Java: Move null values to the right of all non-null elements in 2D array using for loops only

Question

Given String[] [] arr2D = [["sun", "moon"], null, ["black", "white"], ["dog", "cat"], null], how can I loop through arr2D to shift the first null to the immediate right of ["dog", "cat"] or to the end of the array so that arr2D becomes [["sun", "moon"], ["black", "white"], ["dog", "cat"], null, null]?

I have tried adapting this example involving 1D arrays but I ended up with a nullPointerException every time.

// I'm hardcoding i < 4 because there are 4 elements between and including ["sun", "moon"] and ["dog", "cat"]
for(int i = 0; i < 4; i++) {
  if(arr2D[i] == null) {
    for(int j = i + 1; j < 4; j++) {
      arr2D[j - 1][0] = arr2D[j][0];
      arr2D[j- 1][1] = arr2D[j][1];
    }
    arr2D[4 - 1] = null;
    break;
  }
}

Exception in thread "main" java.lang.NullPointerException: Cannot store to object array because "items[...]" is null

If you edit your post to include what you've tried and the stack trace from the NPE we can more easily help you fix your your homework assignment. As it is you are asking people to jump through hoops in order to help you. — The Head Rush
– The Head Rush, Commented Sep 14, 2022 at 18:37

Eritrean · Accepted Answer · 2022-09-14 19:05:19Z

0

The easiest solution using only loops would be to create a new array and copy the non null values:

static String[][] moveNullsRight(String[][] arr){
    String[][] result = new String[arr.length][];
    for (int i = 0, j = 0; i < arr.length; i++){
        if (arr[i] != null){
            result[j++] = arr[i];
        }
    }
    return result;
}

If you need to sort your array in place use something like below:

static String[][] moveNullsRight(String[][] arr){
    for(int i = 0; i < arr.length - 1; i++){
        int m = i;
        for(int j = i+1; j < arr.length; j++){
            if(arr[m] == null && arr[j] != null){
                m = j;
            }
        }
        String[] temp = arr[m];
        arr[m] = arr[i];
        arr[i] = temp;
    }
    return arr;
}

edited Sep 14, 2022 at 19:05

answered Sep 14, 2022 at 18:24

Eritrean

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2 Comments

KLG Over a year ago

I should have been more specific; while Comparator.nullsLast looks very helpful, I was looking for a solution that only uses for loops. Perhaps I could view the source code of nullsLast

KLG Over a year ago

Apologies, check the post again.

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