How to hard copy List> in java?

Question

    public static void main(String[] args) {
     List<List<Integer>> ll1 = new ArrayList<List<Integer>>();
     ll1.add(new ArrayList<Integer>(1));
     System.out.println(ll1.get(0));
     //hard copy don't work
     List<List<Integer>> ll2 = new ArrayList<List<Integer>>(ll1);
     System.out.println(ll2.get(0) == ll1.get(0));
}

Except using for loop to do hard copy for every inner list, do we have another way to do hard copy. Could you explain how List<List<Integer>> ll2 = new ArrayList<List<Integer>>(ll1); work and why it fail?

score 1 · Accepted Answer · answered Jan 11 '15 at 16:02

You also need to copy the inner lists:

private List<List<Integer>> copy(List<List<Integer>> toCopy) {
    List<List<Integer>> copy = new ArrayList<>(toCopy.size());
    for(List<Integer> inner : toCopy) {
        copy.add(new ArrayList<Integer>(inner));
    }
    return copy;
}

How to hard copy List> in java?

1 Answers1