I have an arraylist that looks like:
private ArrayList<ArrayList<Integer>> matrix = new ArrayList<ArrayList<Integer>>(9);
It is filled with these elements:
[[7], [1, 2, 3, 4, 5, 6, 7, 8, 9], [1, 2, 3, 4, 5, 6, 7, 8, 9], [1, 2, 3, 4, 5, 6, 7, 8, 9], [3], [1, 2, 3, 4, 5, 6, 7, 8, 9], [4], [1, 2, 3, 4, 5, 6, 7, 8, 9], [9]]
Now i want to remove the "1" from the second index so my wanted output is:
[[7], [2, 3, 4, 5, 6, 7, 8, 9], [1, 2, 3, 4, 5, 6, 7, 8, 9], [1, 2, 3, 4, 5, 6, 7, 8, 9], [3], [1, 2, 3, 4, 5, 6, 7, 8, 9], [4], [1, 2, 3, 4, 5, 6, 7, 8, 9], [9]]
But when I use the remove method in the form:
matrix.get(1).remove(0);
The output is:
[[7], [2, 3, 4, 5, 6, 7, 8, 9], [2, 3, 4, 5, 6, 7, 8, 9], [2, 3, 4, 5, 6, 7, 8, 9], [3], [2, 3, 4, 5, 6, 7, 8, 9], [4], [2, 3, 4, 5, 6, 7, 8, 9], [9]]
You can see that every "1" in all the ArrayLists of the main ArrayList is removed, instead of only at the second index. How can I change my code so only the "1" of the second index is removed?
This is because you store the same ArrayList instance in outer ArrayList:
outerList = [instance1, instance2, instance2, instance2, ...]
When you delete 1st element from instance2, change is visible in all other indexes.
If you want to avoid these issue, you should create new instance for every sub-list and copy elements from original list.