ArrayList's add(index,object) method throw IndexOutOfBoundException

Question

On creating an arrayList using the below mentioned code snippet:

List arrayList = new ArrayList(16);

The internal implementation of ArrayList creates an array elementData of size 16 and assigns nullat every location. On doing something like arrayList.add(2,"HelloWorld") gives an IndexOutOfBoundException as the index at which the element is being added (i.e 2) is greater than the size attribute of the arrayList .

As clear from the javaDocs, the size attribute of the arrayList is initialized to 0 when the arrayList is initialized and is incremented by 1 everytime a new element is added to the arrayList

Can someone please explain, why the ArrayList dataStructure was designed this way in the first place. Even tho' the internal dataStructure elementData was initialized with 16 null values at the creation of arrayList, still it does not allow to add value at indeces > size; (assuming index <16 in this case). What was the idea to implement the add(index,object) funtionality to be governed by the size attribute of the arrayList?

Answer 1

The purpose of having an internal array with a size greater then List.size() is to avoid re-allocating the array unnecessarily. If the internal array always had the same size as the List, then every time a new element is added, the internal array would have to be re-allocated, causing a performance penalty.