Cast works where the actual instance of the list is an ArrayList. If it is, say, a Vector (which is another extension of List) it will throw a ClassCastException.
The error when changing the definition of your HashMap is due to the elements later being processed, and that process expects a method that is defined only in ArrayList. The exception tells you that it did not found the method it was looking for.
Create a new ArrayList with the contents of the old one.
new ArrayList<String>(myList);