java- how to convert a ArrayList<ArrayList<String>> into String[]][]

A String[][] is nothing more than an array of arrays. For example, this makes a 'triangular matrix' using a 2d array. It doesn't have to be a square (although CSV probably should be square, it doesn't have to be).

String[][] matrix = new String[][5];    

matrix[0] = new String[1];
matrix[1] = new String[2];
matrix[2] = new String[3];
matrix[3] = new String[4];
matrix[4] = new String[5];

So for your purposes

String[][] toMatrix(ArrayList<ArrayList<String>> listOFLists) {
    String[][] matrix = new String[][listOfLists.size()];
    for(int i = 0; i < matrix.length; i++) {
        matrix[i]= listOfLists.get(i).toArray();
    }
    return matrix;
}

Just keep in mind that in this case, it's in matrix[col][row], not matrix[row][col]. You may need to transpose this result, depending on the needs of your library.

Tested and working:

        String[][] arrayOfArraysOfString = new String[arrayListOfArrayListsOfStrings.size()][];

    for (int index = 0; index < arrayListOfArrayListsOfStrings.size(); index++) {
        ArrayList<String> arrayListOfString = arrayListOfArrayListsOfStrings.get(index);
        if (arrayListOfString != null) {
            arrayOfArraysOfString[index] = arrayListOfString.toArray(new String[arrayListOfString.size()]);
        }
    }

Here is an Example of how to convert your multidimesional ArrayList into a multidimensional String Array.

package stuff;

import java.util.ArrayList;

public class ArrayTest {

public static void main(String[] args) throws Exception {
    ArrayList<ArrayList<String>> multidimesionalArrayList = createArrayListContent(); 
    String[][] multidimensionalStringArray = new String[multidimesionalArrayList.size()][];
    int index = 0;

    for (ArrayList<String> strings : multidimesionalArrayList) {
        multidimensionalStringArray[index] = strings.toArray(new String[]{}); 
        index++;
    }

    System.out.println(multidimensionalStringArray);
}

private static ArrayList<ArrayList<String>> createArrayListContent() throws Exception  {
    ArrayList<ArrayList<String>> result = new ArrayList<ArrayList<String>>();
    result.add(createArrayList());
    result.add(createArrayList());
    result.add(createArrayList());
    result.add(createArrayList());
    result.add(createArrayList());
    return result;
}

private static ArrayList<String> createArrayList() throws Exception {
    ArrayList<String> list = new ArrayList<String>();
    list.add(String.valueOf(System.currentTimeMillis()));
    Thread.sleep(10);
    list.add(String.valueOf(System.currentTimeMillis()));
    Thread.sleep(10);
    list.add(String.valueOf(System.currentTimeMillis()));
    Thread.sleep(10);
    list.add(String.valueOf(System.currentTimeMillis()));
    Thread.sleep(10);
    list.add(String.valueOf(System.currentTimeMillis()));
    return list;
}

}

You can create an array of arrays (matrix) with one size at first, and then iterate and add the data as you traverse the list of list

String[][] arr = new String[listOfList.size()][];
int i = 0;
for (List<String> row: listOfList) {
    arr[i++] = row.toArray(new String[row.size()]);
}