Java 2D array into 2D ArrayList with stream

Question

So I have an Integer[][] data that I want to convert into an ArrayList<ArrayList<Integer>> , so I tried using streams and came up with the following line:

ArrayList<ArrayList<Integer>> col = Arrays.stream(data).map(i -> Arrays.stream(i).collect(Collectors.toList())).collect(Collectors.toCollection(ArrayList<ArrayList<Integer>>::new));

But the last part collect(Collectors.toCollection(ArrayList<ArrayList<Integer>>::new)) gives me an error that it cannot convert ArrayList<ArrayList<Integer>> to C .

Answer 1

The inner collect(Collectors.toList() returns a List<Integer> , not ArrayList<Integer> , so you should collect these inner List s into an ArrayList<List<Integer>> :

ArrayList<List<Integer>> col = 
    Arrays.stream(data)
          .map(i -> Arrays.stream(i)
                          .collect(Collectors.toList()))
          .collect(Collectors.toCollection(ArrayList<List<Integer>>::new));

Alternately, use Collectors.toCollection(ArrayList<Integer>::new) to collect the elements of the inner Stream :

ArrayList<ArrayList<Integer>> col = 
     Arrays.stream(data)
           .map(i -> Arrays.stream(i)
                           .collect(Collectors.toCollection(ArrayList<Integer>::new)))
           .collect(Collectors.toCollection(ArrayList<ArrayList<Integer>>::new));