Question

I have got (IMHO) a strange behaviour in my code. I am currently implementing the minimax algorithm for a Tic Tac Toe game. In my “successor” method I want to determine all possible moves. Here’s the code:

private ArrayList<TicTacToeState[][]> successor(final TicTacToeState[][] field, TicTacToeState s) {
    ArrayList<TicTacToeState[][]> returnList = new ArrayList<TicTacToeState[][]>();
    for (int i = 0; i < TicTacToeGame.FIELDSIZE; i++) {
        for (int j = 0; j < TicTacToeGame.FIELDSIZE; j++) {
            if (field[i][j] == TicTacToeState.Empty) {
                TicTacToeState[][] currentCopy = new TicTacToeState[TicTacToeGame.FIELDSIZE][TicTacToeGame.FIELDSIZE];
                System.arraycopy(field, 0, currentCopy, 0, field.length);
                currentCopy[i][j] = s; // <- field seems to be referenced?!
                returnList.add(currentCopy);
            }
        }
    }
    return returnList;
}

As you can see, I want to get all possible moves and save them into an arraylist. Unfortunately, when setting the value in “currentCopy”, the “field” is also changed. But the field shouldn’t be refrenced, because I copied the array. Where is the mistake? I have already tried using the clone() method on the two dimensional array -> same problem.

Thank you for any help.

(FYI, TicTacToeState is an enumeration including “Player1”, “Player2” and “Empty”)

Answer 1

Java uses shallow copies. That is, you get a copy but it isn’t what you want here. You want a deep copy. Try manually copying each element into returnList and see what happens.

Another way to solve this is to make your move, recurse, then unmake the move. Then you don’t need to copy the array at all.