[java] Java 8 lambda Void argument

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?

Add a static method inside your functional interface

package example;

interface Action<T, U> {
       U execute(T t);
       static  Action<Void,Void> invoke(Runnable runnable){
           return (v) -> {
               runnable.run();
                return null;
            };         
       }
    }

public class Lambda {


    public static void main(String[] args) {

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

}

Output

Do nothing!

You can create a sub-interface for that special case:

interface Command extends Action<Void, Void> {
  default Void execute(Void v) {
    execute();
    return null;
  }
  void execute();
}

It uses a default method to override the inherited parameterized method Void execute(Void), delegating the call to the simpler method void execute().

The result is that it's much simpler to use:

Command c = () -> System.out.println("Do nothing!");

That is not possible. A function that has a non-void return type (even if it's Void) has to return a value. However you could add static methods to Action that allows you to "create" a Action:

interface Action<T, U> {
   U execute(T t);

   public static Action<Void, Void> create(Runnable r) {
       return (t) -> {r.run(); return null;};
   }

   public static <T, U> Action<T, U> create(Action<T, U> action) {
       return action;
   } 
}

That would allow you to write the following:

// create action from Runnable
Action.create(()-> System.out.println("Hello World")).execute(null);
// create normal action
System.out.println(Action.create((Integer i) -> "number: " + i).execute(100));

Just for reference which functional interface can be used for method reference in cases method throws and/or returns a value.

void notReturnsNotThrows() {};
void notReturnsThrows() throws Exception {}
String returnsNotThrows() { return ""; }
String returnsThrows() throws Exception { return ""; }

{
    Runnable r1 = this::notReturnsNotThrows; //ok
    Runnable r2 = this::notReturnsThrows; //error
    Runnable r3 = this::returnsNotThrows; //ok
    Runnable r4 = this::returnsThrows; //error

    Callable c1 = this::notReturnsNotThrows; //error
    Callable c2 = this::notReturnsThrows; //error
    Callable c3 = this::returnsNotThrows; //ok
    Callable c4 = this::returnsThrows; //ok

}


interface VoidCallableExtendsCallable extends Callable<Void> {
    @Override
    Void call() throws Exception;
}

interface VoidCallable {
    void call() throws Exception;
}

{
    VoidCallableExtendsCallable vcec1 = this::notReturnsNotThrows; //error
    VoidCallableExtendsCallable vcec2 = this::notReturnsThrows; //error
    VoidCallableExtendsCallable vcec3 = this::returnsNotThrows; //error
    VoidCallableExtendsCallable vcec4 = this::returnsThrows; //error

    VoidCallable vc1 = this::notReturnsNotThrows; //ok
    VoidCallable vc2 = this::notReturnsThrows; //ok
    VoidCallable vc3 = this::returnsNotThrows; //ok
    VoidCallable vc4 = this::returnsThrows; //ok
}

Use Supplier if it takes nothing, but returns something.

Use Consumer if it takes something, but returns nothing.

Use Callable if it returns a result and might throw (most akin to Thunk in general CS terms).

Use Runnable if it does neither and cannot throw.

The lambda:

() -> { System.out.println("Do nothing!"); };

actually represents an implementation for an interface like:

public interface Something {
    void action();
}

which is completely different than the one you've defined. That's why you get an error.

Since you can't extend your @FunctionalInterface, nor introduce a brand new one, then I think you don't have much options. You can use the Optional<T> interfaces to denote that some of the values (return type or method parameter) is missing, though. However, this won't make the lambda body simpler.

I don't think it is possible, because function definitions do not match in your example.

Your lambda expression is evaluated exactly as

void action() { }

whereas your declaration looks like

Void action(Void v) {
    //must return Void type.
}

as an example, if you have following interface

public interface VoidInterface {
    public Void action(Void v);
}

the only kind of function (while instantiating) that will be compatibile looks like

new VoidInterface() {
    public Void action(Void v) {
        //do something
        return v;
    }
}

and either lack of return statement or argument will give you a compiler error.

Therefore, if you declare a function which takes an argument and returns one, I think it is impossible to convert it to function which does neither of mentioned above.

I think this table is short and usefull:

Supplier       ()    -> x
Consumer       x     -> ()
Callable       ()    -> x throws ex
Runnable       ()    -> ()
Function       x     -> y
BiFunction     x,y   -> z
Predicate      x     -> boolean
UnaryOperator  x1    -> x2
BinaryOperator x1,x2 -> x3

As said on the other answers, the appropriate option for this problem is a Runnable