TreeMap没有返回所有键

Question

I have a requirement like I need to sort the keys inside a Map according to some sequence which is defined on a property file entry.So that output should come according to sequence which user defining on the property entry.For this I tried using TreeMap and Comparator .

My property file entry is

seq=People,Object,Environment,Message,Service

Following is my code:

Properties prop=new Properties();
prop.load(new FileInputStream("D:\\vignesh\\sample.properties"));
final String sequence=prop.getProperty("seq");
// Display elements
     final String sequence=prop.getProperty("seq");
     System.out.println("sequence got here is "+sequence); 
      //Defined Comparator
      Comparator<String> comparator = new Comparator<String>() {
          @Override
          public int compare(String key1, String key2) {
              return sequence.indexOf(key1) - sequence.indexOf(key2);
          }
      };
SortedMap<String,String> lhm = new TreeMap<String,String>(comparator);
       // Put elements to the map
          lhm.put("Object", "biu");
          lhm.put("Message", "nuios");
          lhm.put("Service", "sdfe");
          lhm.put("People", "dfdfh");
          lhm.put("Environment", "qwe");
          lhm.put("Other", "names");
          lhm.put("Elements", "ioup");          //Not showing in output
          lhm.put("Rand", "uiy");               //Not showing in output
//Iterating Map
      for(Entry<String, String> entry : lhm.entrySet()) {
            System.out.println(entry.getKey());
        }

Output

sequence got here is People,Object,Environment,Message,Service
Other
People
Object
Environment
Message
Service

Now I have some problems with this code.

• I have nearly 8 elements inside my map.But output showing only six elements.Why last two elements didn't come?

• The values which are not matching with sequence are coming at top
  now.I would like to get those at bottom.Is there a way?

• Here I have declared the string which is reading from properties file as final so that I can't change the property every time.When I
  remove final Identifier it is showing error in my IDE. How can I
  avoid that?

• My keys inside HashMap may not fully equals with the property entry
  sequence.So I need to check whether that sequence are contains in my HashMap key.Do I need to change my Comparator for that?

Answer 1

It's not that your elements are not printed, it's that they are not added in the map !

In a sorted Map you have :

A sorted map performs all key comparisons using its compareTo (or compare) method, so two keys that are deemed equal by this method are, from the standpoint of the sorted map, equal.

But a map cannot have two equal keys :

A map cannot contain duplicate keys; each key can map to at most one value.

But when comparing Element with Other your comparator returns zero. It considers those two strings as equals so the latest is not added to the map.

As suggested in the javadoc of SortedMap :

ote that the ordering maintained by a sorted map (whether or not an explicit comparator is provided) must be consistent with equals if the sorted map is to correctly implement the Map interface.

So I would advise you to make sure that when your comparator returns 0 it is because the two elements compared are indeed equals. A very "naive" way to do that would be :

Comparator<String> comparator = new Comparator<String>() {
      @Override
      public int compare(String key1, String key2) {
          int returned = sequence.indexOf(key1) - sequence.indexOf(key2);

          if (returned == 0 && !key1.equals(key2))
              returned = -1;

          return returned;

      }
  };

Answer 2

First of all, indexOf returns -1 for any substrings it cannot locate in a String .

When you are inserting "Other" , indexOf("Other") returns -1 and indexOf("People") returns 0 , which adds "Other" to the map in the first position.

However, when adding other strings, they are tested against "Other" , which returns -1 . Unfortunately, all other elements are also missing in the property value string and thus return -1 . Which means sequence.indexOf(key1) - sequence.indexOf(key2) is 0 and they all are assumed equal to "Other" .