java内部类并实例化问题

@Data
public class MailOperationContent {
    private String theme;
    private List<String> toMail;
    private List<String> ccMail;
    private List<String> bccMail;
    private List<String> fromMail;
    private BTMailSecretlevelEnum secretLevel;
    private Long mailSize;
    private Integer attachmentNum;
    private List<MailAttachmentInfo> mailAttachmentInfos = new ArrayList<>();

    @Data
    public class MailAttachmentInfo {
        private String attachmentName;
        private String attachmentSecretLevel;
    }

  mailAttachmentInfo = new MailOperationContent.MailAttachmentInfo();

   mailAttachmentInfo = new MailOperationContent().new MailAttachmentInfo();

 @Data
    public static class MailAttachmentInfo {
        private String attachmentName;
        private String attachmentSecretLevel;
    }