TreeMap Constructor using comparator error in Java

Question

I want to make a TreeMap, so that each time an entry is inserted in the TreeMap - the entry is sorted based on value on the run. (needs O(logN) time.) So, I define a TreeMap with its constructor like below :: I dont understand where is the problem... i am confused. Can anyone please explain me the error/problem ?

Code ::

Map<String,Integer> tm = 
  new TreeMap<String,Integer>(new Comparator<Map.Entry<String,Integer>>(){

        @Override
        public int compare(Map.Entry<String, Integer> o1, Map.Entry<String, Integer> o2) {
            throw new UnsupportedOperationException("Not supported yet."); // implement logic here
        }
    });

Answer 1

The comparator is for the key

Map<String,Integer> tm =
                    new TreeMap<String,Integer>(new Comparator<String>(){
                        @Override
                        public int compare(String o1, String o2) {
                            throw new UnsupportedOperationException("Not supported yet."); // implement logic here
                        }
                    });

Answer 2

You should sort entries based on the entry's key. So comparator should compare two strings here. (new Comparator(){...}) as suggested in other answer here.

Take a look at constructor documentation: http://docs.oracle.com/javase/7/docs/api/java/util/TreeMap.html#TreeMap(java.util.Comparator)

Answer 3

If you really need to sort by the value, you should create a new object for the key, also containing your value:

class MyKey {
    String str;
    Integer i;
    ...
}
Map<MyKey,Integer> tm = new TreeMap<MyKey, Integer>(new Comparator<MyKey>(){
    @Override
    public int compare(MyKey o1, MyKey o2) {
         String str1 = o1.str;
         String str2 = o2.str;
         // implement logic here
    }
};

Answer 4

You need to provide comparator, which compares keys, rather then full map entries. So to create TreeMap<String,X> ( X is any type) you need to provide Comparator<String> . This can be done like this:

Map<String,Integer> tm = new TreeMap<String,Integer>(new Comparator<String>(){
                        @Override
                        public int compare(String o1, String o2) {
                            // implement logic here
                            // sample implementation below 
                            return o1.comparateTo(o2)
                        }
                    });

If natural ordering of strings (see: javadoc for String#compareTo) is fine for you, you can just write:

Map<String,Integer> tm = new TreeMap<String,Integer>();

Notice that in both cases, ordering of keys is the same. In both cases String#compareTo method is used to compare keys. Only in the first example we are providing comparator explicitly and in the second one we using the fact that String implements Comparable .