Question

This is not basically how to sort the HashMap based on keys. For that I could directly use TreeMap without a wink 🙂

What I have at the moment is

Map<String, Object> favoritesMap = new HashMap<String, Object>();
and its contents can be
["Wednesdays" : "abcd"]
["Mondays" : "1234"]
["Not Categorized" : "pqrs"]
["Tuesdays" : "5678"]

I want to sort the HashMap based on keys and additional to this I need “Not Categorized” to be the last one to retrieve.

So expected while iterating over keySet is

["Mondays", "Tuesdays", "Wednesdays", "Not Categorized"] i.e. sorted on keys and "Not Categorized" is the last one

Thought of going for HashMap while creating and at the end add ["Not Categorized" : "pqrs"] but HashMap does not guarantee the order 🙂

Any other pointers for the solution?

Answer 1

Are you specifically excluding TreeMap for some external reason? If not you could obviously use TreeMap with a specially made Comparator.

Have you considered any of the other SortedMaps?

If TreeMap is definitely out I would extend HashMap and make it look like there is always one more entry but that is certainly not a trivial piece of work. You should have a very good reason not to use a SortedMap before going down this road.

Added

Here is an example of how you can make a particular entry always sort to the end using a TreeMap:

// This key should always appear at the end of the list.
public static final String AtEnd = "Always at the end";

// A sample map.
SortedMap<String, String> myMap =
        new TreeMap<>(
        new Comparator<String>() {
          @Override
          public int compare(String o1, String o2) {
            return o1.equals(AtEnd) ? 1 : o2.equals(AtEnd) ? -1 : o1.compareTo(o2);
          }
        });

private void test() {
  myMap.put("Monday", "abc");
  myMap.put("Tuesday", "def");
  myMap.put("Wednesday", "ghi");
  myMap.put(AtEnd, "XYZ");

  System.out.println("myMap: "+myMap);
  // {Monday=abc, Tuesday=def, Wednesday=ghi, Always at the end=XYZ}
}

I wonder if you are looking for some variant of that?