Adding list1 to list2 and updating list1 without affecting list2

Question

I'm working on a board game and currently trying to save all surrounding neighbors in a 2D array as lists in a list. The problem that I'm having is that when I have saved list1 to list2, and then move on in the iteration and change list1 to something else, it also affects list2 so I lose the values of the other neighbor.

This is what I have tried:

     private ArrayList<ArrayList> forcedPlays = new ArrayList<ArrayList>();
     private ArrayList<Integer> forcedPlay = new ArrayList<Integer>();   

    private void findNeighbors(){
     for (int y = -1; y <=1 ; y+=2) {
           for (int x = -1; x <= 1; x+=2) {
               if (board[myY+y][myX+x]!=null){
                   enemyY = myY+y;
                   enemyX = myX+x;
                    forcedPlay.addAll(Arrays.asList(
                                     Integer.valueOf(enemyY),
                                     Integer.valueOf(enemyX)));

     forcedPlays.add(forcedPlay);
     forcedPlay.clear()}}}}

If my player is surrounded by two neighbors I would expect the output of the forcedPlays to look like for example: [[2,1],[4,3]], but instead it looks like [[],[]]. So what I what I don't understand is how do i add list1 to list2 and then cut the connection between them so when I clear list1 it wont clear list2?

Answer 1

You can create a copy of forcedPlay before adding it.

forcedPlays.add(new ArrayList<Integer>(forcedPlay));

This way changes made to forcedPlay won't affect the list that was added to the outer list.

Answer 2

You're doing a reference copy. Basically list1 is pointing at some of the elements of list2. You need to add to list2 copies of elements in list1, clones.