Java TreeMap iterator not working correctly for String keys.

Question

I have a TreeMap that maps String keys to a custom City class. Here is how it is instantiated:

TreeMap<String, City> nameDictionary = new TreeMap<String, City>(new CityNameComparator());

CityNameComparator implementation:

    public class CityNameComparator implements Comparator<String>
{
    public int compare (String c1, String c2) {
        return c1.compareTo(c2);
    }
}

I have a method that returns an iterator that should iterate through the map in key-ascii order:

    public Iterator<City> getNameIterator(){
 return nameDictionary.values().iterator();
}

For some reason the values are returned in the order they were added to the TreeMap. Any ideas?

Answer 1

Are you sure a LinkedHashMap hasn't been assigned to the Map reference by mistake? That would preserve the order the entries were added to the map.

Or perhaps there is a bug in the code that is adding entries, putting the wrong value with a key.

Iterate over the entries, and see what is in the map:

for (Map.Entry<String, City> e : dictionary.entrySet()) 
  System.out.println(e.getKey() + " --> " + e.getValue());

Answer 2

It works just fine:

import java.util.Comparator;
import java.util.Iterator;
import java.util.TreeMap;


public class test2 {

    public static class City {
     public final String m_name;

     public City(String aName) {
      m_name = aName;
     }
    }

    public static class CityNameComparator implements Comparator<String>
    {
        public int compare (String c1, String c2) {
            return c1.compareTo(c2);
        }
    }

    public static class CityMap {
     TreeMap<String, City> nameDictionary = new TreeMap<String, City>(new CityNameComparator());

     public Iterator<City> getNameIterator(){
            return nameDictionary.values().iterator();
     }

     public City put(String aName) {
      return nameDictionary.put(aName, new City(aName));
     }
    }

    public static void main(String[] args) {
     CityMap cityMap = new CityMap();
     cityMap.put("d");
     cityMap.put("b");
     cityMap.put("c");
     cityMap.put("a");

     for (Iterator<City> cities = cityMap.getNameIterator(); cities.hasNext(); ) {
      City city = cities.next();
      System.out.println(city.m_name);
     }
    }
}

Output:

a

b

c

d

Answer 3

Sorry, stupid bug. I was assigning a different iterator based on a bug somewhere else. It works fine now.