第四步：双字段分组。

1、此处依然使用了jdk1.8的新特性(转set时)，也可以自己写两个for循环替代。

2、实体类使用的还是Student

3、设计思路就是先将要分组的两个字段分别转为set集合去重，然后借助双层for循环，尽心分组。具体实现代码如下所示：

package com.example.ttdc;

import org.springframework.util.StringUtils;

import java.util.*;

import java.util.stream.Collectors;

public class Test2 {

public static void main(String[] args) {

List list = new ArrayList();

Student student1 = new Student("李四1", "女", "一班");

Student student2 = new Student("李四2", "女", "一班");

Student student3 = new Student("李四3", "女", "一班");

Student student4 = new Student("李四4", "男", "一班");

Student student5 = new Student("李四5", "男", "一班");

Student student6 = new Student("李四6", "男", "二班");

Student student7 = new Student("李四7", "男", "二班");

Student student8 = new Student("李四8", "男", "二班");

Student student9 = new Student("李四9", "男", "二班");

list.add(student1);

list.add(student2);

list.add(student3);

list.add(student4);

list.add(student5);

list.add(student6);

list.add(student7);

list.add(student8);

list.add(student9);

//统计不重复sex

Set sex = new HashSet(list.stream().map(Student::getSex).collect(Collectors.toList()));

Set grade = new HashSet(list.stream().map(Student::getGrade).collect(Collectors.toList()));

//循环分组

Map> map = new HashMap>();

for (String string : sex) {

for (String str : grade) {

if (string == null || str == null) {

continue;

}

List sl = new ArrayList();

for (Student student : list) {

if (string.equals(student.getSex()) && str.equals(student.getGrade())) {

sl.add(student);

}

}

map.put(string + str, sl);

}

}

//查看测试结果

for (Map.Entry> listsList1 : map.entrySet()) {

for (Student st : listsList1.getValue()) {

System.out.println(st.getSex() + " name:" + st.getName() + "  " + st.getGrade());

}

System.out.println("--------------------------");

}

}

}