一個掃描儀對象沒有檢測到寫入程序中的任何文件

Question

我編寫了一個程序，讓用戶不知道，除非他們知道用戶名和密碼。這只是我爲了好玩而編寫的一個程序，並沒有真正的目的。但是，在測試代碼時，我發現在嘗試從最近寫入的文件中讀取時遇到了問題。無論出於何種原因，我爲username.txt文件創建的掃描程序對象userOut並未檢測到它們是文件中的任何內容（即.hasNextLine（）返回false），但如果在程序中寫入則只有。如果我寫一些文本到文件，然後運行該程序，它工作得很好。請注意，password.txt文件完全正常。這是爲什麼發生？

這裏是我的代碼：

import java.io.File; 
import java.io.FileOutputStream; 
import java.io.PrintWriter; 
import java.util.Scanner; 

public class Challenge5 { 

public static void main(String[] args) { 
    try { 
     File usernameFile = new File("username.txt"); 
     File passwordFile = new File("password.txt"); 

     PrintWriter userOut = new PrintWriter(new FileOutputStream(usernameFile, true)); 
     PrintWriter passOut = new PrintWriter(new FileOutputStream(passwordFile, true)); 

     Scanner in = new Scanner(System.in); 

     Scanner userIn = new Scanner(usernameFile, "UTF-8"); 
     Scanner passIn = new Scanner(passwordFile, "UTF-8"); 

     System.out.println("Welcome to Luke's secure program."); 
     if (!userIn.hasNextLine() || !userIn.hasNextLine()) { 
      System.out.println("New users must set up their account: "); 
      if (!userIn.hasNextLine()) { 
       System.out.print("Enter a username: "); 
       String username = in.nextLine().trim(); 
       userOut.print(username); 
       userOut.flush(); 

      } 
      if (!userIn.hasNextLine()) { 
       System.out.print("Enter a password: "); 
       String password = in.nextLine().trim(); 
       passOut.print(password); 
       passOut.flush(); 

      } 
      System.out.println("Your account has been created. You may now log in:"); 
     } 

     System.out.print("Please enter username: "); 
     String username = in.nextLine().trim(); 

     System.out.print("Please enter password: "); 
     String password = in.nextLine().trim(); 

     String fileUsername = "";   
     while (userIn.hasNextLine()) { 
      fileUsername = userIn.nextLine(); 
     } 
     System.out.println(fileUsername); 

     String filePassword = ""; 
     while (passIn.hasNextLine()) { 
      filePassword = passIn.nextLine(); 
     } 
     System.out.println(filePassword); 

     if (username.equals(fileUsername) && password.equals(filePassword)) { 
      System.out.println("Welcome!"); 


      while (true) { 
       System.out.println("What would you like to do? "); 
       System.out.println("1. Change username\n2. Change password\n3. Exit"); 
       System.out.print("> "); 

       int choice; 
       while (!in.hasNextInt()) { 
        System.out.println("What would you like to do? "); 
        System.out.println("1. Change username\n2. Change password\n3. Exit"); 
        System.out.print("> "); 
        in.next(); 
       } 
       choice = in.nextInt(); 
       in.nextLine(); 

       if (choice == 1) { 
        userOut = new PrintWriter(new FileOutputStream(usernameFile, false)); 
        System.out.print("Enter new username: "); 
        String newUsername = in.nextLine().trim(); 
        userOut.print(newUsername); 
        userOut.flush(); 
       } else if (choice == 2) { 
        passOut = new PrintWriter(new FileOutputStream(passwordFile, false));; 
        System.out.print("Enter new password: "); 
        String newPassword = in.nextLine().trim(); 
        passOut.print(newPassword); 
        passOut.flush(); 
       } else if (choice == 3) { 
        System.out.println("Goodbye!"); 
        break; 
       } else { 
        System.out.println("Please try again."); 
       } 
      } 
     } else { 
      System.out.println("INVALID USERNAME OR PASSWORD, EXITING PROGRAM..."); 
     } 

     userOut.close(); 
     passOut.close(); 
     in.close(); 
     userIn.close(); 
     passIn.close(); 

    } catch (Exception ex) { 
     ex.printStackTrace(); 
    } 
} 
} 

我承認我的代碼是不完美的，但我認爲它應該最起碼的工作。任何幫助，將不勝感激。

Answer 1

您需要重新初始化掃描儀對象以打開輸入流以更新文件內容。

System.out.print("Please enter password: "); 
String password = in.nextLine().trim(); 

以上後僅僅兩行，你需要以下條件：

userIn = new Scanner(usernameFile, "UTF-8"); 
passIn = new Scanner(passwordFile, "UTF-8"); 

您無法讀取什麼，是因爲你打開的輸入流連接使用userIn文件的用戶名和密碼的原因和passIn時，有沒有任何文件。添加上面兩行打開新的輸入流連接到更新的文件，因此，您將能夠閱讀內容。

你似乎有一點問題，下面的條件：

//if (!userIn.hasNextLine() || !userIn.hasNextLine()) { 
    if (!userIn.hasNextLine() || !passIn.hasNextLine()) { 
      System.out.println("New users must set up their account: "); 
      if (!userIn.hasNextLine()) { 
       System.out.print("Enter a username: "); 
       String username = in.nextLine().trim(); 
       userOut.print(username); 
       userOut.flush(); 

      } 
      //why again !userIn.hasNextLine(), you should check passIn 
      //if (!userIn.hasNextLine()) { 
       if (!passIn.hasNextLine()) { 
       System.out.print("Enter a password: "); 
       String password = in.nextLine().trim(); 
       passOut.print(password); 
       passOut.flush(); 

      } 
      System.out.println("Your account has been created. You may now log in:"); 
     } 

我有固定的兩個條件和註釋掉的代碼位。