java - 为什么我的 continue 语句不会重新启动我的 while 循环？

Question

我正在尝试为我的最终项目完成一个身份验证程序。我正在检查用户身份验证，如果用户信息与凭据文件不匹配，输出会告诉他们这一点，然后提示输入用户名和密码，同时增加一个尝试计数器。我遇到问题的唯一部分是当我测试和错误尝试之后是正确尝试时，while 循环不会重新启动我的身份验证过程，而是简单地说再次登录不正确。为什么我的 continue 语句没有重新启动我的 while 循环迭代？

我厌倦了在论坛上四处寻找这个答案，并且食物与我的问题无关。我也尝试移动我的条件语句，看看它是否在我继续工作的错误位置，但它没有解决任何问题。编译器说我的两个 continue 语句都是不必要的。

    //prompting user for username
    System.out.println("Please enter your username (\"L\" to logout \"Q\" to quit): ");
    username = scnr.nextLine();
    //evaluating if user wants to quit immediately 
    if(username.equals("Q")) {
        System.exit(0);
    }
    //prompting user for password and storing to password field
    System.out.println("Please enter your password: ");
    password = scnr.nextLine();
    System.out.print("\n");

    //while loop that contains the authentication and authorization logic 
    while (!username.equals("Q") && attemptCounter < 2){
        //calling of hashInput method of MD5Hash class to return the hashed user password
        userHashedPassword = userHash.hashInput(password);
        //while loop to open the credentials for authentication comparison
        while (cfScnr.hasNextLine()) {
            //assigning the files scanned next line to a field for comparison
            line = cfScnr.nextLine();
            //conditional statement to determine if username and password are contained on the line
            //will break file loop as soon as line contains the user's username and password
            //statement logic used to return the role string and remove extra characters and white space
            if (line.contains(username) && line.contains(userHashedPassword)) {
                dqLocation = line.lastIndexOf('"');
                role = line.substring(dqLocation);
                role = role.replaceAll("\\s+", "");
                role = role.replace("\"", "");
                break;
            }
        }
        //conditional statement used to determine if previous loops condtional statement was meant
        //if it wasn't this condition will inform the user of incorrect username and/or password
        //inform them of attempts remaining and prompt them for a new username and password while
        //tracking the attempts and it they want to quit. If Q isn't entered main while loop will restart authentication
        if (role == null){
            attemptCounter++;
            System.out.println("Username or password incorrect. " + (3 - attemptCounter) + " attempts remaining.");
            System.out.println("Please enter your username (\"L\" to logout \"Q\" to quit): ");
            username = scnr.nextLine();
            if(username.equals("Q")) {
                System.exit(0);
            }
            System.out.println("Please enter your password: ");
            password = scnr.nextLine();
            continue;
            }
        //this conditional statement runs only when the user is authenticated
        else {
            //creating new file object and scanner object to scan the role file
            File rFile = new File("src\\zooauthenticationsystem\\" + role + ".txt");
            Scanner rfScnr = new Scanner(rFile);
            //while loop to parse through the role file and output the lines of the file to the console
            while (rfScnr.hasNextLine()){
                rolePrint = rfScnr.nextLine();
                System.out.println(rolePrint);
            }
            //prompting user if they would like to logout or simply quit the program
            System.out.println("\nPress \"L\" to logout and \"Q\" to quit.");
            userDecision = scnr.nextLine();
            //conditional statement to determine their input, and resetting role to null to reset authentication loop conditional statements, restarts main while loop
            if (userDecision.equals("L")){
                System.out.println("Please enter your username: ");
                username = scnr.nextLine();
                if(username.equals("Q")) {
                    System.exit(0);
                }
                System.out.println("Please enter your password: ");
                password = scnr.nextLine();
                System.out.print("\n");
                role = null;
                continue;
            }

标签： java

Answer 1

让我们摆脱你的大部分代码，让我们更好地格式化代码，让我们只留下控制结构来看看continue语句在做什么：

while (!username.equals("Q") && attemptCounter < 2) {
    userHashedPassword = userHash.hashInput(password);
    while (cfScnr.hasNextLine()) {
        line = cfScnr.nextLine();
        if (line.contains(username) && line.contains(userHashedPassword)) {
            // ... do some stuff
            break;
        }
    }
    if (role == null) {
        // ... do some stuff
        continue;  // **** (A) ****
    } else {
        // ... do some stuff
        if (userDecision.equals("L")){
            // ... do some stuff
            continue;  // **** (B) ****
        }
    }
}

如果到达第 (A) 行，则您在if (roll == null)块中，else永远不会进入，并且无论 continue 语句如何， while 循环都会重复，从而变得continue不必要和分散注意力。

同样，如果到达 (B) 行，则您处于 while 循环的最后一个控制块中，因此无论 continue 语句是否continue不必要和分散注意力，循环都将继续。