How to write a custom Comparator for TreeMap in Java?

I want to store key-value pairs in TreeMap and sort the entries based on the value of Key as per following logic:

Sort by the length of the key. If the length of two keys is same then sort them alphabetically. Example, for the following key-value pairs.

IBARAKI MitoCity
TOCHIGI UtunomiyaCity
GUNMA MaehashiCity
SAITAMA SaitamaCity
CHIBA ChibaCity
TOKYO Sinjyuku
KANAGAWA YokohamaCity

The expected output is like this.

CHIBA : ChibaCity
GUNMA : MaehashiCity
TOKYO : Sinjyuku
IBARAKI : MitoCity
SAITAMA : SaitamaCity
TOCHIGI : UtunomiyaCity
KANAGAWA : YokohamaCity

You need to write your own comparator for this and use it in TreeMap, e.g.:

public class StringComparator implements Comparator<String> {

    @Override
    public int compare(String s1, String s2) {
        return s1.length() == s2.length() ? s1.compareTo(s2) : s1.length() - s2.length();
    }

    public static void main(String[] args) throws JsonParseException, JsonMappingException, IOException {
        Map<String, String> map = new TreeMap<>(new StringComparator());
        map.put("IBARAKI", "MitoCity");
        map.put("TOCHIGI", "UtunomiyaCity");
        map.put("GUNMA", "MaehashiCity");
        map.put("SAITAMA", "SaitamaCity");
        map.put("CHIBA", "ChibaCity");
        map.put("TOKYO", "Sinjyuku");
        map.put("KANAGAWA", "YokohamaCity");

        System.out.println(map);
    }

}

This does not handle null values but you can add the handling if you are expecting null values in your use case.