OnJava8-Examples/functional/FunctionalAnnotation.java at master · CSapprenticeWT/OnJava8-Examples · GitHub

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// functional/FunctionalAnnotation.java

// 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;

}