Java：将矩阵转换为列表：我的add方法不起作用，我也不知道为什么

Question

I keep on get an error that says my add method says no suitable method found. The goal of the program is to get all the values of a matrix into a list and I can not get it to work because it keeps on giving me an error at this line:

list.add(mat[i][j]);

The entire code looks like this:

List<Integer> go(int[][] mat) 
{
    ArrayList<String> list = new ArrayList<String>();
    for (int i = 0; i < mat.length; i++) 
    {
        for (int j = 0; j < mat[i].length; j++) 
        {
            list.add(mat[i][j]);
        }
    }
    return list;
}

Answer 1

Your function returns a List<Integer> , but your list object is declared as an ArrayList<String> , which you are trying to add mat[i][j] to. Furthermore, mat[i][j] is itself an int , so list should be an ArrayList<Integer> .

Answer 2

List<Integer> go(int[][] mat) 
{
    int capacity = mat.length * mat[0].length; // Not needed.
    List<Integer> list = new ArrayList<>(capacity);
    for (int i = 0; i < mat.length; i++) 
    {
        for (int j = 0; j < mat[i].length; j++) 
        {
            list.add(mat[i][j]);
        }
    }
    return list;
}

String instead of Integer. And better style is to let the variable already be a List.

Immediately reserving the right amount of list entries is an optimization: no resizing during adding of values, and not reserving too many room.

In fact a linear array might also be an option.