There are many methods in the java.util.ArrayList class in Java. But, two of the overloaded methods set and add at index, are quite interesting. I am writing this post just to tell you a small gotcha of the methods.

ArrayList Class

There a two add methods

1. add(E e)

    This basically adds the element to the ArrayList. Since the ArrayList is dynamic, it automatically increases the size of the ArrayList by 1, and adds the new element to the index.

2. add(int index, E e)

    This is a little confusing because it shifts all the elements after the element has been added to the specified index. Let's say, you have an ArrayList with elements [1, 2, 3, 4] and the size is 4. If you try to add a new element 10 to index 2 where we have element 3 already, it won't replace element 3 with 10, but rather it will add 10 at index 2, and shifts all the remaining elements from 3 onwards to the right of the ArrayList. So, the resulting Arraylist looks something like this:

[1, 2, 10, 3, 4] and the size becomes 5

3. set(int index, E e)

    As the name suggests, the set replaces the existing element in the index with the element that you want to add. So, for the ArrayList [1, 2, 3, 4] with size 4 after set(2, 10) becomes

[1, 2, 10, 4] and the size remains 4

References:

https://docs.oracle.com/javase/8/docs/api/java/util/ArrayList.html