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我正在尝试对包含嵌套键列表的对象列表进行分组，数据结构如下所示：


class BookInformation {
     String accountId;
     String bookName;
     String bookId;
     List<Info> infos;
}

class Info {
    String type;
    String detail;
}

class GroupedInformation {
     String accountId;
     Info info;
     List<String> bookNames;
}

所以说如果输入是 BookInformation 列表，并按帐户和信息分组在一起，输出是 GroupedInformation 列表，我正在尝试使用

List<GroupedInformation> groupBy(List<BookInformation> toGroup) {
    return toGroup.stream.collect(groupingBy(BookInformation::getAccountId), Collectors.??);
}

虽然不确定如何扁平化 List<List> 并按信息再次分组以获得 bookName 的列表，但不引入一些新的数据结构，如 Map<Info, List> 等。有什么想法吗？

为此添加了一个测试：

void test() {
        List<BookInformation> toGroup = List.of(
                new BookInformation("account1", "bookName1", "book1", List.of(new Info("info1", ""), new Info("info2", ""))),
                new BookInformation("account1","bookName2", "book2", List.of(new Info("info2", ""), new Info("info3", ""))),
                new BookInformation("account2","bookName3", "book3", List.of(new Info("info3", ""), new Info("info4", "")))
        );

        List<GroupedInformation> grouped = groupBy(toGroup);

        assertThat(grouped).containsExactlyInAnyOrder(
                new GroupedInformation("account1", new Info("info1",""), List.of("bookName1")),
                new GroupedInformation("account1", new Info("info2",""), List.of("bookName1", "bookName2")),
                new GroupedInformation("account1", new Info("info3",""), List.of("bookName2")),
                new GroupedInformation("account2", new Info("info3",""), List.of("bookName3")),
                new GroupedInformation("account2", new Info("info4",""), List.of("bookName3"))
        );
                
                
    }
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