ParallelStream toArray of higher size Java8

Question

I am trying to create an array with higher size from a parallel stream

myList.parallelStream().map((listElm) -> {
            return someObjectMorpher(listElm);}).toArray(size -> new SomeObject[size+4]);

1. Is it guaranteed that the last 4 elements of the array will always be null? Which means toArray will always populate at first indices of the Array.

2. Is there a possibility of having toArray populate an already created array

   SomeObjet [] arr = new SomeObject[myList.size() + 4];
   arr[0] = new SomeObject(x);
   myList.parallelStream()
       .map((listElm) -> { return someObjectMorpher(listElm);})
       .toArray(**///use arr** );
   

Answer 1

Is it guaranteed that the last 4 elements of the array will always be null?

Essentially, no. The last four elements of the array will retain their previous value. If the value happened to be null, then they will remain null. For example:

Integer[] array = Stream.of(7, 8).parallel().toArray(i -> new Integer[] {1, 2, 3, 4, 5, 6});

System.out.println(Arrays.toString(array));

Output:

[7, 8, 3, 4, 5, 6]

Is there a possibility of having toArray populate an already created array

Yes, see above, as well as another example:

Integer[] array = new Integer[] {1, 2, 3, 4, 5, 6};

Integer[] newArray = Stream.of(7, 8).parallel().toArray(i -> array);

System.out.println(Arrays.toString(newArray));

Output:

[7, 8, 3, 4, 5, 6]

Note: Attempting this with sequential streams will throw an IllegalStateException if the length of the array differs from the size of the stream.