Java中map和flatMap的区别举例详解

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

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

@Data
@NoArgsConstructor
@AllArgsConstructor
@EqualsAndHashCode
public class Library {
    private String name;
    private List<Book> book;
}

@Data
@NoArgsConstructor
@AllArgsConstructor
@EqualsAndHashCode
public class Book {
    private String name;
    private String author;
    private Integer price;
}

public class StreamTest {
    public static void main(String[] args) {
        System.out.println("---------->存储的图书信息: ");
        System.out.println(initInfo());
        System.out.println("---------->测试map方法:");
        testMap();
        System.out.println("---------->测试flatMap方法:");
        testFlatMap();
    }
    private static void testMap() {
        initInfo().stream()
                .map(library -> library.getBook())
                .forEach(book -> System.out.println(book));
    }
 
    private static void testFlatMap() {
        initInfo().stream()
                .flatMap(library -> library.getBook().stream())
                .forEach(book -> System.out.println(book));
    }
 
    public static List<Library> initInfo() {
        Library library1 = new Library("新华图书", null);
        Library library2 = new Library("大家图书", null);
        Library library3 = new Library("瀚海图书", null);
 
        Book book1 = new Book("西游记", "吴承恩", 49);
        Book book2 = new Book("水浒传", "施耐庵", 57);
        Book book3 = new Book("三国演义", "罗贯中", 52);
        Book book4 = new Book("朝花夕拾", "鲁迅", 30);
 
        List<Book> library1Book = new ArrayList<>();
        List<Book> library2Book = new ArrayList<>();
        List<Book> library3Book = new ArrayList<>();
 
        library1Book.add(book1);
        library1Book.add(book2);
 
        library2Book.add(book2);
        library2Book.add(book3);
 
        library3Book.add(book3);
        library3Book.add(book4);
 
        library1.setBook(library1Book);
        library2.setBook(library2Book);
        library3.setBook(library3Book);
 
        return new ArrayList<>(Arrays.asList(library1, library2, library3));
    }
}

val orgList: List<List<List<Char>>> = listOf(
  listOf(
    listOf('a', 'b'),
    listOf('c', 'd')
  ),
  listOf(
    listOf('e', 'f'),
    listOf('g', 'h')
  )
)

val transList = orgList.flatMap { outterList ->
    outterList.flatMap { innerList ->
       innerList.map { charStr ->
           charStr.uppercaseChar()
       }
  }
}