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Java中如何使用lambda表达式分类groupby

分类问题场景

比如一群学生根据性别和年龄排队，Student类有性别gender、年龄age属性，将同性别，同年龄的学生归为一类

gender	age	name	队伍
man	12	Li Hua	队伍1
man	12	Zhang San	队伍1
man	13	Li Si	队伍2
woman	12	Xiao Hua	队伍3
woman	12	Xiao Fang	队伍3

传统手写方式

使用for循环遍历

public static void main(String[] args) {// 读取学生信息List<Student> students = generateStudents();// 排队Map<String, Map<Integer, List<Student>>> queue = queueUp(students);// 打印队伍结果printQueue(queue);
}private static Map<String, Map<Integer, List<Student>>> queueUp(List<Student> students) {// 队伍结果Map<String, Map<Integer, List<Student>>> queue = new HashMap<>();// 传统排队方式for (Student student : students) {if (!queue.containsKey(student.getGender())) {queue.put(student.getGender(), new HashMap<>());}Map<Integer, List<Student>> ageMap = queue.get(student.getGender());if (!ageMap.containsKey(student.getAge())) {List<Student> list = new ArrayList<>();list.add(student);ageMap.put(student.getAge(), list);} else {ageMap.get(student.getAge()).add(student);}}return queue;
}

输出结果

|gender|age|name      |
|------|---|----------|
|woman |12 |Xiao Hua  |
|woman |12 |Xiao Fang |
|man   |12 |Li Hua    |
|man   |12 |Zhang San |
|man   |13 |Li Si     |

lambda使用groupBy()方法一行结束！！！

private static Map<String, Map<Integer, List<Student>>> queueUpWithLambda(List<Student> students) {return students.stream().collect(Collectors.groupingBy(Student::getGender, Collectors.groupingBy(Student::getAge)));
}

完整代码

import lombok.AllArgsConstructor;
import lombok.Getter;
import lombok.NoArgsConstructor;
import lombok.Setter;import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
import java.util.stream.Collectors;public class DemoForGroupBy {@Getter@Setter@NoArgsConstructor@AllArgsConstructorpublic static class Student {private String gender;private int age;private String name;}public static void main(String[] args) {// 读取学生信息List<Student> students = generateStudents();// 排队Map<String, Map<Integer, List<Student>>> queue = queueUp(students);// 打印队伍结果printQueue(queue);queue = queueUpWithLambda(students);printQueue(queue);}private static Map<String, Map<Integer, List<Student>>> queueUp(List<Student> students) {// 队伍结果Map<String, Map<Integer, List<Student>>> queue = new HashMap<>();// 传统排队方式for (Student student : students) {if (!queue.containsKey(student.getGender())) {queue.put(student.getGender(), new HashMap<>());}Map<Integer, List<Student>> ageMap = queue.get(student.getGender());if (!ageMap.containsKey(student.getAge())) {List<Student> list = new ArrayList<>();list.add(student);ageMap.put(student.getAge(), list);} else {ageMap.get(student.getAge()).add(student);}}return queue;}private static Map<String, Map<Integer, List<Student>>> queueUpWithLambda(List<Student> students) {return students.stream().collect(Collectors.groupingBy(Student::getGender, Collectors.groupingBy(Student::getAge)));}private static List<Student> generateStudents() {List<Student> students = new ArrayList<>();String[] genderArray = new String[]{"man", "man", "man", "woman", "woman"};int[] ageArray = new int[]{12, 12, 13, 12, 12};String[] nameArray = new String[]{"Li Hua", "Zhang San", "Li Si", "Xiao Hua", "Xiao Fang"};for (int i = 0; i < nameArray.length; i++) {Student student = new Student(genderArray[i], ageArray[i], nameArray[i]);students.add(student);}return students;}private static void printQueue(Map<String, Map<Integer, List<Student>>> queue) {System.out.printf("|%-6s|%-3s|%-10s|%n", "gender", "age", "name");System.out.printf("|%-6s|%-3s|%-10s|%n", "------", "---", "----------");for (Map.Entry<String, Map<Integer, List<Student>>> entry : queue.entrySet()) {for (Map.Entry<Integer, List<Student>> e : entry.getValue().entrySet()) {for (Student student : e.getValue()) {System.out.printf("|%-6s|%-3d|%-10s|%n", student.getGender(), student.getAge(), student.getName());}}}}}