按值发送二维数组列表

Question

I m trying to send a 2D Arraylist by value So I wrote the following code

    public static void main(String[] args) {
        ArrayList<List<Object>> a = new ArrayList<List<Object>>();

        for (int i = 0; i < 10; i++) {
            List in_a =  Arrays.asList("aa","bb","cc","dd");
            a.add(in_a);
        }

// THE FOLLOWING WOULD WORK IN A 1D ARRAY, BUT NOT 2D
//        ArrayList<List<Object>>  b = new ArrayList < List<Object> > (a);

// THIS IS MY WORKING SOLUTION
        ArrayList < List<Object> > b = new ArrayList < List<Object> >() ;
        for (int i = 0; i < a.size(); i++) {
            List in_b =  new ArrayList(a.get(i));
            b.add(in_b);
        }

        b.get(1).set( 3, "33");

        System.out.println("a.get(1).get(3) = " + a.get(1).get(3));
        System.out.println("b.get(1).get(3) = " + b.get(1).get(3));
    }

However I do not like this solution

is there a better way to solve this issue?

Answer 1

Since Lists can only contain Objects or classes that extend Object (all of them) you will forcibly copy only the reference into your new list. So if you want a 1:1 copy of a "2D-List", you need to loop through the "first dimension" and copy the "second dimension":

ArrayList<List<Object>> c = new ArrayList<>();
for(List in_a : a){
    c.add(new ArrayList<>(in_a));
}

This is just a shorter version from what you already did in your loops. So we create a new ArrayList c and fill it with a copy of each sub-list from a . But be aware that here I use the enhanced loop (without index), so there is no guarantee that c is in the same order as a (although quite frankly, you should be fine)

Answer 2

You can try using clone() method:

ArrayList <List<Object>> originalList = new ArrayList <List<Object>>();
// ...
ArrayList <List<Object>> list2D = new ArrayList <List<Object>>();
for (List<Object> list : originalList) {
    list2D.add(list.clone());
}