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

map = your hashmap;

List<Map.Entry<String, Integer>> list = new LinkedList<Map.Entry<String, Integer>>(map.entrySet());
Collections.sort(list, new cm());//IMP

HashMap<String, Integer> sorted = new LinkedHashMap<String, Integer>();
for(Map.Entry<String, Integer> en: list){
    sorted.put(en.getKey(),en.getValue());
}

System.out.println(sorted);//sorted hashmap

创建新类

class cm implements Comparator<Map.Entry<String, Integer>>{
    @Override
    public int compare(Map.Entry<String, Integer> a, 
                            Map.Entry<String, Integer> b)
    {
        return (a.getValue()).compareTo(b.getValue());
    }
}

其他回答

对键进行排序需要Comparator为每个比较查找每个值。一个更具可扩展性的解决方案将直接使用entrySet,因为这样每次比较都会立即获得该值(尽管我没有用数字来支持)。

这是这样一件事的通用版本:

public static <K, V extends Comparable<? super V>> List<K> getKeysSortedByValue(Map<K, V> map) {
    final int size = map.size();
    final List<Map.Entry<K, V>> list = new ArrayList<Map.Entry<K, V>>(size);
    list.addAll(map.entrySet());
    final ValueComparator<V> cmp = new ValueComparator<V>();
    Collections.sort(list, cmp);
    final List<K> keys = new ArrayList<K>(size);
    for (int i = 0; i < size; i++) {
        keys.set(i, list.get(i).getKey());
    }
    return keys;
}

private static final class ValueComparator<V extends Comparable<? super V>>
                                     implements Comparator<Map.Entry<?, V>> {
    public int compare(Map.Entry<?, V> o1, Map.Entry<?, V> o2) {
        return o1.getValue().compareTo(o2.getValue());
    }
}

有一些方法可以减少上述解决方案的内存旋转。例如,创建的第一个ArrayList可以重新用作返回值;这将需要抑制一些泛型警告,但对于可重用的库代码来说,这可能是值得的。此外,Comparator不必在每次调用时重新分配。

这里有一个更有效但不太吸引人的版本:

public static <K, V extends Comparable<? super V>> List<K> getKeysSortedByValue2(Map<K, V> map) {
    final int size = map.size();
    final List reusedList = new ArrayList(size);
    final List<Map.Entry<K, V>> meView = reusedList;
    meView.addAll(map.entrySet());
    Collections.sort(meView, SINGLE);
    final List<K> keyView = reusedList;
    for (int i = 0; i < size; i++) {
        keyView.set(i, meView.get(i).getKey());
    }
    return keyView;
}

private static final Comparator SINGLE = new ValueComparator();

最后,如果您需要连续访问已排序的信息(而不是偶尔排序一次),可以使用额外的多重映射。如果你需要更多细节,请告诉我。。。

由于TreeMap<>不适用于可以相等的值,因此我使用了以下方法:

private <K, V extends Comparable<? super V>> List<Entry<K, V>> sort(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());
        }
    });

    return list;
}

您可能希望将列表放在LinkedHashMap中,但若您只打算立即对其进行迭代,那个么这是多余的。。。

虽然我同意对地图进行排序的持续需要可能是一种气味,但我认为以下代码是在不使用不同数据结构的情况下进行排序的最简单方法。

public class MapUtilities {

public static <K, V extends Comparable<V>> List<Entry<K, V>> sortByValue(Map<K, V> map) {
    List<Entry<K, V>> entries = new ArrayList<Entry<K, V>>(map.entrySet());
    Collections.sort(entries, new ByValue<K, V>());
    return entries;
}

private static class ByValue<K, V extends Comparable<V>> implements Comparator<Entry<K, V>> {
    public int compare(Entry<K, V> o1, Entry<K, V> o2) {
        return o1.getValue().compareTo(o2.getValue());
    }
}

}

这里有一个令人尴尬的不完整单元测试:

public class MapUtilitiesTest extends TestCase {
public void testSorting() {
    HashMap<String, Integer> map = new HashMap<String, Integer>();
    map.put("One", 1);
    map.put("Two", 2);
    map.put("Three", 3);

    List<Map.Entry<String, Integer>> sorted = MapUtilities.sortByValue(map);
    assertEquals("First", "One", sorted.get(0).getKey());
    assertEquals("Second", "Two", sorted.get(1).getKey());
    assertEquals("Third", "Three", sorted.get(2).getKey());
}

}

结果是Map.Entry对象的排序列表,您可以从中获取键和值。

基于@devinmore代码,一种使用泛型并支持升序和降序排序的map排序方法。

/**
 * Sort a map by it's keys in ascending order. 
 *  
 * @return new instance of {@link LinkedHashMap} contained sorted entries of supplied map.
 * @author Maxim Veksler
 */
public static <K, V> LinkedHashMap<K, V> sortMapByKey(final Map<K, V> map) {
    return sortMapByKey(map, SortingOrder.ASCENDING);
}

/**
 * Sort a map by it's values in ascending order.
 *  
 * @return new instance of {@link LinkedHashMap} contained sorted entries of supplied map.
 * @author Maxim Veksler
 */
public static <K, V> LinkedHashMap<K, V> sortMapByValue(final Map<K, V> map) {
    return sortMapByValue(map, SortingOrder.ASCENDING);
}

/**
 * Sort a map by it's keys.
 *  
 * @param sortingOrder {@link SortingOrder} enum specifying requested sorting order. 
 * @return new instance of {@link LinkedHashMap} contained sorted entries of supplied map.
 * @author Maxim Veksler
 */
public static <K, V> LinkedHashMap<K, V> sortMapByKey(final Map<K, V> map, final SortingOrder sortingOrder) {
    Comparator<Map.Entry<K, V>> comparator = new Comparator<Entry<K,V>>() {
        public int compare(Entry<K, V> o1, Entry<K, V> o2) {
            return comparableCompare(o1.getKey(), o2.getKey(), sortingOrder);
        }
    };

    return sortMap(map, comparator);
}

/**
 * Sort a map by it's values.
 *  
 * @param sortingOrder {@link SortingOrder} enum specifying requested sorting order. 
 * @return new instance of {@link LinkedHashMap} contained sorted entries of supplied map.
 * @author Maxim Veksler
 */
public static <K, V> LinkedHashMap<K, V> sortMapByValue(final Map<K, V> map, final SortingOrder sortingOrder) {
    Comparator<Map.Entry<K, V>> comparator = new Comparator<Entry<K,V>>() {
        public int compare(Entry<K, V> o1, Entry<K, V> o2) {
            return comparableCompare(o1.getValue(), o2.getValue(), sortingOrder);
        }
    };

    return sortMap(map, comparator);
}

@SuppressWarnings("unchecked")
private static <T> int comparableCompare(T o1, T o2, SortingOrder sortingOrder) {
    int compare = ((Comparable<T>)o1).compareTo(o2);

    switch (sortingOrder) {
    case ASCENDING:
        return compare;
    case DESCENDING:
        return (-1) * compare;
    }

    return 0;
}

/**
 * Sort a map by supplied comparator logic.
 *  
 * @return new instance of {@link LinkedHashMap} contained sorted entries of supplied map.
 * @author Maxim Veksler
 */
public static <K, V> LinkedHashMap<K, V> sortMap(final Map<K, V> map, final Comparator<Map.Entry<K, V>> comparator) {
    // Convert the map into a list of key,value pairs.
    List<Map.Entry<K, V>> mapEntries = new LinkedList<Map.Entry<K, V>>(map.entrySet());

    // Sort the converted list according to supplied comparator.
    Collections.sort(mapEntries, comparator);

    // Build a new ordered map, containing the same entries as the old map.  
    LinkedHashMap<K, V> result = new LinkedHashMap<K, V>(map.size() + (map.size() / 20));
    for(Map.Entry<K, V> entry : mapEntries) {
        // We iterate on the mapEntries list which is sorted by the comparator putting new entries into 
        // the targeted result which is a sorted map. 
        result.put(entry.getKey(), entry.getValue());
    }

    return result;
}

/**
 * Sorting order enum, specifying request result sort behavior.
 * @author Maxim Veksler
 *
 */
public static enum SortingOrder {
    /**
     * Resulting sort will be from smaller to biggest.
     */
    ASCENDING,
    /**
     * Resulting sort will be from biggest to smallest.
     */
    DESCENDING
}
public class SortedMapExample {

    public static void main(String[] args) {
        Map<String, String> map = new HashMap<String, String>();

        map.put("Cde", "C");
        map.put("Abc", "A");
        map.put("Cbc", "Z");
        map.put("Dbc", "D");
        map.put("Bcd", "B");
        map.put("sfd", "Bqw");
        map.put("DDD", "Bas");
        map.put("BGG", "Basd");

        System.out.println(sort(map, new Comparator<String>() {
            @Override
            public int compare(String o1, String o2) {
                    return o1.compareTo(o2);
            }}));
    }

    @SuppressWarnings("unchecked")
    public static <K, V> Map<K,V> sort(Map<K, V> in, Comparator<? super V> compare) {
        Map<K, V> result = new LinkedHashMap<K, V>();
        V[] array = (V[])in.values().toArray();
        for(int i=0;i<array.length;i++)
        {

        }
        Arrays.sort(array, compare);
        for (V item : array) {
            K key= (K) getKey(in, item);
            result.put(key, item);
        }
        return result;
    }

    public static <K, V>  Object getKey(Map<K, V> in,V value)
    {
       Set<K> key= in.keySet();
       Iterator<K> keyIterator=key.iterator();
       while (keyIterator.hasNext()) {
           K valueObject = (K) keyIterator.next();
           if(in.get(valueObject).equals(value))
           {
                   return valueObject;
           }
       }
       return null;
   }

}

//请在这里尝试。我正在修改值排序的代码。