java List 如何进行分组
1、第一步：javalist分组的需求实现思路。
一班情况下我们遇到这种情况都是在List集合中放的是对象的情况下，我们需要将根据对象的某一个属性值或多个属性值分组。
实现方式还是比较多的，
1、借助jdk1.8新提供的Collectors.groupingBy
2、借助于for或者请它循环，当然原始for循环的执行速度是最快的。
2、第二步：jdk1.8新方法实现。
先编写实体类：Student
public class Student {
public Student(String name, String sex, String grade) {
this.name = name;
this.sex = sex;
this.grade = grade;
}
private String name;
private String sex;
// 班级
private String grade;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this.sex = sex;
}
public String getGrade() {
return grade;
}
public void setGrade(String grade) {
this.grade = grade;
}
}
在编写测试类：Test
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;
public class Test {
public static void main(String[] args) {
List<Student> list = new ArrayList<Student>();
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);
List<List<Student>> groupList = new ArrayList<List<Student>>();
list.stream().collect(Collectors.groupingBy(Student::getGrade, Collectors.toList())).forEach((grade, fooListByAge) -> {
studentList.forEach((student) -> System.out.println(student.getGrade()));
});
}
}
3、第三步：测试jdk1.8分组代码：
1、在Test类中main方法左边，工具类中左键单击，弹出运行命令选择--》选择运行。如下图所示：
2、java jdk1.8新特性Lambda表达式
3、java jdk1.8新特性Stream API
4、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<Student> list = new ArrayList<Student>();
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<String> sex = new HashSet<String>(list.stream().map(Student::getSex).collect(Collectors.toList()));
Set<String> grade = new HashSet<String>(list.stream().map(Student::getGrade).collect(Collectors.toList()));
//循环分组
Map<String, List<Student>> map = new HashMap<String, List<Student>>();
for (String string : sex) {
for (String str : grade) {
if (string == null || str == null) {
continue;
}
List<Student> sl = new ArrayList<Student>();
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<String, List<Student>> listsList1 : map.entrySet()) {
for (Student st : listsList1.getValue()) {
System.out.println(st.getSex() + " name:" + st.getName() + " " + st.getGrade());
}
System.out.println("--------------------------");
}
}
}
5、第五步：测试双属性分组。
测试步骤同第三步基本相同，测试结结果如下图所示：
