Convert nested list to 2d array

问题

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.

回答1:

There is no simple builtin way to do what you want because your 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[nestedList.size()]);
}

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

回答2:

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[][].

回答3:

Slightly briefer than Alex's answer:

final List<List<String>> list = ...;
final String[][] array = list.stream().map(List::toArray).toArray(String[][]::new);

来源：https://stackoverflow.com/questions/26726366/convert-nested-list-to-2d-array

标签

java

multidimensional-array

nested-lists