Converting a 1D array to a 2D array

Got an array:

String[] array = song.split("");
[W, U, B, W, U, B, A, B, C, W, U, B]

I need to convert it in two dimension:

String[][] arr = new String[4][3];
[[W, U, B], [W, U, B], [A, B, C], [W, U, B]]

Below code:

for (int i = 0; i < arr.length; i++) {
    for (int j = 0; j < arr[j].length; j++) {
        arr[i][j] = array[j];
    }
}

gives the result:

[[W, U, B], [W, U, B], [W, U, B], [W, U, B]]

How to get:

[[W, U, B], [W, U, B], [A, B, C], [W, U, B]]

You're assigning the wrong value to arr[i][j]:

for (int i = 0; i <arr.length ; i++) {
    for (int j = 0; j <arr[j].length ; j++) {
        arr[i][j] = array[i * arr[j].length + j];
    }
}

You can do it with a single loop, iterating over array, and calculating the indexes into arr using division and remainder operators.

for (int i = 0; i < array.length; i++)
	arr[i / 3][i % 3] = array[i];

Test

String song = "WUBWUBABCWUB";
String[] array = song.split("");
System.out.println(Arrays.toString(array));

String[][] arr = new String[4][3];
for (int i = 0; i < array.length; i++)
	arr[i / 3][i % 3] = array[i];
System.out.println(Arrays.deepToString(arr));

Output

[W, U, B, W, U, B, A, B, C, W, U, B]
[[W, U, B], [W, U, B], [A, B, C], [W, U, B]]

The problem is in arr[i][j] = array[j]. The variable j should continue. Instead use a variable that doesn't assign 0 again when the outer loop pass to the next iteration because the inner loop do the assing j = 0 again. Use another counter outside of the loops to maintain the value, in this case k.

public class Main
{
public static void main(String[] args)
{
String[] array = { "W", "U", "B", "W", "U", "B", "A", "B", "C", "W", "U", "B" };
String array2[][] = new String[4][3];
// One dimension array into a two dimension array.
int k = 0;
for (int i = 0; i  <array2.length ; i++)
{
for (int j = 0; j < array2[j].length; j++)
{
array2[i][j] = array[k++];
}
}
// Print 1 dimension array.
System.out.println("array:");
for (int i = 0; i < array.length; i++)
{
System.out.print(array[i] + " ");
}
// Print 2 dimension array.
System.out.println("\n\narray2:");
for (int i = 0; i < array2.length; i++)
{
for (int j = 0; j < array2[i].length; j++)
{
System.out.print(array2[i][j] + " ");
}
System.out.println();
}
}
}

You can process this array as follows:

String[] arr = "WUBWUBABCWUB".split("");

String[][] arr2d = IntStream
        .iterate(0, i -> i < arr.length, i -> i + 3)
        .mapToObj(i -> Arrays
                .stream(arr, i, Math.min(i + 3, arr.length))
                .toArray(String[]::new))
        .toArray(String[][]::new);

System.out.println(Arrays.deepToString(arr2d));
// [[W, U, B], [W, U, B], [A, B, C], [W, U, B]]

^{See also: Copying a 1d array to a 2d array}