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

In the same way of @rado answer with the parameters and the description:

/*----------------------
Represents an operation
that accepts two input
arguments and returns no
result.
*/
BiConsumer<T,U>         (T x, U y)  -> ()


/*----------------------
Represents a function
that accepts two arguments
and produces a result.
*/
BiFunction<T,U,R>       (T x, U y)   -> R z


/*----------------------
Represents an operation
upon two operands of the
same type, producing a
result of the same type
as the operands.
*/
BinaryOperator<T>       (T x1, T x2) -> T x3


/*----------------------
A task that returns a
result and may throw an
exception.
*/
Callable<V>             ()    -> V x   throws ex


/*----------------------
Represents an operation
that accepts a single
input argument and returns
no result.
*/
Consumer<T>             (T x)   -> ()


/*----------------------
Represents a function that
accepts one argument and
produces a result.
*/
Function<T,R>           (T x)   -> R y


/*----------------------
Represents a predicate
(boolean-valued function)
of one argument.
*/
Predicate<T>            (T x)   -> boolean


/*----------------------
Represents a portion of
executable code that
don't recieve parameters
and returns no result.
*/ 
Runnable                ()    -> ()


/*----------------------
Represents a supplier of
results.
*/
Supplier<T>             ()      -> T x

/*----------------------
Represents an operation 
on a single operand that
produces a result of the
same type as its operand.
*/
UnaryOperator<T>        (T x1)  -> T x2

fonts:

[1] https://docs.oracle.com/javase/8/docs/api/java/util/function/package-summary.html

[2] https://docs.oracle.com/javase/8/docs/api/java/lang/Runnable.html

[3] https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/Callable.html

Answer 2

I think this table is short and usefull:

Supplier       ()    -> x
Consumer       x     -> ()
BiConsumer     x, y  -> ()
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

Answer 3

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.

Answer 4

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
}

Answer 5

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!");

Answer 6

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!

Answer 7

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"));

Answer 8

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));

Answer 9

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.

Answer 10

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.