如何对HashMap按键值排序

川长思鸟来 2022-04-12 04:41 300阅读 0赞

Java中HashMap是一种用于存储“键”和“值”信息对的数据结构。不同于LinkedHashMap，它不会维持插入元素的顺序。因此，在键或值的基础上排序HashMap是一个很难的面试问题，如果你不知道如何解决的话。下面让我们看看如何解决这个问题。

1.创建一个简单的HashMap，并插入一些键和值。

Map<String,Integer> aMap = new HashMap<String,Integer>();
        //adding keys and values
        aMap.put("Five", 5);
        aMap.put("Seven", 7);
        aMap.put("Four", 4);
        aMap.put("Eight", 8);
        aMap.put("One", 1);
        aMap.put("Two", 2);
        aMap.put("Three", 3);

2.利用Set entrySet(): 返回Map.Entry对象的视图集，即映像中的关键字/值对

Set<Map.Entry<String,Integer>> mapEntries = aMap.entrySet();

3.从上述mapEntries创建LinkedList。我们将排序这个链表来解决顺序问题

List<Map.Entry<String,Integer>> aList = new LinkedList<Map.Entry<String,Integer>>(mapEntries);
    
        // sorting the List
        Collections.sort(aList, new Comparator<Map.Entry<String,Integer>>(){
            @Override
            public int compare(Map.Entry<String, Integer> ele1, Map.Entry<String, Integer> ele2){
                return ele1.getValue().compareTo(ele2.getValue());
            }
        });

4.Collections.sort()是一个内置方法，仅排序值的列表。它在Collections类中重载。这两种个方法是

public static <T extends Comparable<? super T>> void sort(List<T> list)
         
        public static <T> void sort(List<T> list, Comparator<? super T> c)

5.完整代码：

package cn.edu.ahui;
        import java.util.*;
         
        /*
         * @author leo
         * @data 2017.3.10
        */
         
        public class SortHashMapByValues {
             private static void sortMapByValues(Map<String, Integer> aMap){
                 Set<Map.Entry<String,Integer>> mapEntries = aMap.entrySet();
                 System.out.println("Values and Keys before sorting ");
                 for(Map.Entry<String,Integer> entry : mapEntries)
                     System.out.println(entry.getKey() + " - "+ entry.getValue());
                 //use LinkedList to sort, because insertion of elements in linked list is faster than ArrayList.
                 List<Map.Entry<String,Integer>> aList = new LinkedList<Map.Entry<String,Integer>>(mapEntries);
                 // sorting the List
                 Collections.sort(aList, new Comparator<Map.Entry<String,Integer>>(){
                     @Override
                     public int compare(Map.Entry<String, Integer> ele1, Map.Entry<String, Integer> ele2){
                         return ele1.getValue().compareTo(ele2.getValue());
                    }
                 });
                 // Storing the list into Linked HashMap to preserve the order of insertion.
                 Map<String,Integer> aMap2 = new LinkedHashMap<String, Integer>();
                 for(Map.Entry<String,Integer> entry: aList){
                     aMap2.put(entry.getKey(), entry.getValue());
                 }
                 // printing values after sorting of map
                 System.out.println("Values and Keys after sorting ");
                 for(Map.Entry<String,Integer> entry : aMap2.entrySet()){
                     System.out.println(entry.getKey() + " - " + entry.getValue());
                 }
            }
            public static void main(String[] args){
                Map<String,Integer> aMap = new HashMap<String,Integer>();
                //adding keys and values
                aMap.put("Five", 5);
                aMap.put("Seven", 7);
                aMap.put("Four", 4);
                aMap.put("Eight", 8);
                aMap.put("One", 1);
                aMap.put("Two", 2);
                aMap.put("Three", 3);
                sortMapByValues(aMap);
            }  
        }

6.运行结果：

Values and Keys before sorting  
Eight - 8  
Five - 5  
Four - 4  
One - 1  
Seven - 7  
Two - 2  
Three - 3  
Values and Keys after sorting  
One - 1  
Two - 2  
Three - 3  
Four - 4  
Five - 5  
Seven - 7  
Eight - 8