一眼看穿flatMap和map的区别

函数

功能

输出结果

List<String> names = Arrays.asList("Alice", "Bob");

List<Integer> nameLengths = names.stream()

.map(String::length) // 每个名字映射成长度

.collect(Collectors.toList());

System.out.println(nameLengths); // [5, 3]

Map<String, List<String>> hobbies = new HashMap<>();

hobbies.put("Alice", Arrays.asList("reading","swimming"));

hobbies.put("Bob", Arrays.asList("running","gaming"));

List<String> allHobbies = hobbies.values().stream()

.flatMap(List::stream) // 每个 List 扁平化成一个 Stream

.collect(Collectors.toList());

System.out.println(allHobbies); // [reading, swimming, running, gaming]

方法

输入

映射结果

最终 Stream

[元素1, 元素2]

map -> [结果1, 结果2]

flatMap -> [结果1a, 结果1b, 结果2a, 结果2b]