排序地图 <String, Integer> 最大的先入名单 <String> 有效率的？

Question

I have a ConcurrentMap<String, Integer> , and I'd like to get a List<String> from it with the String mapping to the largest Integer first, second largest second, etc.

Right now I have something along the lines of this:
Loop through the keySet of the map
In that loop, loop through a "sorted" List<String>
Keep looping until the key String's respective value is less than element i of the "sorted" List , and insert it.

Now this will work, but I doubt it's very efficient. Does Java 8 have any built in sorting algorithms that could help me here?

Answer 1

Using streams, you could write it like this:

List<String> sorted = map.entrySet().stream()
                         .sorted(reverseOrder(comparing(Entry::getValue)))
                         .map(Entry::getKey)
                         .collect(toList());

Or as commented by Holger:

List<String> sorted = map.entrySet().stream()
                         .sorted(comparingByValue(reverseOrder()))
                         .map(Entry::getKey)
                         .collect(toList());

Note static imports:

import static java.util.Collections.reverseOrder;
import static java.util.Comparator.comparing;
import static java.util.stream.Collectors.toList;

I don't know if that's more efficient than your method but you can profile both and decide based on actual measurement.

Answer 2

Yes, it does. You could do something like this:

ConcurrentMap<String, Integer> map = /* something */;

// entrySet just returns a view of the entries, so copy it into a new List
List<Map.Entry<String, Integer>> entries = new ArrayList<>(yourMap.entrySet());

// sort entries by their int values
Collections.sort(entries, (entry1, entry2) -> Integer.compare(entry1.getValue(), entry2.getValue()));

// copy just the keys into a new List
List<String> result = new LinkedList<>();
for (Map.Entry<String, Integer> entry : entries) {
  result.add(entry.getKey());
}

Answer 3

import java.util. TreeSet;

import java.util.Set; import java.util.concurrent.ConcurrentHashMap;

public class Test {

public static void main(String[] args){
    ConcurrentHashMap< String, Integer> map = new ConcurrentHashMap<String , Integer>();
    map.put("Fish", 1);
    map.put("Bird", 100);
    map.put("Reptile", 2);
    map.put("Mammal", 10);
    TreeSet <Integer> sortedList = new TreeSet <Integer>(map.values());
    for(Integer i :sortedList){
        System.out.println(i);
    }
}

}