A functional interface is an interface with exactly one abstract method but can have any number of default, static methods. A functional interface can extend another interface only when it does not have any abstract method.
@FunctionalInterfaceThe @FunctionalInterface annotation is an informative annotation that indicates whether or not an interface type declaration is meant to be a functional interface.
We can create a custom functional interface using the @FunctionalInterface annotation. Lambda expressions, method references, and constructor references can create functional interface instances.
public class Main {@FunctionalInterfacepublic interface Talkable {void talk(String text);}public static void main(String[] args){Talkable talkable = (text -> System.out.println("Talking - " + text));String textToTalk = "Hello Edpresso";talkable.talk(textToTalk);}}
Talkable with only one abstract method, i.e., talk(). We annotate the interface with the @FunctionalInterface annotation. This indicates that it’s a functional interface.Talkable interface called talkable. This prints the given text to the console.talk() method is called with the text defined in line 11.The code throws an error if we introduce another abstract method to the Talkable interface. The error indicates multiple non-overriding abstract methods in the Talkable interface.