 
    
13. void show();
14. }

• Can we create functional interface with @FunctionalInterface annotation without any single abstract method?
• No. Functional interfaces must have single abstract method inside it.
• It will throw compile time error : Invalid '@FunctionalInterface' annotation; FunctionalInterfaceExample is not a functional interface.
• Lets see an example program on this so that we can conclude it.
• How to use these functional interfaces we will discuss in next post on Lamda expressions.
• Implement java 8 functional interface using lambda example program

#2: What will happend if we define functional interface without a single abstract method in it?