Java 8 lambda Void argument

Question

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

interface Action {
   U execute(T t);
}

And for some cases I ne

Answer 1

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 {
   U execute(T t);

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

   public static  Action create(Action 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));