How change an element of 2D ArrayList without change all the elements of the column-CodePudding

I want to change the value of an element of the list. I have to tried but I changed all the elements of the column. How can I do this?

My code is this:

    ArrayList<ArrayList<Integer>> matrix = new ArrayList<ArrayList<Integer>>();
    ArrayList<Integer> array = new ArrayList<Integer>();
    for(int i = 0; i < 3;   i){
        array.add(0);
    }
    for(int i = 0; i < 3;   i){
        matrix.add(i,array);
    }

    showMatrix(matrix);
    
    matrix.get(0).set(0,1);
    
    showMatrix(matrix);

Matrix initialized is:

E/Matrix: [[0, 0, 0], [0, 0, 0], [0, 0, 0]]

Matrix change the position [0][0] to 1:

E/Matrix: [[1, 0, 0], [1, 0, 0], [1, 0, 0]]

How can I change only the position[0][0]? And not change [1][0] and [2][0].

Thank you.

CodePudding user response：

problem is with:

for(int i = 0; i < 3;   i){
    matrix.add(i,array);
}

array is instantiated at the start with ArrayList<Integer> array = new ArrayList<Integer>(); and never get instantiated again. That means array always refer to the same object. So, there is always the only copy!

Instead, try:

for(int i = 0; i < 3;   i){
        matrix.add(i, new ArrayList<>(array));
}