Deleting the treeMap entries with null values

Question

I am attempting to remove all the nulls, but if the last key's treeSet is null then it remains there. So I was thinking how to delete the last entry if it is null. Since this is a treeMap I thought that I can obtain the last element by accessing it with tm.lastKey() but that method does not seem to exist. So this question is twofold. First, is there a way to delete all the nulls including the last one and the second is, where is the .lastKey() method?

public class Timing {
    private static Map<String, SortedSet> tm = new TreeMap<String, SortedSet>();

    public static Map manipulate() {
        SortedSet ss = new TreeSet();
        ss.add("APPL");
        ss.add("VOD");
        ss.add("MSFT");

        tm.put("2019-09-18",null);
        tm.put("2019-09-21",ss);
        tm.put("2019-09-22", null);
        tm.put("2019-09-20",ss);
        tm.put("2019-09-19", null);
        tm.put("2019-09-23",null);

        return tm;
    }

    public static void printMap() {
        for (String s: tm.keySet()) {
            System.out.println(s + ": " + tm.get(s));
        }
    }

    // Will delete all but the last one
    public static void deleteNull() {
        Set set = tm.entrySet();
        Iterator i = set.iterator();
        Map.Entry me = (Map.Entry) i.next();
        // there is no tm.lastKey()??
        while(i.hasNext()) {
            if (me.getValue() == null) {
                i.remove();
            }
            me = (Map.Entry) i.next();
        }
    }
}

Answer 1

To remove all entries with a value of null from your map you can replace the deleteNull method with

tm.values().removeIf(Objects::isNull);