public abstract int indexOf (Object o)

Returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element. More formally, returns the lowest index i such that Objects.equals(o, get(i)), or -1 if there is no such index.

Parameters:
o    element to search for

Returns:  the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element

Exceptions:
ClassCastException    if the type of the specified element is incompatible with this list (optional)
NullPointerException    if the specified element is null and this list does not permit null elements (optional)