如何按值对带有列表的地图排序？

Question

How can I sort Map with list by value?I have in list always one element but i must use list. I have:

 Map<String,List<String>> map = new HashMap<>();

And

import java.util.*;

public class MapUtil
{
    public static <K, V extends Comparable<? super V>> Map<K, V>
        sortByValue( Map<K, V> map )
    {
        List<Map.Entry<K, V>> list =
            new LinkedList<Map.Entry<K, V>>( map.entrySet() );
        Collections.sort( list, new Comparator<Map.Entry<K, V>>()
        {
            public int compare( Map.Entry<K, V> o1, Map.Entry<K, V> o2 )
            {
                return (o1.getValue()).compareTo( o2.getValue() );
            }
        } );

        Map<K, V> result = new LinkedHashMap<K, V>();
        for (Map.Entry<K, V> entry : list)
        {
            result.put( entry.getKey(), entry.getValue() );
        }
        return result;
    }
}

and when i try this:

MapUtil.sortByValue(map);

I get this error:

The method sortByValue(Map<K,V>) in the type MapUtil is not applicable for the arguments (Map<String,List<String>>)

Answer 1

V needs to implement comparable, but List<String> does not. You need to change the sorting method to use explicit comparator .