How to specify function types for void not Void methods in Java8

Question

I m playing around with Java 8 to find out how functions as first class citizens  I have the following snippet   package test   import java util    import java util function     public class Test        public static void myForEach List lt Integer gt  list  Function lt Integer  Void gt  myFunction          list forEach functionToBlock myFunction               public static void displayInt Integer i          System out println i               public static void main String   args          List lt Integer gt  theList   new ArrayList lt  gt           theList add 1         theList add 2         theList add 3         theList add 4         theList add 5         theList add 6         myForEach theList  Test  displayInt             What I m trying to do is pass method displayInt to method myForEach using a method reference  To compiler produces the following error   src test Test java 9  error  cannot find symbol       list forEach functionToBlock myFunction                           symbol    method functionToBlock Function lt Integer Void gt     location  class Test src test Test java 25  error  method myForEach in class Test cannot be applied to given ty pes        myForEach theList  Test  displayInt             required  List lt Integer gt  Function lt Integer Void gt    found  List lt Integer gt  Test  displayInt   reason  argument mismatch  bad return type in method reference       void cannot be converted to Void   The compiler complains that void cannot be converted to Void  I don t know how to specify the type of the function interface in the signature of myForEach such that the code compiles  I know I could simply change the return type of displayInt to Void and then return null  However  there may be situations where it s not possible to alter the method I want to pass somewhere else  Is there an easy way to reuse displayInt as it is

User · Answer

Set return type to Void instead of void and return null     Modify existing method public static Void displayInt Integer i        System out println i       return null      OR     Or use Lambda myForEach theList  i - gt   System out println i  return null

User · Answer

I feel you should be using the Consumer interface instead of Function lt T  R gt    A Consumer is basically a functional interface designed to accept a value and return nothing  i e void   In your case  you can create a consumer elsewhere in your code like this   Consumer lt Integer gt  myFunction   x - gt        System out println  processing value      x                do some more things with  x  which returns nothing        Then you can replace your myForEach code with below snippet   public static void myForEach List lt Integer gt  list  Consumer lt Integer gt  myFunction       list forEach x- gt myFunction accept x        You treat myFunction as a first-class object

User · Answer

You are trying to use the wrong interface type  The type Function is not appropriate in this case because it receives a parameter and has a return value  Instead you should use Consumer  formerly known as Block   The Function type is declared as   interface Function lt T R gt      R apply T t       However  the Consumer type is compatible with that you are looking for   interface Consumer lt T gt       void accept T t       As such  Consumer is compatible with methods that receive a T and return nothing  void   And this is what you want   For instance  if I wanted to display all element in a list I could simply create a consumer for that with a lambda expression   List lt String gt  allJedi   asList  Luke   Obiwan   Quigon    allJedi forEach  jedi - gt  System out println jedi       You can see above that in this case  the lambda expression receives a parameter and has no return value   Now  if I wanted to use a method reference instead of a lambda expression to create a consume of this type  then I need a method that receives a String and returns void  right     I could use different types of method references  but in this case let s take advantage of an object method reference by using the println method in the System out object  like this   Consumer lt String gt  block   System out  println   Or I could simply do  allJedi forEach System out  println     The println method is appropriate because it receives a value and has a return type void  just like the accept method in Consumer   So  in your code  you need to change your method signature to somewhat like   public static void myForEach List lt Integer gt  list  Consumer lt Integer gt  myBlock       list forEach myBlock       And then you should be able to create a consumer  using a static method reference  in your case by doing   myForEach theList  Test  displayInt     Ultimately  you could even get rid of your myForEach method altogether and simply do   theList forEach Test  displayInt     About Functions as First Class Citizens  All been said  the truth is that Java 8 will not have functions as first-class citizens since a structural function type will not be added to the language  Java will simply offer an alternative way to create implementations of functional interfaces out of lambda expressions and method references  Ultimately lambda expressions and method references will be bound to object references  therefore all we have is objects as first-class citizens  The important thing is the functionality is there since we can pass objects as parameters  bound them to variable references and return them as values from other methods  then they pretty much serve a similar purpose

User · Answer

When you need to accept a function as argument which takes no arguments and returns no result  void   in my opinion it is still best to have something like    public interface Thunk   void apply        somewhere in your code  In my functional programming courses the word  thunk  was used to describe such functions  Why it isn t in java util function is beyond my comprehension   In other cases I find that even when java util function does have something that matches the signature I want - it still doesn t always feel right when the naming of the interface doesn t match the use of the function in my code  I guess it s a similar point that is made elsewhere here regarding  Runnable  - which is a term associated with the Thread class - so while it may have he signature I need  it is still likely to confuse the reader

[java] How to specify function types for void (not Void) methods in Java8?

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