2021-11-04

• Collectors.groupingBy()根据某个字段分组转成list
• 输出结果

`package java8;

import Dto.Student;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class GroupByDemo {
public static void main(String[] args) {
Student student = new Student();
student.setCode(“123”);
student.setName(“xiongchao”);
student.setAge(27);

 Student student1 = new Student();
 student1.setCode("123");
 student1.setName("zhangsan");
 student1.setAge(28);

 Student student2 = new Student();
 student2.setCode("124");
 student2.setName("xiongchao2");
 student2.setAge(29);

 Student student3 = new Student();
 student3.setCode("125");
 student3.setName("xiongchao3");
 student3.setAge(30);

 Student student4 = new Student();
 student4.setCode("125");
 student4.setName("xiongchao3");
 student4.setAge(31);

 List list = new ArrayList(){
     {
         add(student);
         add(student1);
         add(student2);
         add(student3);
         add(student4);
     }
 };

 Map> map = list.stream().collect(Collectors.groupingBy(
         Student::getCode, HashMap::new,Collectors.toList()));

 for(Map.Entry> entry:map.entrySet()){
     System.out.println(entry.getKey());
     System.out.println(entry.getValue());
 }

}
}
`

123
[Student{name=‘xiongchao’, age=27, code=‘123’}, Student{name=‘zhangsan’, age=28, code=‘123’}]
124
[Student{name=‘xiongchao2’, age=29, code=‘124’}]
125
[Student{name=‘xiongchao3’, age=30, code=‘125’}, Student{name=‘xiongchao3’, age=31, code=‘125’}]