Java how to copy values from 2D array to regular array row by row

This will copy any size 2D array of ints.

int[][] testData = { { 1, 2, 3 },{}, null, { 4, 5, 6, 7 },null,{ 8, 9 },
        { 10, 11, 12 } };
int[] result = copy2DArrays(testData);
System.out.println(Arrays.toString(result));

prints

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]

• First, compute size of result array. This also handles null rows.
• if a null row is encountered, replace with an empty array for copy phase.
• Allocate the return array of computed size
• Then for each row
  • iterate thru the row, copying the value to result array indexed by k
• When done, return the resultant array.

public static int[] copy2DArrays(int[][] array) {
    int size = 0;
    for (int i = 0; i < array.length; i++) {
        if (array[i] == null) {
            // replace null row with empty one
            array[i] = new int[]{};
           continue;
        }
        size += array[i].length;
    }

    int k = 0;
    int[] result = new int[size];
    for (int[] row : array) {
        for (int v : row) {
            result[k++] = v;
        }
    }
    
    return result;
}

Another, simpler option is using streams.

• stream the "2D" array
• only pass nonNull rows
• flatten each row into a single stream
• then gather them into an array.

int[] array = Arrays.stream(testData)
        .filter(Objects::nonNull)
        .flatMapToInt(Arrays::stream)
        .toArray();

You can create a new array and then copy the values into it:

for (int i = 0; i < testList.length; i++) {
    int[] copy = new int[testList[i].length];
    int copyIndex = 0;
    
    for (int j = testList[i].length-1; j >=0; j--) {
        System.out.print(testList[i][j] + " ");
        copy[copyIndex++] = testList[i][j];
    }
}

The Arrays class gives you a lot of nifty features:

public static void main(String[] args) {
    Integer[][] testList = new Integer[][]{{1,2,3},{4,5,6},{7,8,9}, {10,11,12}};
    int max = 0;
    for (Integer[] row : testList) {
        max = Math.max(max, row.length);
    }
    
    Integer[][] copy = new Integer[testList.length][max];
    
    for (int i = 0; i < testList.length; i++) {
        copy[i] = Arrays.copyOf(testList[i], copy[i].length);
    }
    
    for (int i = 0; i < testList.length; i++) {
        System.out.println("Source " + i + ": " + Arrays.toString(testList[i]));
        System.out.println("Copy " + i + ": " + Arrays.toString(copy[i]));
    }
}