CODEWITHSUNDEEP
javamultidimensional-arraynested-lists
Ypnypn
Ypnypn

Reputation: 931

Convert nested list to 2d array

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.

Upvotes: 18

Views: 38376

Answers (3)

Pshemo
Pshemo

Reputation: 124225

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)

Upvotes: 18

Alican OZER
Alican OZER

Reputation: 1

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);

Upvotes: 0

Alex - GlassEditor.com
Alex - GlassEditor.com

Reputation: 15507

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[][].

Upvotes: 40

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