difference on list.add() AND list.add(new ArrayList<>())? [duplicate]

问题

This question already has answers here:

Why does my ArrayList contain N copies of the last item added to the list? (5 answers)

Closed 3 years ago.

the following code:

List<List<Integer>> res = new ArrayList<>();
List<Integer> row = new ArrayList<>();

for (int i = 1; i <= 3; i++) {
  row.add(i);
  res.add(row);
}

res: [[1,2,3],[1,2,3],[1,2,3]]

wrote in this way:

for (int i = 1; i <= 3; i++) {
  row.add(i);
  res.add(new ArrayList<>(row));
}

res: [[1],[1,2],[1,2,3]]

回答1:

In the first case, you've only create 2 objects (called new twice). You've added the second to the first 3 times, resulting in the second object appearing 3 times in the first.

In the second case, you've created 5 objects: res, a work area row, and 3 copies of row taken a 3 different moments in time. The 3 copies are added to res.

来源：https://stackoverflow.com/questions/45929473/difference-on-list-add-and-list-addnew-arraylist