Java面试题：如何对HashMap按键值排序

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