How to convert nested array to nested list using lambda?

Question

Update: use Integer[][] as as src array type could make the following code work.

I want to convert int[][] to List<List<Integer>> and tried with:

int[][] arr = new int[][]{{2}, {3, 4}, {6, 5, 7}, {4, 1, 8, 3}};
List<List<Integer>> ll = Arrays.stream(arr)
        .map(Arrays::asList) //  I expect this produces Stream<List<Integer>> but it was actually a Stream<List<int[]>>.
        .collect(Collectors.toList());

the compiler emits an error:

|  Error:
|  incompatible types: inference variable T has incompatible bounds
|      equality constraints: java.util.List<java.lang.Integer>
|      lower bounds: java.util.List<int[]>
|          List<List<Integer>> ll = Arrays.stream(arr).map(Arrays::asList).collect(Collectors.toList());
|                                   ^-----------------------------------------------------------------^

@Naman I don't see how this question is a duplicate of any of the linked questions. — Unmitigated
– Unmitigated, Commented Jul 15, 2020 at 3:56

Andy Turner · Accepted Answer · 2020-07-14 13:51:54Z

5

Arrays.asList doesn't work with primitives (at least, not in the way you intend: Arrays.asList(new int[n]) is a List<int[]> with one element, not a List<Integer> with n elements).

Instead, map to an IntStream (to give you an IntStream), box the elements (to give you a Stream<Integer>), and collect to a list (to give you a List<Integer>:

List<List<Integer>> ll = Arrays.stream(arr)
    .map(a -> IntStream.of(a).boxed().collect(toList()))
    .collect(toList());

Note that if you're using Guava, you could use Ints.asList:

List<List<Integer>> ll = Arrays.stream(arr)
    .map(Ints::asList)
    .collect(toList());

Other libraries may well also have int[] -> List<Integer> methods; this is merely one I know exists.

answered Jul 14, 2020 at 13:51

Andy Turner

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