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ArrayList allows duplicate values while HashSet doesn't allow duplicates values. Ordering : ArrayList maintains the order of the object in which they are inserted while HashSet is an unordered collection and doesn't maintain any order.
List allows duplicates while Set doesn't allow duplicate elements . All the elements of a Set should be unique if you try to insert the duplicate element in Set it would replace the existing value. List permits any number of null values in its collection while Set permits only one null value in its collection.
CopyOnWriteArrayList is thread safe. ArrayList iterator is fail-fast and ArrayList throws ConcurrentModificationException if concurrent modification happens during iteration. CopyOnWriteArrayList is fail-safe and it will never throw ConcurrentModificationException during iteration.
There's no difference between list implementations in both of your examples. There's however a difference in a way you can further use variable myList in your code.
When you define your list as:
List myList = new ArrayList();
you can only call methods and reference members that are defined in the List interface. If you define it as:
ArrayList myList = new ArrayList();
you'll be able to invoke ArrayList-specific methods and use ArrayList-specific members in addition to those whose definitions are inherited from List.
Nevertheless, when you call a method of a List interface in the first example, which was implemented in ArrayList, the method from ArrayList will be called (because the List interface doesn't implement any methods).
That's called polymorphism. You can read up on it.
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