In Java, sort hash map by its key.length()

The compiler is complaining because you cannot call compareTo on an int. The correct way to sort the map is the following:

Map<String, Integer> treeMap = new TreeMap<String, Integer>(
    new Comparator<String>() {
        @Override
        public int compare(String s1, String s2) {
            if (s1.length() > s2.length()) {
                return -1;
            } else if (s1.length() < s2.length()) {
                return 1;
            } else {
                return s1.compareTo(s2);
            }
        }
});

The first two conditions compare the lengths of the two Strings and return a positive or a negative number accordingly. The third condition would compare the Strings lexicographically if their lengths are equal.

You call String#length(), which returns a primitive int. You need the static method Integer.compare(int,int). If you are on Java 8, you can save yourself a lot of typing:

Map<String,Integer> treeMap = new TreeMap<>(
        Comparator.comparingInt(String::length)
                  .thenComparing(Function.identity()));

public int compare(String o1, String o2) {
  return o1.length() == o2.length() ? o1.compareTo(o2) : o1.length() - o2.length();
}