Java中如何使用lambda表達式分類groupby

分類問題場景

比如一群學生根據性別和年齡排隊，Student類有性別gender、年齡age屬性，將同性別，同年齡的學生歸為一類

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

傳統手寫方式

使用for循環遍歷

public static void main(String[] args) {// 讀取學生信息List<Student> students = generateStudents();// 排隊Map<String, Map<Integer, List<Student>>> queue = queueUp(students);// 打印隊伍結果printQueue(queue);
}private static Map<String, Map<Integer, List<Student>>> queueUp(List<Student> students) {// 隊伍結果Map<String, Map<Integer, List<Student>>> queue = new HashMap<>();// 傳統排隊方式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;
}

輸出結果

|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()方法一行結束！！！

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

完整代碼

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) {// 讀取學生信息List<Student> students = generateStudents();// 排隊Map<String, Map<Integer, List<Student>>> queue = queueUp(students);// 打印隊伍結果printQueue(queue);queue = queueUpWithLambda(students);printQueue(queue);}private static Map<String, Map<Integer, List<Student>>> queueUp(List<Student> students) {// 隊伍結果Map<String, Map<Integer, List<Student>>> queue = new HashMap<>();// 傳統排隊方式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());}}}}}