Question

The value type of my map is a List<Pair<Integer, String>>

I want to sort it by the left side of first pair in the list. But there is an error said:

The constructor
TreeMap<String,List<Pair<Integer,String>>> (Comparator<List<Pair<Integer,String>>>)
is undefined

Here is my code:

import java.util.TreeMap;
import org.apache.commons.lang3.tuple.Pair;

Map<String, List<Pair<Integer, String>>> data = new HashMap<String, List<Pair<Integer,String>>>();  
            ....
Comparator<List<Pair<Integer, String>>> valueComparator = new Comparator<List<Pair<Integer, String>>>() {
            @Override
            public int compare(List<Pair<Integer, String>> o1,
                    List<Pair<Integer, String>> o2) {
                int rzt = o1.get(0).getLeft() - o2.get(0).getLeft();
                return rzt;
            }  
           };

           TreeMap<String, List<Pair<Integer, String>>> sortedByValues = new TreeMap<String, List<Pair<Integer, String>>>(valueComparator);
           sortedByValues.putAll(data);

Answer 1

The Comparator should compare keys and not values. The constructor of TreeMap has the following signature:

public TreeMap(Comparator<? super K> comparator)

where K is the type parameter of the Key: Class TreeMap<K,V>