How to preserve insertion order in HashMap?

HashMap is unordered per the second line of the documentation:

This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time.

Perhaps you can do as aix suggests and use a LinkedHashMap, or another ordered collection. Take a look on javapractices.com's guide on Choosing the right Collection.

LinkedHashMap is precisely what you're looking for.

It is exactly like HashMap, except that when you iterate over it, it presents the items in the insertion order.