IndexofBoundsException is thrown when initializing array of List

Question

I am new to Java and have below code.

List<List<TreeNode>> dp = new ArrayList<List<TreeNode>>(N + 1);
System.out.println(dp.isEmpty());

for (int i = 0; i <= N; i++) {
    dp.set(i, new ArrayList<TreeNode>());
}

dp.isEmpty() returns true; and I have Java.lang.IndexofBoundsException for line dp.set()...

I thought after new.. the dp already has N+1 entries, but it turned out wrong....dp is still empty..

so How can populate dp with N+1 arraylist

https://stackoverflow.com/questions/27946920/distinction-between-the-capacity-of-an-array-list-and-the-size-of-an-array — Ryuzaki L, Nov 24 '19 at 03:13
https://stackoverflow.com/questions/8896758/initial-size-for-the-arraylist — Ryuzaki L, Nov 24 '19 at 03:17

score 2 · Answer 1 · answered Nov 24 '19 at 03:12

When you are writing new ArrayList<List<TreeNode>>(N + 1);, this is telling java that create arraylist with initial size of N + 1 elements.

As arraylist is dynamically increases as you add elements this number increases also internally.

Here, N+1 does not mean there are N+1 elements in the List, it just means N+1 memory is allocated by the List.

To add elements you need to do as follows,

for (int i = 0; i <= N; i++) {
    dp.add(i, new ArrayList<TreeNode>());
}

System.out.println(dp.isEmpty()); // false here

IndexofBoundsException is thrown when initializing array of List

1 Answers1