OnJava8-Examples/functional/FunctionalAnnotation.java

// functional/FunctionalAnnotation.java
// (c)2021 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;
  }
}