Convert two-dimensional ArrayList to two-dimensional array

Question

I have a two-dimensional ArrayList of Integers (say list)
ArrayList<ArrayList<Integer>> list=new ArrayList<ArrayList<Integer>>() //assume values are assigned
I want to convert it to a two-dimensional array of primitive int in Java. How can I achieve that using steam as we do in the one-dimensional case

ArrayList<Integer> x =  new ArrayList<Integer>();
int[] arr = x.stream().mapToInt(i -> i).toArray();

Shouldn't `x` be a list of lists? – Murat Karagöz May 24 '20 at 05:27 — Murat Karagöz, May 24 '20 at 05:27

score 1 · Accepted Answer · answered May 24 '20 at 05:27

You can do it as follows:

int[][] arr = list.stream()
                  .map(l -> l.stream()
                             .mapToInt(Integer::intValue)
                             .toArray())
                  .toArray(int[][]::new);

Each inner List is mapped to an int[] (by first converting it to an IntStream), and then you convert your Stream<int[]> to an int[][].

Convert two-dimensional ArrayList to two-dimensional array

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