Java中如何使用lambda表达式分类groupby

Java中如何使用lambda表达式分类groupby

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

java 复制代码
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

shell 复制代码
|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!!!

java 复制代码
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

java 复制代码
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
    @AllArgsConstructor
    public 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());
                }
            }
        }
    }

}
相关推荐
ybq1951334543117 分钟前
Redis-主从复制-分布式系统
java·数据库·redis
weixin_472339461 小时前
高效处理大体积Excel文件的Java技术方案解析
java·开发语言·excel
小毛驴8501 小时前
Linux 后台启动java jar 程序 nohup java -jar
java·linux·jar
枯萎穿心攻击1 小时前
响应式编程入门教程第二节:构建 ObservableProperty<T> — 封装 ReactiveProperty 的高级用法
开发语言·unity·c#·游戏引擎
DKPT2 小时前
Java桥接模式实现方式与测试方法
java·笔记·学习·设计模式·桥接模式
Eiceblue3 小时前
【免费.NET方案】CSV到PDF与DataTable的快速转换
开发语言·pdf·c#·.net
好奇的菜鸟3 小时前
如何在IntelliJ IDEA中设置数据库连接全局共享
java·数据库·intellij-idea
m0_555762903 小时前
Matlab 频谱分析 (Spectral Analysis)
开发语言·matlab
程序视点4 小时前
Window 10文件拷贝总是卡很久?快来试试这款小工具,榨干硬盘速度!
windows
wuk9984 小时前
基于MATLAB编制的锂离子电池伪二维模型
linux·windows·github