Java Stream中map与flatMap对比超详细教程

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class MapExample {
    public static void main(String[] args) {
        // 示例1：字符串转大写
        List<String> words = Arrays.asList("hello", "world");
        List<String> upperCaseWords = words.stream()
                .map(String::toUpperCase)
                .collect(Collectors.toList());
        System.out.println(upperCaseWords); // [HELLO, WORLD]

        // 示例2：提取对象属性
        class User {
            String name;
            User(String name) { this.name = name; }
            String getName() { return name; }
        }
        
        List<User> users = Arrays.asList(
            new User("Alice"),
            new User("Bob")
        );
        
        List<String> names = users.stream()
                .map(User::getName)
                .collect(Collectors.toList());
        System.out.println(names); // [Alice, Bob]
    }
}

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

public class FlatMapExample {
    public static void main(String[] args) {
        // 示例1：展开嵌套集合
        List<List<String>> nestedList = Arrays.asList(
            Arrays.asList("Java", "Python"),
            Arrays.asList("C++", "Go"),
            Arrays.asList("JavaScript", "TypeScript")
        );
        
        List<String> languages = nestedList.stream()
                .flatMap(List::stream)
                .collect(Collectors.toList());
        System.out.println(languages); 
        // [Java, Python, C++, Go, JavaScript, TypeScript]

        // 示例2：拆分字符串为单词
        List<String> sentences = Arrays.asList(
            "Hello World",
            "Java Stream API"
        );
        
        List<String> words = sentences.stream()
                .flatMap(sentence -> Arrays.stream(sentence.split(" ")))
                .collect(Collectors.toList());
        System.out.println(words); 
        // [Hello, World, Java, Stream, API]
    }
}

import java.util.*;
import java.util.stream.*;

class Student {
    String name;
    List<String> courses;
    
    Student(String name, List<String> courses) {
        this.name = name;
        this.courses = courses;
    }
    
    List<String> getCourses() { return courses; }
}

public class CompareExample {
    public static void main(String[] args) {
        List<Student> students = Arrays.asList(
            new Student("Alice", Arrays.asList("Math", "Physics")),
            new Student("Bob", Arrays.asList("History", "Art"))
        );

        // 错误用法：map处理嵌套集合
        List<List<String>> wrongResult = students.stream()
                .map(Student::getCourses)
                .collect(Collectors.toList());
        System.out.println("错误结果：" + wrongResult); 
        // [[Math, Physics], [History, Art]]

        // 正确用法：flatMap展开嵌套集合
        List<String> correctResult = students.stream()
                .flatMap(student -> student.getCourses().stream())
                .collect(Collectors.toList());
        System.out.println("正确结果：" + correctResult); 
        // [Math, Physics, History, Art]

        // 综合应用：统计所有课程数量
        long courseCount = students.stream()
                .flatMap(student -> student.getCourses().stream())
                .count();
        System.out.println("总课程数：" + courseCount); // 4
    }
}

list.stream()
    .map(...)    // 初步转换
    .flatMap(...) // 展开处理
    .collect(...)

List<List<List<String>>> deepNested = ...;
deepNested.stream()
    .flatMap(List::stream)  // 展开第一层
    .flatMap(List::stream)  // 展开第二层
    .collect(...);