Question

Consider this example:

int[] a = new int[] {0,0};
ArrayList<int[]> b = new ArrayList<int[]>();

b.add(a);
a[0] = 1;
a[1] = 1;
b.add(a);

b is now {[1,1],[1,1]}.
How can I ensure it will be {[0,0],[1,1]} without allocating another array?

Answer 1

Each element of an ArrayList<int[]> is a reference to an array of ints. If you add the same array twice (as you are doing), it will be repeated in the list. Changing an element of the int array will be reflected in every occurrence of it in the ArrayList. To have different arrays, you need to have separate arrays allocated. Something like this will do:

int[] a = new int[] {0,0};
ArrayList<int[]> b = new ArrayList<int[]>();

b.add(a.clone()); // add a distinct array
a[0] = 1;
a[1] = 1;
b.add(a);

Then the contents of b will be {[0,0],[1,1]}.