Convert nested list to 2d array

Question

I'm trying to convert a nested list into a 2d array.

List<List<String>> list = new ArrayList<>();

list.add(Arrays.asList("a", "b", "c"));
list.add(Arrays.asList("dd"));
list.add(Arrays.asList("eee", "fff"));

I want to make this a String[][]. I've tried the following:

String[][] array = (String[][]) list.toArray();      // ClassCastException

String[][] array = list.toArray(new String[3][3]);   // ArrayStoreException

String[][] array = (String[][]) list.stream()        // ClassCastException
    .map(sublist -> (String[]) sublist.toArray()).toArray();

Is there a way that works? Note that I won't know the size of the list until runtime, and it may be jagged.

You haven't tried to do it manually with two nested for loops? — Tom
– Tom, Commented Nov 4, 2014 at 1:44

Alex - GlassEditor.com · Accepted Answer · 2014-11-04 02:56:12Z

41

You could do this:

String[][] array = list.stream()
    .map(l -> l.stream().toArray(String[]::new))
    .toArray(String[][]::new);

It creates a Stream<List<String>> from your list of lists, then from that uses map to replace each of the lists with an array of strings which results in a Stream<String[]>, then finally calls toArray(with a generator function, instead of the no-parameter version) on that to produce the String[][].

edited Nov 4, 2014 at 2:56

answered Nov 4, 2014 at 1:47

Alex - GlassEditor.com

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3 Comments

fuat Over a year ago

How about primitive types like "double[][]" ?

Alex - GlassEditor.com Over a year ago

@fuat list.stream() .map(l -> l.stream().mapToDouble(Double::doubleValue).toArray()) .toArray(double[][]::new) if list is a List<List<Double>>. It will throw a NullPointerException if any of the Doubles are null though.

fuat Over a year ago

i use this .stream().map(p -> new double[] {p.get(0), p.get(1)}).toArray(double[][]::new) but your solution looks better. thanks

Pshemo · Accepted Answer · 2022-10-03 12:16:58Z

18

There is no simple builtin way to do what you want because your list.toArray() can return only array of elements stored in list which in your case would also be lists.

Simplest solution would be creating two dimensional array and filling it with results of toArray from each of nested lists.

String[][] array = new String[list.size()][];

int i = 0;
for (List<String> nestedList : list) {
    array[i++] = nestedList.toArray(new String[0]);
}

(you can shorten this code if you are using Java 8 with streams just like Alex did)

edited Oct 3, 2022 at 12:16

answered Nov 4, 2014 at 1:46

Pshemo

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Comments

Alican OZER · Accepted Answer · 2019-11-30 16:58:55Z

0

This is the best and most efficient way to convert 2d list to 2d array;

List<List<Integer>>  list2d = new ArrayList<>();
Integer[][] array2d;

array2d = list2d.stream().map(x->x.toArray(new Integer[x.size()])).toArray(Integer[][]::new);

answered Nov 30, 2019 at 16:58

Alican OZER

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