How can I convert List<List<String>> into String[][]?

Question

In my project I have a List that contains Lists of Strings (List<List<String>>) and now I have to convert this List into an array of String arrays (String[][]).

If I had a single List<String> (for example myList) then I could do this to convert to String[]:

String[] myArray = new String[myList.size()];
myArray = myList.toArray(myArray);

But in this case I have lots of List of String inside a List (of List of Strings). How can I do this? I've spent lots of time but I didn't find a solution..

Questions are supposed to show some effort, so I suggest taking what you consider to be your best effort and posting some information about what you tried and what went wrong. — Patricia Shanahan
– Patricia Shanahan, Commented Aug 23, 2013 at 16:17

Rohit Jain · Accepted Answer · 2013-08-23 23:48:00Z

10

I wrote a generic utility method few days back for my own usage, which converts a List<List<T>> to T[][]. Here's the method. You can use it for your purpose:

public static <T> T[][] multiListToArray(final List<List<T>> listOfList, 
                                         final Class<T> classz) {
    final T[][] array = (T[][]) Array.newInstance(classz, listOfList.size(), 0);

    for (int i = 0; i < listOfList.size(); i++) {
        array[i] = listOfList.get(i).toArray((T[]) Array.newInstance(classz, listOfList.get(i).size()));
    }

    return array;
}

There I've created the array using Array#newInstance() method, since you cannot create an array of type parameter T directly.

Since we're creating a 2-D array, and we don't yet know how many columns you will be having, I've given the column size as 0 initially. Anyways, the code is initializing each row with a new array inside the for loop. So, you don't need to worry about that.

This line:

array[i] = listOfList.get(i).toArray((T[]) Array.newInstance(classz, listOfList.get(i).size()));

uses the List#toArray(T[]) method to convert the inner list to an array, as you already did. I'm just passing an array as a parameter to the method so that the return value which I get is T[], which I can directly assign to the current row - array[i].

Now you can use this method as:

String[][] arr = multiListToArray(yourList, String.class);

Thanks to @arshaji, you can modify the generic method to pass the uninitialized array as 2^nd parameter, instead of Class<T>:

public static <T> void multiListToArray(final List<List<T>> listOfList, 
                                        final T[][] arr) {
    for (int i = 0; i < listOfList.size(); ++i) {
        arr[i] = listOfList.get(i).toArray(arr[i]);
    }
}

But then, you have to pass the array to this method like this:

List<List<String>> list = new ArrayList<>();
// Initialize list
String[][] arr = new String[list.size()][0];
multiListToArray(list, arr);

Note that, since now we are passing the array as argument, you no longer need to return it from your method. The modification done in the array will be reflected to the array in the calling code.

edited Aug 23, 2013 at 23:48

answered Aug 23, 2013 at 16:17

Rohit Jain

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6 Comments

Marc-Andre Over a year ago

When I saw the question, I remember your code review. Very nice answer (method).

arshajii Over a year ago

+1 Although if I were writing this, I would pass an array to be filled instead of a Class instance to the method.

Rohit Jain Over a year ago

@arshajii. Nice idea. I'll try out that, and post it too, if get it to work. Thanks :)

Rohit Jain Over a year ago

@arshajii. That will still require me to pass Class<T> as parameter. That will be needed to create the inner arrays, which is of type T.

arshajii Over a year ago

You shouldn't have to. array is the parameter and, in the for-loop, array[i] = listOfList.get(i).toArray(array[i]).

|

Marcelo · Accepted Answer · 2013-08-23 16:28:01Z

5

 String[][] myArray  = new String[myList.size()][];
 for (int i = 0; i < myList.size(); i++) {
         List<String> row = myList.get(i);
         myArray[i] = row.toArray(new String[row.size()]);
 }

edited Aug 23, 2013 at 16:28

Marcelo

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answered Aug 23, 2013 at 16:22

Prabhakaran Ramaswamy

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1 Comment

Marcelo Over a year ago

I would just change ArrayList<String> to List<String>.

Eng.Fouad · Accepted Answer · 2013-08-23 16:19:36Z

2

Here is one way

List<List<String>> list = new ArrayList<List<String>>();
String[][] array = new String[list.size()][];

int counter1 = 0;

for(List<String> outterList : list)
{
    array[counter1] = new String[outterList.size()];
    int counter2 = 0;

    for(String s : outterList)
    {
        array[counter1][counter2] = s;
        counter2++;
    }

    counter1++;
}

answered Aug 23, 2013 at 16:19

Eng.Fouad

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1 Comment

Eng.Fouad Over a year ago

oops, people don't like the traditional way :)

user2711733 · Accepted Answer · 2013-08-23 16:23:05Z

-1

To init String[][], you should init the list(array) of String[]

So you need to define a list

List<String[]>  string = new List<String[]>();
for (int i = 0;....)
{
String[] _str = new String[numbers];
_str[0] = ...
string.append(_str);
}

answered Aug 23, 2013 at 16:23

user2711733

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