Standard Java arrays are of a fixed length. After arrays are created, they cannot grow or shrink, which means that you must know in advance how many elements an array will hold.
ArrayList are created with an initial size. When this size is exceeded the collection is automatically enlarged. When objects are removed, the array may be shrunk.
If the capacity is exceeded, this grow function is called. This method implements copying original array with new capacity.
add(E e) function
The time complexity of the
add(E e) function is 0(1) basically. This is because, ArrayList has internally standard array. But if the capacity of ArrayList is exceeded, it takes 0(N) time in order to copy original array with new capacity.
add(int index, E element) function
In this function case, we have to shift the elements in order to insert the element into middle of the ArrayList. So it takes O(N) time.
remove(int index) function
This method takes O(N) time in order to shift elements as well as
add(int index, E element).
E get(int index) function
This method takes O(1) time because we can access the element by index