How to convert an ArrayList of String arrays into a 2D string array

Question

I have an ArrayList<String[]> array_list
I want to convert it into a 2D String array say result such that first string array in array_list is in result[0]. Also I want to have first string in result to be equal to string equal.
i.e. If first array in array_list has ["A","B","C"] and my string equal is "D" my result[0] should be ["D","A","B","C"]

I tried:

ArrayList<String[]> result = new ArrayList<String[]>();
// I'm getting value of result from a function
String[][] array = new String[result.size()][]; 
int j =0;
for (String[] arrs : result)
{
    System.out.println(j);
    String arr = n;//I have taken n as parameter for the function
    array[j][0] = arr;
    int i =1;
    for(String o : arrs)
    {
        array[j][i] = o;
        i++;
    }
    j++;
}

but this gives java.lang.NullPointerException.

At what line is the nullpointerexception thrown?

Tim van der Lippe
– Tim van der Lippe

2015-05-04 19:45:30 +00:00
Commented May 4, 2015 at 19:45 — Tim van der Lippe
– Tim van der Lippe, Commented May 4, 2015 at 19:45

Chronio · Accepted Answer · 2015-05-04 19:48:29Z

3

array[j] is never initialized. You have to first assign a String[] instance to array[j] before using it.

Add the following line at the beginning of your outer for-loop (before array[j][0] = arr):

array[j] = new String[arrs.length + 1];

answered May 4, 2015 at 19:48

Chronio

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Comments

Jyr · Accepted Answer · 2015-05-04 20:18:24Z

0

Here is an alternative (to your question in the title).

My initial thought was to do the following;

String[][] array = (String[][]) result.toArray()

But it turns out to give a nasty ClassCastException telling we can't cast an Object array to a String array.

So, this is where the Arrays class (java.util.Arrays) comes in handy.

String[][] array = Arrays.copyOf(result.toArray(), result.toArray().length, String[][].class);

Or fancy Java 8;

String[][] array = Arrays.stream(result.toArray()).toArray(String[][]::new);

edited May 4, 2015 at 20:18

answered May 4, 2015 at 20:10

Jyr

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