Convert a list of functions to can passed to a varargs method arguments

Question

  public Email myMethod(Function<MyObject, String>... functions) { ...  }

I created a list of functions and I want to passed to myMethod:

     List<Function<MyObject, String>>  functions= new ArrayList<>();
        if (condition1) {
            functions.add(myObject->myObject.getStringX());
        } else {
            functions.add(myObject->myObject.getStringY());
        }
        myMethod(functions);//Does not compile, I must find a solution here 
        to convert my list of functions to an array that can be accepted as 
        myMethod argument

Eugene · Answer 1 · 2018-09-04T12:55:48.927

1

It's an array of functions, so create an array of such functions and pass it.

Or you can call it with:

Function<MyObject, String>[] array = new Function[functions.size()];
functions.toArray(array);
myMethod(array);

Just notice that you can not create a generic array, but you can declare one as such.

edited Sep 04 '18 at 12:55

answered Sep 04 '18 at 12:42

Eugene

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I tried this solution but I have a compilation error: T[ ] is not a functional interface. I tried also this code: `myMethod(new Function[functions.size()]); ` But still not work – Imar Sep 04 '18 at 12:51
@Imar no idea where you got that from. did you try this with `javac`? – Eugene Sep 04 '18 at 12:52
You need the stream: `...locations.stream().toArray(...` – MaanooAk Sep 04 '18 at 12:52
1

@MaanooAk oh shoot, I have java-11 as the compiler, and it resolves to `Collection::toArray` that was added in 11. My bad – Eugene Sep 04 '18 at 12:53
1

`functions.toArray(array);` return type not accounted? – Naman Sep 04 '18 at 12:57
@nullpointer *If the list fits in the specified array, it is returned therein* `List list = List.of(1, 2, 3, 4); Integer[] arr = new Integer[list.size()]; Integer[] newArr = list.toArray(arr); System.out.println(Arrays.toString(arr)); System.out.println(arr == newArr);` – Eugene Sep 04 '18 at 13:00
@nullpointer [see this also](https://stackoverflow.com/questions/51902362/collections-emptylist-singleton-singletonlist-list-set-toarray) – Eugene Sep 04 '18 at 13:45
@Eugene yup, I've gone through that thread and Stuart's explanation via another linked question I guess. Thanks for sharing though. – Naman Sep 04 '18 at 13:54

score 1 · Answer 2 · answered Sep 04 '18 at 12:59

1

A cleaner code I believe could've been

Function [] functionsArray = new Function[functions.size()];
for (int i=0;i< functions.size();i++) {
    functionsArray[i] = functions.get(i);
}
myMethod(functionsArray); // unchecked assignment here ofcourse

which then my IDE suggests me to write as

myMethod(functions.toArray(new Function[0]));

answered Sep 04 '18 at 12:59

Naman

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“cleaner” in which regard? A clean solution would be to implement the target method to accept a `List>` as you can convert an array to a `List` easily and efficient via `Arrays.asList`, while the opposite direction incorporates copying overhead and unchecked operations… – Holger Sep 04 '18 at 14:50
@HolgerI meant simplicity of the code, but I do borrow the point that the argument type change in the target method contract is a better way to deal with. – Naman Sep 04 '18 at 15:04
Can be improved by using: `Arrays.setAll(functionsArray, functions::get);` – Lino Sep 04 '18 at 15:07

Convert a list of functions to can passed to a varargs method arguments

2 Answers2