Trying to copy an element in a 2D array into another 2D array

Question

I have 2 Arrays, both are 4*4 arrays. I want to copy 1 element from the first array and put it into the second array, then display the second array with the new element in it. However, I'm getting an error.

I'm using a deepToString call to print the Arrays. Below is my code:

 public static void main(String [] args) { System.out.print("Enter a row and column # for your first selection?"); row = scan.nextInt(); //user enters row # column = scan.nextInt(); //user enters column # service.show(row, column); //row and column # passed as parameters System.out.println(Arrays.deepToString(board1)); //this will display board1 //with the new element, leaving the rest of the elements untouched } Public void show(int row, int column) { Int a = row; Int b = column; board1[a][b] = board2[a][b]; //destination on left, source on right //board1 is taking an element from index [a][b] in board2 }

The line board1[a][b] = board2[a][b]; is where I'm getting a "NullPointerException". I thought it was just an assignment statement to copy one element into another array. Is there a more efficient way to copy one element and display the new array? Does anybody have an idea on how to fix this?

show the declarations of your arrays

Ahsan Mahboob Shah
– Ahsan Mahboob Shah

2013-09-19 13:35:53 +00:00
Commented Sep 19, 2013 at 13:35 — Ahsan Mahboob Shah
– Ahsan Mahboob Shah, Commented Sep 19, 2013 at 13:35
are you instantiate your arrays board1 and board2?

user902383
– user902383

2013-09-19 13:36:53 +00:00
Commented Sep 19, 2013 at 13:36 — user902383
– user902383, Commented Sep 19, 2013 at 13:36

Zagrev · Accepted Answer · 2013-09-19 13:38:05Z

2

Clearly, you have global variables 'board1' and 'board2'. Look where they are declared and verify that you allocated the memory. In Java, you have to allocate the array memory, something like this:

Int board1[][] = new Int[256][8];

answered Sep 19, 2013 at 13:38

Zagrev

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1 Comment

Dan S. Over a year ago

the board1 and board2 are arrays. the declarations are: char[] [] board1 = new char[4][4]; char[] [] board2 = new char[4][4]; I have random numbers placed into both arrays. I'm fairly certain the memory is allocated, I'm not adding to either array, I'm just trying to swap an element.

mgokgoz · Accepted Answer · 2013-09-19 14:10:11Z

I'm thinking your class name is service (or it is an instance of the class you posted) from the look of code, but your show() method is not static so that it couldn't be reach from a static content if you didn't init service as a new object. Could you post the full code, so that we can check?

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