Functional interface with multiple methods in java 8

• Functional interfaces will contains single abstract method with @FunctionalInterface annotation.
• Check below two posts for defining functional interface and implementing functional interface method using Lamda in java 8.
• Java 8 functional interface with example
• Implement java 8 functional interface using lambda example program
• Now the question is can we declare or define multiple abstract methods in one functional interface in java 8.
• No. Functional interface should contain only one abstract method so that Lamda will implement it.
• Lets see what happens if we define multiple abstract methods inside a functional interface
• Note: all methods inside any interface by default abstract methods.

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. //compile time error: Invalid '@FunctionalInterface' annotation; FunctionalInterfaceExample is
13. not a functional interface
14.  
15. void show();
16. void calculate();
17. }

• If we define multiple abstract methods inside a functional interface it will throw a compile time error.
• Invalid '@FunctionalInterface' annotation; FunctionalInterfaceExample is not a functional interface