n
Teacher
+ ClassID+ ClassName
－ ArrangeHomework()－ DownloadHomework()－ AddNotice()＋ ViewClassList()
User
+ UserName+ UID+ Password+ Nickname+ PreviousFile
+ Login()+ UploadFile()+ DownloadFile()+ DeleteFile()+ ModifyUserInformation()+ EnterClass()+ QuitClass()
File
+ FileSize+ FileType+ Text+ FileName+ PublisherID+ UploadTime+ ClassID+ FileIntroduction+ ValidTime+ FileName+ ValidUser
－ DeleteFile()－ GenerateFileLink（）
Student
+ Class
+ SearchNotice()－ SubmitHomework()－ CallForHelp()－ MeetDemand()－ MakeComment()＋ ViewClassList()
Class
+ ClassID+ ClassName+ UserList
－ AddUser()－ DeleteUser()
