CODEWITHSUNDEEP
javaarraysstringlistarraylist
Enigmo96
Enigmo96

Reputation: 111

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

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..

Upvotes: 1

Views: 261

Answers (4)

Rohit Jain
Rohit Jain

Reputation: 213391

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 2nd 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.

Upvotes: 10

Prabhakaran Ramaswamy
Prabhakaran Ramaswamy

Reputation: 26084

 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()]);
 }

Upvotes: 5

user2711733
user2711733

Reputation: 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);
}

Upvotes: -1

Eng.Fouad
Eng.Fouad

Reputation: 117665

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++;
}

Upvotes: 2

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