indexOf(value: MemberType): Integer abstract;
The indexOf method of the Collection class returns the index of the entry specified in the value parameter if it exists in the collection or it returns zero (0) if it does not exist.
If the specified value occurs more than once in the collection, the index of the first occurrence is returned.
Use the indexOf64 method instead of the indexOf method, if the number of entries in the collection could exceed the maximum integer value of 2,147,483,647.