ArrayList index out of bounds

Question

Why java.lang.IndexOutOfBoundsException is raised in this example, if the size of ArrayList has been predefined? How to solve this problem?

int size = 2:
ArrayList<Integer[]> nums = new ArrayList<Integer[]>(size);
Integer[] value1 = {1,2,3};
Integer[] value2 = {1,2};
nums.add(1,value1); // java.lang.IndexOutOfBoundsException
nums.add(0,value2);

Answer 1

You cannot put an item in an ArrayList before the other one is set. If you want to do it you'll have to assign null values to the item on place 0 first.

It will work if you do:

int size = 2:
ArrayList<Integer[]> nums = new ArrayList<Integer[]>(size);
Integer[] value1 = {1,2,3};
Integer[] value2 = {1,2};
nums.add(0, null);
nums.add(1,value1);
nums.set(0,value2);

edit/ Replaced add by set to replace the null object

Answer 2

The argument to the ArrayList constructor isn't the size of the list, as your code is assuming; it's the capacity of the underlying storage used by the data structure.

The capacity will grow as required as you add elements to the list. The only reason to specify the initial capacity in the constructor is to pre-allocate a larger capacity if you know you're going to be adding lots of elements. This means that the underlying array doesn't havwe to be resized too often as you add them.

Regardless of what value you specify in the ArrayList constructor, the size of the list is governed solely by what you put into it, so you can't fetch the item with index of 1 until you've added at least 2 elements.