Java TreeMap (comparator) and get method ignoring the comparator

Question

public final Comparator<String> ID_IGN_CASE_COMP = new Comparator<String>() {

        public int compare(String s1, String s2) {
            return s1.compareToIgnoreCase(s2);
        }
    };

private Map< String, Animal > _animals = new TreeMap< String, Animal >(ID_IGN_CASE_COMP);

My problem is, how to use method get(id) ignoring the given comparator. I want the map to be order by Case Insensitive but, I want it to be case sensitive when I fetch the values by a given key.

Answer 1

I think the answer is easy. Implement your own comparator that does a case insensitive sort but does NOT return 0 for "A" and "a"... sort them too.

The issue is that your comparator returns 0 for the compare( "A", "a" ) case which means it is the same key as far as the map is concerned.

Use a comparator like:

public final Comparator<String> ID_IGN_CASE_COMP = new Comparator<String>() {

    public int compare(String s1, String s2) {
        int result = s1.compareToIgnoreCase(s2);
        if( result == 0 )
            result = s1.compareTo(s2);
        return result;
    }
};

Then all keys will go in regardless of case and "a" and "A" will still be sorted together.

In other words, get("a") will give you a different value from get("A")... and they will both show up in keySet() iterators. They will just be sorted together.

Answer 2

In a TreeMap, adding two keys a and b (in that order) so that compare(a, b) returns 0 will result in that the latest added entry (b) will overwrite the first one (a).

In your case, this means that there will never be any use for case insensitive get(id).

quoting http://java.sun.com/javase/6/docs/api/java/util/TreeMap.html

Note that the ordering maintained by a sorted map (whether or not an explicit comparator is provided) must be consistent with equals if this sorted map is to correctly implement the Map interface. (See Comparable or Comparator for a precise definition of consistent with equals.) This is so because the Map interface is defined in terms of the equals operation, but a 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. The behavior of a sorted map is well-defined even if its ordering is inconsistent with equals; it just fails to obey the general contract of the Map interface.

This is probably not what you want.

If the map is comparably small and you don't need to fetch the sorted entries very many times, a solution is to use a HashMap (or a TreeMap without explicitly setting the comparator), and sort the entries case-insensitively when you need them ordered.