Java ArrayList学生登录编译错误

我的代码有问题。我正在尝试创建一个学生日志，如果输入正确，将允许用户访问memberEnter（）部分，如果输入错误，系统将退出

我一直在尝试编译代码，但它给了我一个错误。有人能帮我修复这个问题吗

我得到的错误是：

cannot find symbol - variable studentidlogin 在

MainSystem

的

main

方法中（也在代码本身中标记）

主要系统代码：

import java.io.*;//imports the io package
import java.util.Scanner;//imports scanner
import java.util.*;//imports the util package

public class MainSystem {

    static String fileName = null;
    static Library lib = new Library();
    static Scanner in = new Scanner(System.in);
    static Boolean running = true;
    static Member mem = new Member();//static variables that can be initialised at compile time, but can be modified at run time

    public static void main(String[] args) throws IOException {// there has to be an input/output
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));//allows the user to type in the script.
        String user, password;
        while (running) {
            System.out.println("\nEnter 0 To login as a Librarian"
                + "\nEnter 1 to Login"
                + "\nEnter 2 to Sign Up");//first display screen in the interface.

            int answer = in.nextInt();
            switch (answer) {
                case 0:

                System.out.println("\nEnter Username: ");
                user = br.readLine();

                System.out.println("\nEnter Password: ");
                password = br.readLine();//if '0' is selected this is displayed.

                if (user.matches("enter") && (password.matches("Password")))// if 0 is entered this has to be entered.
                {
                    librairanEnter();
                    break;
                }
                else
                {
                    System.exit(0);
                }  

                case 1:
                String studentidlogin;
                System.out.println("\nEnter Student ID: ");
                studentidlogin = br.readLine();//if '1' is pressed this is displayed           
                System.out.println(mem.studentidlogin(studentidlogin));

                /************* ERROR OCCURS HERE **************/
                if(mem.studentidlogin = ("\n Not a member "))
                {
                    System.exit(0);
                }else
                {
                    memberEnter();
                    break;
                }

                case 2:
                newMember();//if '2' is selected this is displayed.
                break;

            }
        }

    }

    private static void newMember() throws IOException
    {
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));

        int libraryNumber;
        String StudentID, Username, FullName, Address, email, PhoneNumber;

        System.out.println("\nEnter Library Number: ");
        libraryNumber= Integer.parseInt(br.readLine());;//prompts user to enter library number.

        System.out.println("\nEnter Student ID: ");//prompts user to enter student id.
        StudentID = br.readLine();

        System.out.println("\nEnter Full Name: ");//prompts user to enter full name
        FullName = br.readLine();

        System.out.println("\nEnter Username: ");//prompts user to enter username
        Username = br.readLine();

        System.out.println("\nEnter E-Mail Address: ");//prompts user to enter an email address
        email = br.readLine();

        System.out.println("\nEnter Home Address: ");//prompts user to enter home address
        Address = br.readLine();

        System.out.println("\nEnter Phone Number: ");//prompts user to enter phone number
        PhoneNumber = br.readLine();

        StudentSignUp b = new StudentSignUp(libraryNumber, FullName, StudentID, Username, Address, email, PhoneNumber);
        mem.newMember(b);

        System.out.println("\nThankyou. You can now Login.: ");// prompts user that they can now sign in.
    }

    private static void memberEnter() throws IOException {
        while (running) {
            System.out.println("\nEnter 0 for load a library."
                + "\nEnter 1 for save and quit"
                + "\nEnter 2 for list all books in library"
                + "\nEnter 3 for Search For A Book");//options available if member is enters.

            int answer = in.nextInt();
            switch (answer) {
                case 0:
                System.out.println("Enter the file name to load");
                loadScript(in.next());//loads previously saved script
                break;
                case 1:
                saveAndQuit();//saves and quits from the system
                break;
                case 2:
                System.out.println(lib.toString());//lists all the books in the library
                break;
                case 3:
                searchBook();//searches for a book
                break;

            }
        }
        System.exit(0);
    }

    private static void librairanEnter() throws IOException {
        while (running) {
            System.out.println("\nEnter 0 for load a library."
                + "\nEnter 1 for save and quit"
                + "\nEnter 2 for list all books in library"
                + "\nEnter 3 for add book to library"
                + "\nEnter 4 for Search For A Book"
                + "\nEnter 5 for list of current members"
                + "\nEnter 6 to borrow a book"
                + "\nEnter 7 to return a book");//options available if the librarian logs in.

            int answer = in.nextInt();
            switch (answer) {
                case 0:
                System.out.println("Enter the file name to load");
                loadScript(in.next());//loads previously saved libraries
                break;
                case 1:
                saveAndQuit();//saves and quits
                break;
                case 2:
                System.out.println(lib.toString());//prints current books in library
                break;
                case 3:
                addBook();//adds a book to the system
                break;
                case 4:
                searchBook();//searches for a book in the system
                case 5:
                System.out.println(mem.toString());//list of current members
                break;
                case 6:
                borrowBook();//allows to borrow a book.
                case 7:
                returnBook();//allows to return a book.

            }
        }
        System.exit(0);//exits from the system/
    }

    private static void addBook() throws IOException
    {
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        int isbn, numcopies;
        String author, title, genre;// initializes the variables

        System.out.println("\nEnter ISBN: ");
        isbn = Integer.parseInt(br.readLine());//prompts user to enter isbn

        System.out.println("\nEnter Author: ");//prompts user to enter the auth
        author = br.readLine();

        System.out.println("\nEnter Title: ");//prompts user to enter title
        title = br.readLine();

        System.out.println("\nEnter Genre: ");//prompts user to enter genre
        genre = br.readLine();

        System.out.println("\nEnter Number Of Copies: ");//prompts user to enter number of copies
        numcopies = Integer.parseInt(br.readLine());

        Book b = new Book(isbn, author, title, genre, numcopies);//Creates the new book and adds it to the library
        lib.addBook(b);
    }

    private static void saveAndQuit() {//saves and quits
        // TODO Auto-generated method stub
        System.out.println("Enter file name: ");//prompts the user to enter the file name.
        fileName = in.next() + ".ser";//saves the file name
        running = false;
        FileOutputStream fos = null;
        ObjectOutputStream out = null;
        try {
            fos = new FileOutputStream(fileName);
            out = new ObjectOutputStream(fos);
            out.writeObject(lib);
            fos.close();
            out.close();
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }

    private static void loadScript(String name) {//loads a previously saved script
        // TODO Auto-generated method stub
        FileInputStream fis = null;
        ObjectInputStream in = null;
        File file = new File(name + ".ser");
        if (file.exists()) {
            try {
                fis = new FileInputStream(file);
                in = new ObjectInputStream(fis);
                lib = (Library) in.readObject();
                fis.close();
                in.close();

            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (ClassNotFoundException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

        } else {
            System.out.println("\nThe file does not exist!");
        }
    }

    private static void searchBook() throws IOException//allows searching for a book
    {
        String titleSearch;
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        while(running) {
            System.out.println("\nEnter 0 to search on title."////prompts the user to answer 0
                + "\nEnter 3 to go back");//promts the user to enter 3

            int answer = in.nextInt();
            switch (answer) {
                case 0:
                System.out.println("Enter the book Title");//promts the user to enter the title of the book
                titleSearch = br.readLine();
                System.out.println(lib.searchTitle( titleSearch));

                break;
                case 3:
                librairanEnter();//takes the user back to the librarian home page.

            }
        }
    }

    private static void borrowBook() throws IOException
    {
        String titleBorrow;
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        while(running) {
            System.out.println("\nEnter 0 to search on title."//prompts the user to answer 0
                + "\nEnter 3 to go back");//prompt the user to answer 3

            int answer = in.nextInt();
            switch (answer) {
                case 0:
                System.out.println("Enter the book Title");//promts the user ot enter the book title
                titleBorrow = br.readLine();
                System.out.println(lib.borrowBook( titleBorrow));

                break;
                case 3:
                librairanEnter();//takes the user back to the librarian home page.

            }
        }
    }

    private static void returnBook() throws IOException
    {
        String returnedBook;
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        while(running) {
            System.out.println("\nEnter 0 to retrun a book."//promts the user to enter 0
                + "\nEnter 3 to go back");//prompts the user to enter 3

            int answer = in.nextInt();
            switch (answer) {
                case 0:
                System.out.println("Enter the book Title");//promts the user to enter the book title.
                returnedBook = br.readLine();
                System.out.println(lib.returnBook( returnedBook));

                break;
                case 3:
                librairanEnter();//takes the user back to the librarian home page.

            }
        }
    }

}

成员类别：

import java.util.*;
import java.io.*;
/**
 * Write a description of class Member here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */

public class Member
{
   private List<StudentSignUp> memberList;//creates the array list


    public Member()
    {
        memberList = new ArrayList<StudentSignUp>(); //makes the 'memberList' the array list. 
    }
    public void newMember(StudentSignUp student)
    {
        memberList.add(student);//adds a student to the array list.
    }
    public String toString()
    {
        String totalmem = "\n ";
        for (int i=0; i<memberList.size(); i++)
        {
            StudentSignUp b = memberList.get(i);
            totalmem = totalmem + b.toString(); //prints out all students
        }

        return totalmem;
    }

        public String studentidlogin(String studentidlogin) {
        if (studentidlogin == null) return "\n Not a member ";
        for(int i = 0; i < memberList.size(); i++){
            if(studentidlogin.equalsIgnoreCase(memberList.get(i).getStudentID())){
                return memberList.get(i).toString();//allows user to search for a book in the system
            }
        }
        return "\n Not a member "; //reachable only if no book found
    }





}

import java.util.*；
导入java.io.*；
/**
*在这里写一个类成员的描述。
* 
*@author（你的名字）
*@version（版本号或日期）
*/
公共班级成员
{
私有列表memberList；//创建数组列表
公职人员（）
{
memberList=new ArrayList（）；//使“memberList”成为数组列表。
}
公共无效新成员（学生注册学生）
{
memberList.add（student）；//将学生添加到数组列表中。
}
公共字符串toString（）
{
字符串totalmem=“\n”；
对于（int i=0；i

对于，您在if语句中执行了mem.studentidlogin而不是studentidlogin：

public static void main(String[] args) throws IOException {// there has to be an input/output
    BufferedReader br=new BufferedReader(new InputStreamReader(System.in));//allows the user to type in the script.
    String user, password;
    while (running) {
        System.out.println("\nEnter 0 To login as a Librarian"
            + "\nEnter 1 to Login"
            + "\nEnter 2 to Sign Up");//first display screen in the interface.

        int answer = in.nextInt();
        switch (answer) {
            case 0:

            System.out.println("\nEnter Username: ");
            user = br.readLine();

            System.out.println("\nEnter Password: ");
            password = br.readLine();//if '0' is selected this is displayed.

            if (user.matches("enter") && (password.matches("Password")))// if 0 is entered this has to be entered.
            {
                librairanEnter();
                break;
            }
            else
            {
                System.exit(0);
            }  

            case 1:
            String studentidlogin;
            System.out.println("\nEnter Student ID: ");
            studentidlogin = br.readLine();//if '1' is pressed this is displayed           
            System.out.println(mem.studentidlogin(studentidlogin));

            if(studentidlogin.equals("\n Not a member "))
            {
                System.exit(0);
            }else
            {
                memberEnter();
                break;
            }

            case 2:
            newMember();//if '2' is selected this is displayed.
            break;

        }
    }

}

你的职能

 public String studentidlogin(String studentidlogin)

接受一个字符串参数，但您是这样调用它的：

mem.studentidlogin = ("\n Not a member ")

编辑：
只是为了扩展我的答案。你还有一些问题要解决。也许阅读java基础知识会有所帮助

e、 g

要比较两个字符串并使用赋值运算符（=），
但在你的情况下，你需要equals方法

String str = "anything";
if (str.equals("anything")) {
//do something
}

我想您的意思是使用==运算符，但这只比较了两个内存点。
您的代码：

        System.out.println(mem.studentidlogin(studentidlogin));

        if(mem.studentidlogin = ("\n Not a member "))

println（）
调用中的
mem.studentidlogin
的第一次使用是正确的。由于
studentidlogin
是
成员类的方法，因此应将其作为方法调用，并带有与其参数匹配的括号和参数

第二种用法是不正确的。您将其视为一个字段。我想您可能期望这是从上一次调用返回的结果，但如果是这样，Java就不能这样工作

此外，我相信您正在尝试将其与字符串
“\n非成员”
进行比较。只能使用
equals（）
或
equalsIgnoreCase（）
对字符串进行比较

从代码的角度看，您似乎在尝试为一个字段（不存在）赋值，该字段位于只需要布尔值的
if
语句中（比较将是
=
，正如我所说，您无论如何都不应该使用它，但您使用的是赋值
=
）

您有两种选择。请正确地再次调用：

        System.out.println(mem.studentidlogin(studentidlogin));

        if(mem.studentidlogin().equals("\n Not a member "))

或者，为了避免所有工作重复两次，最好定义一个变量：

        String loginResult = mem.studentidlogin(studentidlogin);
        System.out.println(loginResult);

        if( loginResult.equals("\n Not a member "))

错误是什么？OO找不到symbol-变量studentidlogin表示错误，但永远不要使用“+”表示字符串。请使用StringBuilder。如果（mem.studentidlogin=（“\n非成员”））在更改该错误时出现另一个错误，则在主系统部件中找不到symbol-变量studentidlogin代码行：“不兼容的类型：java.lang.string无法转换为布尔值”
        System.out.println(mem.studentidlogin(studentidlogin));

        if(mem.studentidlogin = ("\n Not a member "))

        System.out.println(mem.studentidlogin(studentidlogin));

        if(mem.studentidlogin().equals("\n Not a member "))

        String loginResult = mem.studentidlogin(studentidlogin);
        System.out.println(loginResult);

        if( loginResult.equals("\n Not a member "))