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如何通过key或者value值对map排序_Java指南

如何通过key或者value值对map排序？这是在Stackoverflow网站问的比较普遍的一个问题。在这里我们将给你一个处理方案。

顺序不能保证

最常用的map类型是HashMap,官方的api明确指出“makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time”意思是插入数据的顺序随着时间的推移并不是一直不变的。

LinkedHashMap 维持的插入的顺序

当你使用EntrySet集合存储数据时候，它通常使用LinkedHashMap来保证顺利一致性，官方文档有一句话“it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is normally the order in which keys were inserted into the map (insertion-order)” ，但是仍旧没有提供方法或者处理方式来对整个实体进行排序。

TreeMap 是通过key排序

TreeMap默认是通过key对插入的entry进行排序，官方api指出“The map is sorted according to the natural ordering of its keys, or by a Comparator provided at map creation tim” 然而为了解决通过value来排序，我们需要找一个办法

TreeMap有一个构造器接受Comparator，它能够通过key来对map排序
方法就是让comparator能够访问到values代替key
因此，我们通过map的方法 usingmap.get(key)得到values值，然后传入comparator
注意value必须实现Comparable类型，values对象必须实现Comparable接口，这里例子里面的String属于是Comparable类型

package com.collections.one; import java.util.Map; import java.util.TreeMap; public class MapSort { public static Map sortByValue(Map unsortedMap){ Map sortedMap = new TreeMap(new ValueComparator(unsortedMap)); sortedMap.putAll(unsortedMap); return sortedMap; } public static Map sortByKey(Map unsortedMap){ Map sortedMap = new TreeMap(); sortedMap.putAll(unsortedMap); return sortedMap; } }

package com.collections.one; import java.util.Comparator; import java.util.Map; public 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); System.out.println(valueA +" - "+valueB); return valueA.compareTo(valueB); } }

package com.collections.one; import java.util.HashMap; import java.util.Map; public class Test { public static void main(String[] args){ Map map = new HashMap(); //*value Class should implements the Comparable interface //*String implements Comparable by default. map.put("Z", "3"); map.put("D", "4"); map.put("A", "1"); map.put("B", "2"); map.put("F", "6"); map.put("E", "5"); System.out.println("Unsorted Map: "+map); System.out.println("Sorted Map By Values: "+MapSort.sortByValue(map)); System.out.println("Sorted Map By Keys: "+MapSort.sortByKey(map)); } }

输出结果：

Unsorted Map: {D=4, E=5, F=6, A=1, B=2, Z=3} 5 - 4 6 - 4 6 - 5 1 - 5 1 - 4 2 - 5 2 - 4 2 - 1 3 - 5 3 - 2 3 - 4 Sorted Map By Values: {A=1, B=2, Z=3, D=4, E=5, F=6} Sorted Map By Keys: {A=1, B=2, D=4, E=5, F=6, Z=3}

源码下载：https://github.com/Harries/JAVA-TUTORIAL/tree/master/src/com/collections/one

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