Java 3d ArrayList subMatrix修改会更改3d ArrayList的其他子矩阵

Question

I am relatively new to Java and am running into an issue. I have a 3d ArrayList and I send an item of this ArrayList to a method where it is modified. But this seems to alter other items of the 3d ArrayList .

So I create a matrix ArrayList<ArrayList<ArrayList<Integer>>> and print out each item.

for(int i =0; i<solutionsMatrix.size(); i++)
{
    //System.out.println(i);
    System.out.println(solutionsMatrix.get(i));
}

I then send this 3d ArrayList to another method. Where each subMatrix (i) is modified. But when doing this somehow the modification of a matrix (i) also alters other (non-i) matrices.

private ArrayList<ArrayList<ArrayList<Integer>>> siftSolutions(ArrayList<ArrayList<ArrayList<Integer>>> solutionsMatrix)
{
    for(int n=0; n<solutionsMatrix.size();n++){
    //System.out.println(solutionsMatrix.get(n));
    ArrayList<ArrayList<Integer>> currentMatrix = mirrorMatrix(solutionsMatrix.get(n));
    System.out.println(currentMatrix);
}

The issue is that the printed out matrix (commented out portion in second code block) does not match the printed out matrix of the first code block. I have fiddled with the code and determined that the method mirrorMatrix is somehow altering the 3d matrix solutionsMatrix .

public ArrayList<ArrayList<Integer>> mirrorMatrix(ArrayList<ArrayList<Integer>> unmirrored) {
    //every value of ij needs to equal ji, thus simply transmit all non-zeros
    ArrayList<ArrayList<Integer>> mirror = new ArrayList<ArrayList<Integer>>(unmirrored);
    for (int i = 0; i < mirror.size(); i++) {
        for (int j = 0; j < mirror.size(); j++) {
            if (mirror.get(i).get(j) != 0) {
                mirror.get(j).set(i, mirror.get(i).get(j));
            }
        }
    }
    return mirror;
}

I have tried searching for an answer online but I could not find an explanation (which may simply be a result that I do not know the proper terminology to describe my problem well). I hope someone can explain to me why this is occurring so I can resolve the issue and avoid it in the future.

Answer 1

ArrayList<ArrayList<Integer>> mirror = new ArrayList<ArrayList<Integer>>(unmirrored);

This line does not make a deep copy of all the elements in it. It does create a new instance of an ArrayList , but all its the elements are the same instances of ArrayList<Integer> as were present in the unmirrored instance.

So changing those elements with mirror.get(j).set(i, mirror.get(i).get(j)); does change the same instance mirror.get(j) and unmirrored.get(j) . Ie, mirror.get(j) == unmirrored.get(j) .