Java 8 lambda Void argument

Question

Let s say I have the following functional interface in Java 8   interface Action lt T  U gt       U execute T t       And for some cases I need an action without arguments or return type  So I write something like this   Action lt Void  Void gt  a      - gt    System out println  Do nothing          However  it gives me compile error  I need to write it as  Action lt Void  Void gt  a    Void v  - gt    System out println  Do nothing     return null      Which is ugly  Is there any way to get rid of the Void type parameter

User · Answer

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 lt T  U gt       U execute T t       public static Action lt Void  Void gt  create Runnable r           return  t  - gt   r run    return null             public static  lt T  U gt  Action lt T  U gt  create Action lt T  U gt  action           return action            That would allow you to write the following      create action from Runnable Action create   - gt  System out println  Hello World    execute null      create normal action System out println Action create  Integer i  - gt   number      i  execute 100

User · Answer

Add a static method inside your functional interface  package example   interface Action lt T  U gt           U execute T t          static  Action lt Void Void gt  invoke Runnable runnable              return  v  - gt                   runnable run                    return null                                          public class Lambda         public static void main String   args             Action lt Void  Void gt  a   Action invoke    - gt  System out println  Do nothing              Void t   null          a execute t              Output  Do nothing

User · Answer

The lambda      - gt    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 lt T gt  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

User · Answer

The syntax you re after is possible with a little helper function that converts a Runnable into Action lt Void  Void gt   you can place it in Action for example    public static Action lt Void  Void gt  action Runnable runnable        return  v  - gt            runnable run            return null               Somewhere else in your code  Action lt Void  Void gt  action   action    - gt  System out println  foo

User · Answer

I think this table is short and usefull   Supplier             - gt  x Consumer       x     - gt     Callable             - gt  x throws ex Runnable             - gt     Function       x     - gt  y BiFunction     x y   - gt  z Predicate      x     - gt  boolean UnaryOperator  x1    - gt  x2 BinaryOperator x1 x2 - gt  x3   As said on the other answers  the appropriate option for this problem is a Runnable

User · Answer

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

User · Answer

You can create a sub-interface for that special case   interface Command extends Action lt Void  Void gt      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      - gt  System out println  Do nothing

User · Answer

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 lt Void gt         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

User · Answer

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

[java] Java 8 lambda Void argument

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