class FontAlphabetizer
implements Comparator<Font> {
    @Override
    public int compare(Font font1, Font font2) {
        return font1.getName().compareTo(font2.getName());
    }
}

class MapValueComparator<K, V extends Comparable<V>>
implements Comparator<Map<K, V>> {
    final K key;

    MapValueComparator(K key) {
        this.key = key;
    }

    @Override
    public int compare(Map<K, V> map1, Map<K, V> map2) {
        return map1.get(key).compareTo(map2.get(key));
    }
}

static void sortByTown(List<Map<String, String>> list) {
    Collections.sort(list, new MapValueComparator<String, String>("town"));
}

class PopulationComparator
implements Comparator<Map<String, String>> {
    @Override
    public int compare(Map<String, String> map1, Map<String, String> map2) {
        final Long pop1 = Long.valueOf(map1.get("population"));
        final Long pop2 = Long.valueOf(map2.get("population"));

        return pop1.compareTo(pop2);
   }
}

public class CustomComparator implements Comparator<MyObjectType>
 {
  public int compare(MyObjectType ob1 , MyObjectType ob2)
  {
     //code to compare the 2 objects
  }
 }

final List<Map<String, Object>> towns = new ArrayList<Map<String, Object>>();

final Map<String, Object> toronto = new HashMap<String, Object>();
toronto.put("town", "Toronto");
toronto.put("population", 2500000);
toronto.put("age", 147);
towns.add(toronto);

final Map<String, Object> ottawa = new HashMap<String, Object>();
ottawa.put("town", "Ottawa");
ottawa.put("population", 883000);
ottawa.put("age", 159);
towns.add(ottawa);

final Map<String, Object> montreal = new HashMap<String, Object>();
montreal.put("town", "Montreal");
montreal.put("population", 1600000);
montreal.put("age", 372);
towns.add(montreal);

final Map<String, Object> quebec = new HashMap<String, Object>();
quebec.put("town", "Quebec City");
quebec.put("population", 600000);
quebec.put("age", 406);
towns.add(quebec);

final Map<String, Object> vancouver = new HashMap<String, Object>();
vancouver.put("town", "Vancouver");
vancouver.put("population", 600000);
vancouver.put("age", 128);
towns.add(vancouver);

Collections.sort(towns, new Comparator<Map<String, Object>>() {
    @Override
    public int compare(final Map<String, Object> o1, final Map<String, Object> o2) {
        if (o1.get("population") instanceof Integer && o2.get("population") instanceof Integer && !((Integer)o1.get("population")).equals((Integer)o2.get("population"))) {
            return ((Integer)o1.get("population")).compareTo((Integer)o2.get("population"));
        }
        if (o1.get("age") instanceof Integer && o2.get("age") instanceof Integer) {
            return ((Integer)o1.get("age")).compareTo((Integer)o2.get("age"));
        }
        // Default if there is no population/no age, shouldn't happen.
        // TODO : do something else.
        return o1.toString().compareTo(o2.toString());
    }
});

for (final Map<String, Object> town: towns) {
    System.out.println(town.get("population")+"t"+town.get("age")+"t"+town.get("town"));
}

600000  128 Vancouver
600000  406 Quebec City
883000  159 Ottawa
1600000 372 Montreal
2500000 147 Toronto