java中Map按值排序

在许多场合下，经常需要对Map进行排序，排序的依据是Map中的值。简单的例子比如根据

Map<Integer,Integer>

代表学生分数，想要根据分数从高到低排列该Map。更复杂的例子中，

Map<String,List<Flight>>

代表一组共享航班，key为共享航班号，value为共享该航班号的所有航班，其中执飞航班位于第一个元素，我们现在想根据执飞航班的时间排序。

解决方案如下：

private LinkedHashMap<String, List<Flight>> sortHashMapByValues(
Map<String, List<Flight>> passedMap , Comparator<List<Flight>> comparator) {
List<String> mapKeys = new ArrayList<>(passedMap.keySet());
List<List<Flight>> mapValues = new ArrayList<>(
passedMap.values());
Collections.sort(mapValues, comparator);
Collections.sort(mapKeys);

LinkedHashMap<String, List<Flight>> sortedMap = new LinkedHashMap<>();

Iterator<List<Flight>> valueIt = mapValues.iterator();
while (valueIt.hasNext()) {
List<Flight> val = valueIt.next();
Iterator<String> keyIt = mapKeys.iterator();

while (keyIt.hasNext()) {
String key = keyIt.next();
List<Flight> comp1 = passedMap.get(key);
List<Flight> comp2 = val;

if (comp1.equals(comp2)) {
keyIt.remove();
sortedMap.put(key, val);
break;
}
}
}
return sortedMap;
}

Comparator的一个实现如下：

public class RTFlightArrivalComparator implements
Comparator<List<Flight>> {

@Override
public int compare(List<Flight> o1,
List<Flight> o2) {

Flight first1 = o1.get(0);
Flight first2 = o2.get(0);

return first1.getSta().compareTo(first2.getSta());
}

}

LinkedHashMap维护的是一个双向链表，其顺序通过与插入顺序相关（Insertion-Order），这种方案效率比较低。 下面是使用TreeMap来实现排序的一个例子。TreeMap会对其key进行排序，可以接受自定义的比较器Comparator：

import java.util.Comparator;
import java.util.HashMap;
import java.util.TreeMap;

public class SortMapByValue {

public static void main(String[] args) {
HashMap<String, Integer> map = new HashMap<String, Integer>();
map.put("a", 10);
map.put("b", 30);
map.put("c", 50);
map.put("d", 40);
map.put("e", 20);
System.out.println(map);

TreeMap<String, Integer> sortedMap = sortMapByValue(map);
System.out.println(sortedMap);
}

public static TreeMap<String, Integer> sortMapByValue(HashMap<String, Integer> map){
Comparator<String> comparator = new ValueComparator(map);
//TreeMap is a map sorted by its keys.
//The comparator is used to sort the TreeMap by keys.
TreeMap<String, Integer> result = new TreeMap<String, Integer>(comparator);
result.putAll(map);
return result;
}
}

在实现key的比较器时，因为我们想要根据值排序，所以其顺序依赖于值，因此在比较器构造的时候，需要传入待排序的Map：

// a comparator that compares Strings
class ValueComparator implements Comparator<String>{

HashMap<String, Integer> map = new HashMap<String, Integer>();

public ValueComparator(HashMap<String, Integer> map){
this.map.putAll(map);
}

@Override
public int compare(String s1, String s2) {
if(map.get(s1) >= map.get(s2)){
return -1;
}else{
return 1;
}
}
}

上述代码只能处理特定类型的Map，下面使用泛型来实现更通用的方法：

import java.util.Comparator;
import java.util.HashMap;
import java.util.Map;
import java.util.TreeMap;

public class Solution {
public static void main(String[] args) {
HashMap<String, Integer> map = new HashMap<String, Integer>();
map.put("a", 10);
map.put("b", 30);
map.put("c", 50);
map.put("d", 40);
map.put("e", 20);
System.out.println(map);

Map sortedMap = sortByValue(map);
System.out.println(sortedMap);
}

public static Map sortByValue(Map unsortedMap) {
Map sortedMap = new TreeMap(new ValueComparator(unsortedMap));
sortedMap.putAll(unsortedMap);
return sortedMap;
}

}

class ValueComparator implements Comparator {
Map map;

public ValueComparator(Map map) {
this.map = map;
}

public int compare(Object keyA, Object keyB) {
Comparable valueA = (Comparable) map.get(keyA);
Comparable valueB = (Comparable) map.get(keyB);
return valueB.compareTo(valueA);
}
}

// a comparator using generic type
class ValueComparator<K, V extends Comparable<V>> implements Comparator<K>{

HashMap<K, V> map = new HashMap<K, V>();

public ValueComparator(HashMap<K, V> map){
this.map.putAll(map);
}

@Override
public int compare(K s1, K s2) {
return -map.get(s1).compareTo(map.get(s2));//descending order
}
}

public class SortMapByValue {

public static void main(String[] args) {
// <String, Integer> Map
HashMap<String, Integer> map = new HashMap<String, Integer>();
map.put("a", 10);
map.put("b", 30);
map.put("c", 50);
map.put("d", 40);
map.put("e", 20);
System.out.println(map);

Comparator<String> comparator = new ValueComparator<String, Integer>(map);
TreeMap<String, Integer> result = new TreeMap<String, Integer>(comparator);
result.putAll(map);

System.out.println(result);

// <Integer, Integer> Map

HashMap<Integer, Integer> map2 = new HashMap<Integer, Integer>();
map2.put(1, 10);
map2.put(2, 30);
map2.put(3, 50);
map2.put(4, 40);
map2.put(5, 20);
System.out.println(map2);

Comparator<Integer> comparator2 = new ValueComparator<Integer, Integer>(map2);
TreeMap<Integer, Integer> result2 = new TreeMap<Integer, Integer>(comparator2);
result2.putAll(map2);

System.out.println(result2);

}

}

（完）