Why is the variable of STORED being changing after the method solve is called?

Question

The variable puzzle is set to have the same initial value as the variable STORED, but is then changed through the solve method. The STORED variable is never used in any of the methods. If STORED is printed before the solve method it returns its initial 2D array, but in the if statement it prints out the new solved value of puzzle instead.

public class CA_Cindy {
public static boolean isSafe(int[][] board, int row, int col, int num)
{
    int sqrt;
    int boxRowStart;
    int boxColStart;
    for (int d = 0; d < board.length; d++)
    {
        if (board[row][d] == num)
            return false;
    }
    for (int r = 0; r < board.length; r++)
    {
        if (board[r][col] == num)
            return false;
    }
    sqrt = (int) Math.sqrt(board.length);
    boxRowStart = row - row % sqrt;
    boxColStart = col - col % sqrt;
    for (int r = boxRowStart; r < boxRowStart + sqrt; r++)
    {
        for (int d = boxColStart; d < boxColStart + sqrt; d++)
        {
            if (board[r][d] == num)
                return false;
        }
    }
    return true;
}

public static boolean solve(int[][] board, int n)
{
    int row = -1;
    int col = -1;
    boolean isEmpty = true;
    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < n; j++)
        {
            if (board[i][j] == 0)
            {
                row = i;
                col = j;
                isEmpty = false;
                break;
            }
        }
        if (!isEmpty)
            break;
    }
    if (isEmpty)
        return true;
    for (int num = 1; num <= n; num++)
    {
        if (isSafe(board, row, col, num))
        {
            board[row][col] = num;
            if (solve(board, n))
                return true;
            else
                board[row][col] = 0;
        }
    }
    return false;
}

public static void print(int[][] board, int N)
{
    for (int r = 0; r < N; r++)
    {
        for (int d = 0; d < N; d++)
        {
            System.out.print(board[r][d]);
            System.out.print(" ");
        }
        System.out.print("\n");
        if ((r + 1) % (int) Math.sqrt(N) == 0)
            System.out.print("");
    }
}
public static void main(String[] args) throws IOException {
    final int[][] STORED = {{5, 3, 0, 0, 7, 0, 0, 0, 0},
            {6, 0, 0, 1, 9, 5, 0, 0, 0},
            {0, 9, 8, 0, 0, 0, 0, 6, 0},
            {8, 0, 0, 0, 6, 0, 0, 0, 3},
            {4, 0, 0, 8, 0, 3, 0, 0, 1},
            {7, 0, 0, 0, 2, 0, 0, 0, 6},
            {0, 6, 0, 0, 0, 0, 2, 8, 0},
            {0, 0, 0, 4, 1, 9, 0, 0, 5},
            {0, 0, 0, 0, 8, 0, 0, 7, 9}};
    int[][] puzzle = STORED;
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    int c, r, d;
    int N = puzzle.length;
    int[][] board = new int[9][9];
    for (int i = 0; i < 9; i++) {
        for (int j = 0; j < 9; j++)
            board[i][j] = STORED[i][j];
    }
    for (int i = 0; i < 9; i++) {
        for (int j = 0; j < 9; j++){}
            //System.out.print(board[i][j] + " ");
        System.out.println();
    }
    System.out.println();
    if (solve(puzzle, N)) {
        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++)
                System.out.print(STORED[i][j] + " ");
            System.out.println();
        }
    }

score 2 · Accepted Answer · answered May 20 '20 at 01:26

puzzle isn't set to the same value as STORED, it's literally referring to the same object, so when you change one you change the other. Consider the following code:

public class HelloWorld{

     public static void main(String []args){
        final int[] a = {1, 2, 3};

        int[] b = a;

        System.out.println(a[1]); // Prints 2

        b[1] = 3;

        System.out.println(a[1]); // Prints 3
     }
}

Also, final doesn't do what you'd necessarily expect in this case. All it does is prevent you from changing the reference, it doesn't make the array itself immutable. It's a constant reference to a mutable object. Put another way, you can change the content of the array as long as it's still the same array.

Why is the variable of STORED being changing after the method solve is called?

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