// functional/FunctionalAnnotation.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.

@FunctionalInterface
interface Functional {
 String goodbye(String arg);
}

interface FunctionalNoAnn {
 String goodbye(String arg);
}

/*
@FunctionalInterface
interface NotFunctional {
 String goodbye(String arg);
 String hello(String arg);
}
Produces error message:
NotFunctional is not a functional interface
multiple non-overriding abstract methods
found in interface NotFunctional
*/

public class FunctionalAnnotation {
 public String goodbye(String arg) {
 return "Goodbye, " + arg;
 }
 public static void main(String[] args) {
 FunctionalAnnotation fa =
 new FunctionalAnnotation();
 Functional f = fa::goodbye;
 FunctionalNoAnn fna = fa::goodbye;
 // Functional fac = fa; // Incompatible
 Functional fl = a -> "Goodbye, " + a;
 FunctionalNoAnn fnal = a -> "Goodbye, " + a;
 }
}