Why does copying an array into another array change the original array?

Question

When I copy a 2D array into a different temporary array, it changes my original when I perform operations on the temporary.

Here is a part of my code to show what I mean:

public int getPossibleMoves(int color, int turn) {
  int x = 0;
  int blankI;
  blankI = -1;
  int pBoard[][];
  pBoard = new int[board.length][board.length];
  System.arraycopy(board, 0, pBoard, 0, board.length);

  //if its the first turn and color is black, then there are four possible moves
  if(turn == 0 && color == BLACK) {       
    pBoard[0][0] = BLANK;
    current.addChild(pBoard);
    current.children.get(x).setParent(current);
    System.arraycopy(board, 0, pBoard, 0, board.length);
    x++;

    pBoard[pBoard.length-1][pBoard.length-1] = BLANK;
    current.addChild(pBoard);
    current.children.get(x).setParent(current);
    System.arraycopy(board, 0, pBoard, 0, board.length);
    x++;

    pBoard[pBoard.length/2][pBoard.length/2] = BLANK;
    current.addChild(pBoard);
    current.children.get(x).setParent(current);
    System.arraycopy(board, 0, pBoard, 0, board.length);
    x++;

    pBoard[(pBoard.length/2)-1][(pBoard.length/2)-1] = BLANK;
    current.addChild(pBoard);
    current.children.get(x).setParent(current);
    System.arraycopy(board, 0, pBoard, 0, board.length);
    x++;
  }

On the line that says pBoard[0][0] = BLANK; and the similar ones, it changes board as well as pBoard and I need board to stay the same for my program to work correctly.

I have found an answer similar to this, which is where I got the idea to use System.arraycopy() instead of pBoard = board. The System.arraycopy() works in a different program that I used it in, but not in this one.
Any help is greatly appreciated.

One more thing:
This is part of a homework assignment. However, solving this small issue will not even bring me close to the final product that I need. This is just a tiny chunk of my code so far, but I need to get past this to move on.

Looks like you are copying references and not object. This is shallow copying — RNJ
– RNJ, Commented Oct 11, 2012 at 10:21
I doubt very much that this code modifies board in any way, perhaps somewhere else in your code? — David Soroko
– David Soroko, Commented Oct 11, 2012 at 10:44
I used the netbeans debugger and it changes both pBoard and board when it goes through the pBoard[0][0] = BLANK; line. BLANK = -1 by the way. — Tyler Pohl
– Tyler Pohl, Commented Oct 11, 2012 at 16:48

OldCurmudgeon · Accepted Answer · 2012-10-11 13:11:57Z

3

You need to do a deep copy.

Instead of:

pBoard = new int[board.length][board.length];
System.arraycopy(board, 0, pBoard, 0, board.length);

Try:

pBoard = new int[board.length][];
for ( int i = 0; i < pBoard.length; i++ ) {
  pBoard[i] = new int[board[i].length];
  System.arraycopy(board[i], 0, pBoard[i], 0, board[i].length);
}

edited Oct 11, 2012 at 13:11

answered Oct 11, 2012 at 10:52

OldCurmudgeon

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Tyler Pohl Over a year ago

Thank you so much! I did something similar to that and it worked perfectly.

Alexei Kaigorodov · Accepted Answer · 2012-10-11 10:46:44Z

1

int board[][] is array of references to arrays of type int[]. System.arraycopy(board, 0, pBoard, 0, board.length) copies the array of references but not the referenced arrays, which now are accessible in two ways. To make deep copy you have to make copies of the referenced one-dimensional arrays also. Note, to make a copy of an array you can use array.clone(). Consider also to use a one-dimensional array of size N*N with access array[x+N*y].

answered Oct 11, 2012 at 10:46

Alexei Kaigorodov

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