ArrayList out of bounds exception

Question

I have the following code:

ArrayList<Integer> arr = new ArrayList<Integer>(10);
arr.set(0,5);

I am getting an index out of bounds error, and I don't know why. I have declared the ArrayList of size 10. Why do I get this error?

Answer 1

You declared an ArrayList, that has the initial capacity of 10 elements, but you did not add an element to this list, i.e. the list is empty. set will replace an existing element, but since there is no element in the list, the exception is thrown. You have to add elements before, using the add method.

Initial capacity means that the array that the list maintains internally is of size 10 at the beginning. While adding more elements to the list, the size of this internal array might change.

Answer 2

looking into JDK source code of ArrayList.set(int, E) gives a hint: you need to have at least N elements in your list before you can call set(N-1, E).

Add your elements via add() method.