Sorted entries
Duplicates elements not allowed

TreeMap<Integer, String> tm = new TreeMap<>();

tm.clear();
// tm.put(null, "x"); // java.lang.NullPointerException
// tm.put(0, null); // OK
tm.put(0, "a");
tm.put(4444, "c");
tm.put(55555, "b");
tm.put(333, "d");
tm.put(11, "f");
tm.put(222, "e");
tm.put(666666, "g");

for (Map.Entry<Integer, String> entry : tm.entrySet()) {
    System.out.println(entry.getKey() + " = " + entry.getValue());
}

Output:

0 = a
11 = f
222 = e
3333 = d
44444 = c
555555 = b
6666666 = g