Java 8 functional interface with no arguments and no return value

Question

What is the Java 8 functional interface for a method that takes nothing and returns nothing?

I.e., the equivalent to to the C# parameterless Action wit

Answer 1

@FunctionalInterface allows only method abstract method Hence you can instantiate that interface with lambda expression as below and you can access the interface members

        @FunctionalInterface
        interface Hai {

            void m2();

            static void m1() {
                System.out.println("i m1 method:::");
            }

            default void log(String str) {
                System.out.println("i am log method:::" + str);
            }

        }

    public class Hello {
        public static void main(String[] args) {

            Hai hai = () -> {};
            hai.log("lets do it.");
            Hai.m1();

        }
    }


output:
i am log method:::lets do it.
i m1 method:::

Answer 2

Just make your own

@FunctionalInterface
public interface Procedure {
    void run();

    default Procedure andThen(Procedure after){
        return () -> {
            this.run();
            after.run();
        };
    }

    default Procedure compose(Procedure before){
        return () -> {
            before.run();
            this.run();
        };
    }
}

and use it like this

public static void main(String[] args){
    Procedure procedure1 = () -> System.out.print("Hello");
    Procedure procedure2 = () -> System.out.print("World");

    procedure1.andThen(procedure2).run();
    System.out.println();
    procedure1.compose(procedure2).run();

}

and the output

HelloWorld
WorldHello

Answer 3

If I understand correctly you want a functional interface with a method void m(). In which case you can simply use a Runnable.