背景
现在需要对一个有序的手机列表按照品牌进行分组，那么我们使用java8中的groupingBy的时候默认返回的是无序的Map，如果想输出有序的Map，需要使用三参数的groupingBy，指定返回有序的LinkedHashMap。
LinkedHashMap<String,List<Mobile>> linkedHashMap = mobileList.stream().collect(Collectors.groupingBy(Mobile::getBrand, LinkedHashMap::new,Collectors.toList()));
代码如下
package com.lingyejun.blog;
import java.util.Arrays;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;
public class MobileMain {
public static void main(String[] args) {
List<Mobile> mobileList = getMobileList();
Map<String,List<Mobile>> hashMap = mobileList.stream().collect(Collectors(Mobile::getBrand));
LinkedHashMap<String,List<Mobile>> linkedHashMap = mobileList.stream().collect(Collectors.groupingBy(Mobile::getBrand, LinkedHashMap::new,Collectors.toList())); } public static List<Mobile> getMobileList() { Mobile mobile1 = new Mobile("华为Mate40","华为",1); Mobile mobile2 = new Mobile("华为Mate30","华为",2); Mobile mobile3 = new Mobile("小米MIX7","小米",3); Mobile mobile4 = new Mobile("小米11畅玩版","小米",4); Mobile mobile5 = new Mobile("小米11青春版","小米",5); Mobile mobile6 = new Mobile("Iphone11","Iphone",6); Mobile mobile7 = new Mobile("Oppo Reno6","Oppo",7); Mobile mobile8 = new Mobile("Oppo K7x","Oppo",8); return Arrays.asList(mobile1, mobile2, mobile3, mobile4, mobile5, mobile6, mobile7, mobile8); } }
原始的list是按照sequence顺序排列的
按照常规的groupingBy分组后得到的结果是无序的
Map<String,List<Mobile>> hashMap = mobileList.stream().collect(Collectors.groupingBy(Mobile::getBrand));
使用新的方式
LinkedHashMap<String,List<Mobile>> linkedHashMap = mobileList.stream().collect(Collectors.groupingBy(Mobile::getBrand, LinkedHashMap::new,Collectors.toList()));
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