一文详细解析Java 8 Stream API中的flatMap方法

<R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);

List<List<String>> nestedList = Arrays.asList(
    Arrays.asList("apple", "banana"),
    Arrays.asList("cherry", "date"),
    Arrays.asList("elderberry", "fig", "grape")
);

List<String> flatList = nestedList.stream()
    .flatMap(List::stream)
    .collect(Collectors.toList());

System.out.println("Flat List: " + flatList);

List<String> words = Arrays.asList("apple", "banana", "cherry");

List<String> characters = words.stream()
    .flatMap(word -> Arrays.stream(word.split("")))
    .collect(Collectors.toList());

System.out.println("Characters: " + characters);

class User {
    String name;
    List<String> addresses;

    User(String name, List<String> addresses) {
        this.name = name;
        this.addresses = addresses;
    }

    public List<String> getAddresses() {
        return addresses;
    }
}

List<User> users = Arrays.asList(
    new User("Alice", Arrays.asList("123 Main St", "456 Oak St")),
    new User("Bob", Arrays.asList("789 Pine St")),
    new User("Charlie", Arrays.asList("101 Maple St", "202 Birch St", "303 Cedar St"))
);

List<String> allAddresses = users.stream()
    .flatMap(user -> user.getAddresses().stream())
    .collect(Collectors.toList());

System.out.println("All Addresses: " + allAddresses);