Here and elsewhere, use interface types (List, Map, etc.) where appropriate.
For example:
List<String> strings = new ArrayList<>();
Note that the type on the left is List, not ArrayList. When we do this, we're more flexible to be able to change our code to use a different type of list later.

Similarly for maps:
Map<String, String> myMap = new HashMap<>();
Note that on the left we use Map instead of HashMap.

And sets:
Set<String> strings = new HashSet<>();
Note that on the left we use Set instead of HashSet.

In summary:

• interface type on left to declare type (List, Map, etc.)
• Concrete type on right to instantiate instance (HashMap, ArrayList etc.)