A Tree Map's keys are iterated in natural order. : Map « Utility Classes « SCJP

1. SCJP
2. Utility Classes
3. Map

import java.util.Map;
import java.util.Set;
import java.util.TreeMap;

public class MainClass {
  public static void main(String[] argv) {
    Map map = new TreeMap();
    map.put("key", "value");
    map.put("key2", "value2");
    map.put("key3", "value3");

    Set set = map.keySet();

    System.out.println(set);
  }
}

[key, key2, key3]