java - 根据键和值对映射进行排序

Question

这个问题已经有答案了:

Sort a Map<Key, Value> by values (64 个回答)

已关闭 10 年前。

我开发了下面的代码，我们可以在其中对 Hash Map 进行排序基于键以及值的基础。

主要逻辑是我们可以对Map进行排序，就是HashMap或Hashtable将 key 复制到 List比排序List通过使用Collections.sort()方法，这里您可以使用 Comparator或Comparable根据您想要按自定义顺序还是自然顺序排序。一次List键已排序，我们可以创建另一个 Map，特别是 LinkedHashMap按排序顺序插入键。 LinkedHashMap将维护键插入的顺序，结果是基于键排序的 Map

我的问题是，如果您愿意，请告诉我其他更好的方法或你们可以建议的任何改进。

public class MapSortingExample {


    public static void main(String args[]) {

        //creating Hashtable for sorting
        Map<String, Integer> olympic2012 = new HashMap<String, Integer>();

        olympic2012.put("England", 3);
        olympic2012.put("USA", 1);
        olympic2012.put("China", 2);
        olympic2012.put("Russia", 4);
        //olympic2012.put("Australia", 4); //adding duplicate value

        //printing hashtable without sorting
        System.out.println("Unsorted Map in Java : " + olympic2012);

        //sorting Map e.g. HashMap, Hashtable by keys in Java
        Map<String, Integer> sorted = sortByKeys(olympic2012);
        System.out.println("Sorted Map in Java by key: " + sorted);


        //sorting Map like Hashtable and HashMap by values in Java
        sorted = sortByValues(olympic2012);
        System.out.println("Sorted Map in Java by values: " + sorted);


        //Sorting Map in Java by keys using TreeMap
        Map<String, Integer> sortedMapByKeys = new TreeMap<String,Integer>();
        sortedMapByKeys.putAll(olympic2012);
        System.out.println("Sorted Map in Java by key using TreeMap : " + sortedMapByKeys);


        //Sorting Map by keys in Java using Google Collections (Guava)
        //Main benefit is you can specify any ordering like natural or toString or arbitrary
        Map<String, Integer> sortingUsingGuava = Maps.newTreeMap(Ordering.natural());
        sortingUsingGuava.putAll(olympic2012);
        System.out.println("Example to sort Map in Java using Guava : " + sortingUsingGuava);



    }

    /*
     * Paramterized method to sort Map e.g. HashMap or Hashtable in Java
     * throw NullPointerException if Map contains null key
     */
    public static <K extends Comparable,V extends Comparable> Map<K,V> sortByKeys(Map<K,V> map){
        List<K> keys = new LinkedList<K>(map.keySet());
        Collections.sort(keys);

        //LinkedHashMap will keep the keys in the order they are inserted
        //which is currently sorted on natural ordering
        Map<K,V> sortedMap = new LinkedHashMap<K,V>();
        for(K key: keys){
            sortedMap.put(key, map.get(key));
        }

        return sortedMap;
    }

    /*
     * Java method to sort Map in Java by value e.g. HashMap or Hashtable
     * throw NullPointerException if Map contains null values
     * It also sort values even if they are duplicates
     */
    public static <K extends Comparable,V extends Comparable> Map<K,V> sortByValues(Map<K,V> map){
        List<Map.Entry<K,V>> entries = new LinkedList<Map.Entry<K,V>>(map.entrySet());

        Collections.sort(entries, new Comparator<Map.Entry<K,V>>() {

            @Override
            public int compare(Entry<K, V> o1, Entry<K, V> o2) {
                return o1.getValue().compareTo(o2.getValue());
            }
        });

        //LinkedHashMap will keep the keys in the order they are inserted
        //which is currently sorted on natural ordering
        Map<K,V> sortedMap = new LinkedHashMap<K,V>();

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

        return sortedMap;
    }

}

输出将是..

Unsorted Map in Java : {USA=1, England=3, Russia=4, China=2}
Sorted Map in Java by key: {China=2, England=3, Russia=4, USA=1}
Sorted Map in Java by values: {USA=1, China=2, England=3, Russia=4}
Sorted Map in Java by key using TreeMap : {China=2, England=3, Russia=4, USA=1}
Example to sort Map in Java using Guava : {China=2, England=3, Russia=4, USA=1}

Answer 1

更好的方法是使用 TreeMap 和给定的比较器，而不是“手动”排序并将结果放入 LinkedHashMap