今天碰到的一个的操作， 如果是同一个作者，假设书名是一样的情况下，对应的日期修改为指定日期，但是没有弄明白，为什么可以这么操作，下面是例子:

@Data
@Builder
public class Book {
    private String name;
    private String author;
    private String day;
}

@Slf4j
public class Test {
 public static void main(String[] args) {
        Book book1 = Book.builder().name("Java").author("a").day("2020-01-01").build();
        Book book2 = Book.builder().name("Python").author("b").day("2020-06-01").build();
        Book book3 = Book.builder().name("PHP").author("c").day("2020-07-01").build();
        Book book4 = Book.builder().name("Java").author("a").day("2020-02-01").build();
        Map<String, Book> map = new HashMap<>();
        List<Book> books = Arrays.asList(book1, book2, book3, book4);
        List<Book> result = books.stream().map(x -> {
            if (map.containsKey(x.getAuthor())) {
                x.setDay("2020-05-01");
                map.get(x.getAuthor()).setDay("2020-05-01");
            } else {
                map.put(x.getAuthor(), x);
            }
            return x;
        }).collect(Collectors.toList());
        log.info(JSON.toJSONString(result));
    }

结果:
   [{"author":"a","day":"2020-05-01","name":"Java"},
    {"author":"b","day":"2020-06-01","name":"Python"}, 
    {"author":"c","day":"2020-07-01","name":"PHP"},
    {"author":"a","day":"2020-05-01","name":"Java"}]