我对Java比较陌生,经常发现需要对值进行Map<Key,Value>排序。

由于这些值不是唯一的,我发现自己将keySet转换为一个数组,并使用自定义比较器通过数组排序对该数组进行排序,该比较器根据与该键关联的值进行排序。

有没有更简单的方法?


当前回答

这还有一个额外的好处,即可以使用Java8进行升序或降序排序

import static java.util.Comparator.comparingInt;
import static java.util.stream.Collectors.toMap;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.stream.Collectors;
import java.util.stream.Stream;

class Utils {
    public static Map<String, Integer> sortMapBasedOnValues(Map<String, Integer> map, boolean descending) {
        int multiplyBy = (descending) ? -1: 1;
        Map<String, Integer> sorted =  map.entrySet().stream()
                .sorted(comparingInt(e -> multiplyBy * e.getValue() ))
                .collect(toMap(
                        Map.Entry::getKey, 
                        Map.Entry::getValue,
                        (a, b) -> { throw new AssertionError();},
                        LinkedHashMap::new
                    ));
        return sorted;
    }
}

其他回答

我合并了user157196和Carter Page的解决方案:

class MapUtil {

    public static <K, V extends Comparable<? super V>> Map<K, V> sortByValue( Map<K, V> map ){
        ValueComparator<K,V> bvc =  new ValueComparator<K,V>(map);
        TreeMap<K,V> sorted_map = new TreeMap<K,V>(bvc);
        sorted_map.putAll(map);
        return sorted_map;
    }

}

class ValueComparator<K, V extends Comparable<? super V>> implements Comparator<K> {

    Map<K, V> base;
    public ValueComparator(Map<K, V> base) {
        this.base = base;
    }

    public int compare(K a, K b) {
        int result = (base.get(a).compareTo(base.get(b)));
        if (result == 0) result=1;
        // returning 0 would merge keys
        return result;
    }
}

对于Java 8,您可以使用streams api以一种明显不那么冗长的方式来实现:

Map<K, V> sortedMap = map.entrySet().stream()
                         .sorted(Entry.comparingByValue())
                         .collect(Collectors.toMap(Entry::getKey, Entry::getValue, (e1, e2) -> e1, LinkedHashMap::new));

在Java 8及以上版本中对任何地图进行排序的简单方法

Map<String, Object> mapToSort = new HashMap<>();

List<Map.Entry<String, Object>> list = new LinkedList<>(mapToSort.entrySet());

Collections.sort(list, Comparator.comparing(o -> o.getValue().getAttribute()));

HashMap<String, Object> sortedMap = new LinkedHashMap<>();
for (Map.Entry<String, Object> map : list) {
   sortedMap.put(map.getKey(), map.getValue());
}

如果您使用的是Java 7及以下版本

Map<String, Object> mapToSort = new HashMap<>();

List<Map.Entry<String, Object>> list = new LinkedList<>(mapToSort.entrySet());

Collections.sort(list, new Comparator<Map.Entry<String, Object>>() {
    @Override
    public int compare(Map.Entry<String, Object> o1, Map.Entry<String, Object> o2) {
       return o1.getValue().getAttribute().compareTo(o2.getValue().getAttribute());      
    }
});

HashMap<String, Object> sortedMap = new LinkedHashMap<>();
for (Map.Entry<String, Object> map : list) {
   sortedMap.put(map.getKey(), map.getValue());
}

这种方法正好能达到目的。(“挫折”是Values必须实现java.util.Comparable接口)

  /**

 * Sort a map according to values.

 * @param <K> the key of the map.
 * @param <V> the value to sort according to.
 * @param mapToSort the map to sort.

 * @return a map sorted on the values.

 */ 
public static <K, V extends Comparable< ? super V>> Map<K, V>
sortMapByValues(final Map <K, V> mapToSort)
{
    List<Map.Entry<K, V>> entries =
        new ArrayList<Map.Entry<K, V>>(mapToSort.size());  

    entries.addAll(mapToSort.entrySet());

    Collections.sort(entries,
                     new Comparator<Map.Entry<K, V>>()
    {
        @Override
        public int compare(
               final Map.Entry<K, V> entry1,
               final Map.Entry<K, V> entry2)
        {
            return entry1.getValue().compareTo(entry2.getValue());
        }
    });      

    Map<K, V> sortedMap = new LinkedHashMap<K, V>();      

    for (Map.Entry<K, V> entry : entries)
    {
        sortedMap.put(entry.getKey(), entry.getValue());

    }      

    return sortedMap;

}

http://javawithswaranga.blogspot.com/2011/06/generic-method-to-sort-hashmap.html

    Map<String, Integer> map = new HashMap<>();
    map.put("b", 2);
    map.put("a", 1);
    map.put("d", 4);
    map.put("c", 3);
    
    // ----- Using Java 7 -------------------
    List<Map.Entry<String, Integer>> entries = new ArrayList<>(map.entrySet());
    Collections.sort(entries, (o1, o2) -> o1.getValue().compareTo(o2.getValue()));
    System.out.println(entries); // [a=1, b=2, c=3, d=4]


    // ----- Using Java 8 Stream API --------
   map.entrySet().stream().sorted(Map.Entry.comparingByValue()).forEach(System.out::println); // {a=1, b=2, c=3, d=4}