【Lambda】 Map＜String,Map＜String,List＜String＞＞＞ 分组

今天业务上有一个分组的操作，如图：

要求按省份，城市分组，显示内容，第一反应想到了Java 8的Lambda表达式，下面用一个例子来说明一下：

首先定义一个类：

package org.blackholeframework.common.test;

import lombok.AllArgsConstructor;
import lombok.Data;

@Data
@AllArgsConstructor
public class TestVO {

    private String a;

    private String b;

    private String c;
}

然后Test类：

public static void main(String[] args) {
        List<TestVO> list = new ArrayList<>();
        list.add(new TestVO("A","A1","1"));
        list.add(new TestVO("A","A1","2"));
        list.add(new TestVO("A","A2","3"));
        list.add(new TestVO("A","A2","4"));
        list.add(new TestVO("A","A3","4"));
        list.add(new TestVO("B","B1","1"));
        list.add(new TestVO("B","B1","2"));
        list.add(new TestVO("B","B2","3"));
        list.add(new TestVO("B","B2","4"));
        list.add(new TestVO("B","B3","4"));
        list.add(new TestVO("C","",""));
        list.add(new TestVO("D","D1",""));
        Map<String,Map<String,List<String>>>  map = list.stream().collect(Collectors.groupingBy(TestVO::getA,Collectors.groupingBy(TestVO::getB, Collectors.mapping(TestVO::getC,Collectors.toList()))));
        System.out.println(map);
    }

直接看输出：

{A={A1=[1, 2], A2=[3, 4], A3=[4]}, B={B2=[3, 4], B3=[4], B1=[1, 2]}, C={=[]}, D={D1=[]}}

另外如果要保证顺序性，我们会想到Map集合有一个LinkedHashMap

public static void main(String[] args) {
        List<TestVO> list = new ArrayList<>();
        list.add(new TestVO("A","A1","1"));
        list.add(new TestVO("A","A1","2"));
        list.add(new TestVO("A","A2","3"));
        list.add(new TestVO("A","A2","4"));
        list.add(new TestVO("A","A3","4"));
        list.add(new TestVO("B","B1","1"));
        list.add(new TestVO("B","B1","2"));
        list.add(new TestVO("B","B2","3"));
        list.add(new TestVO("B","B2","4"));
        list.add(new TestVO("B","B3","4"));
        list.add(new TestVO("C","",""));
        list.add(new TestVO("D","D1",""));
        LinkedHashMap<String,LinkedHashMap<String,List<String>>>  map = list.stream().collect(Collectors.groupingBy(TestVO::getA,LinkedHashMap::new,  Collectors.groupingBy(TestVO::getB,  LinkedHashMap::new, Collectors.mapping(TestVO::getC,Collectors.toList()))));
        System.out.println(map);
    }

直接看输出：

{A={A1=[1, 2], A2=[3, 4], A3=[4]}, B={B1=[1, 2], B2=[3, 4], B3=[4]}, C={=[]}, D={D1=[]}}

大家可以根据以上的代码灵活运用，取其精华，以上内容只作为参考，看看你是否也遇到过这种业务场景呢