Java 8 lambda Void argument

Question

Let's say I have the following functional interface in Java 8:

interface Action<T, U> {

   U execute(T t);

}

And for some cases I need an action without arguments or return type. So I write something like this:

Action<Void, Void> a = () -> { System.out.println("Do nothing!"); };

However, it gives me compile error, I need to write it as

Action<Void, Void> a = (Void v) -> { System.out.println("Do nothing!"); return null;};

Which is ugly. Is there any way to get rid of the Void type parameter?

Answer 1

The syntax you're after is possible with a little helper function that converts a Runnable into Action<Void, Void> (you can place it in Action for example):

public static Action<Void, Void> action(Runnable runnable) {

    return (v) -> {

        runnable.run();

        return null;

    };

}

// Somewhere else in your code

 Action<Void, Void> action = action(() -> System.out.println("foo"));