Java中Integer [] []（矩阵）的数组列表

Question

I defined an ArrayList of matrix ( Integer[][] ) in Java. When I add a new matrix into the ArrayList , it changes all of the variables to the last matrix.

I mean when I add

0 4 4 2
0 4 4 2
1 2 3 4
4 5 9 7

and then add

1 4 7 8
0 1 2 3
4 5 6 7
4 1 2 3

When I print the elements like that:

1 4 7 8
0 1 2 3
4 5 6 7
4 1 2 3
-------
1 4 7 8
0 1 2 3
4 5 6 7
4 1 2 3

So what should I do for this? this is my code :

private Integer matrix[][] = new Integer[4][4];

public Integer[][] right(Integer[][] M) {
        for (int k = 0; k < 4; k++)
            for (int i = 0; i < 4; i++)
                for (int j = 0; j < 3; j++) {
                    if (M[i][j] != 0 && M[i][j + 1] == 0) {
                        M[i][j + 1] += M[i][j];
                        M[i][j] = 0;
                        new_tile = true;
                    }
                }
        for (int i = 0; i < 4; i++)
            for (int j = 3; j > 0; j--) {
                if (M[i][j] == M[i][j - 1] && M[i][j] != 0 && M[i][j - 1] != 0) {
                    M[i][j] += M[i][j - 1];
                    M[i][j - 1] = 0;
                    new_tile = true;
                }
            }
        for (int k = 0; k < 4; k++)
            for (int i = 0; i < 4; i++)
                for (int j = 0; j < 3; j++) {
                    if (M[i][j] != 0 && M[i][j + 1] == 0) {
                        M[i][j + 1] += M[i][j];
                        M[i][j] = 0;
                        new_tile = true;
                    }
                }
        return M;
    }


static void printMatrix(Integer[][] matrix) {
    for (int i = 0; i < matrix.length; i++) {
        for (int j = 0; j < matrix[0].length; j++) {
            System.out.print(matrix[i][j]);
        }
        System.out.println();
    }
}

/**
 * Print the elements in a matrix list
 */
static void printMatrices(ArrayList<Integer[][]> matrices) {
    for (Integer[][] matrix : matrices) {
        printMatrix(matrix);
        System.out.println("--------");
    }
}
public void solve() {

    Integer[][] temp = right(matrix);
    printMatrix(temp);
    if (new_tile)
        visited_nodes.add(temp);
    else {
        printMatrices(visited_nodes);
    }
    refresh();
}

    @Override
    public void actionPerformed(ActionEvent e) {

        solve();
        repaint();
    }

and the solve method called every 100ms.

Answer 1

You actually modify always the same instance of matrix.

Change the right() method for it to create a new matrix each time it is called:

public Integer[][] right(Integer[][] M) {
    Integer[][] newM = new Integer[4][4];

    // modify newM in your algo instead of M
    // Showing only the first bloc
    for (int k = 0; k < 4; k++)
        for (int i = 0; i < 4; i++)
            for (int j = 0; j < 3; j++) {
                if (M[i][j] != 0 && M[i][j + 1] == 0) {
                    newM[i][j + 1] = M[i][j + 1] + M[i][j];
                    newM[i][j] = 0;
                    new_tile = true;
                }
            }
    //...
    return newM;
}

Answer 2

When you do:

private Integer matrix[][] = new Integer[4][4];

You create an object and make the variable matrix point to it. When you call right(matrix) , you pass the original object to the right() method, so it gets changed. Instead, you should pass a copy of the current matrix or create a new matrix, depending on what you need.

Breaking your code into smaller methods would make it easier to reason about. Something like this:

/**
 * Print a single matrix
 */
static void printMatrix(Integer[][] matrix) {
    for (int i = 0; i < matrix.length; i++) {
        for (int j = 0; j < matrix[0].length; j++) {
            System.out.print(matrix[i][j]);
        }
        System.out.println();
    }
}

/**
 * Print the elements in a matrix list
 */
static void printMatrices(List<Integer[][]> matrices) {
    for (Integer[][] matrix : matrices) {
        printMatrix(matrix);
        System.out.println("--------");
    }
}

Answer 3

Adding an element to a list adds a reference to that element. If the element is subsequently changed, the changed element will be seen in the list.

It cannot be confirmed from your code sample that this is the problem because you have not posted the code for the right() method. My suspicion is that it returns the same, altered matrix every time instead of a new matrix with different values.

This question shows how to clone a 2D array which you need to do to avoid repeatedly adding the same one to your list.