java在嵌套“if”语句时打破了这个“for”循环

Question

我试图使用break;语句退出for循环

final int NUM_USERS = 6;                     // Max Number of Users.
UserInfo[] users = new UserInfo[NUM_USERS];  // Array of Users.
int loginCounter = 0;                        // Counts bad login attempts.
int i = 0;                                   // Loop index variable.
String userRole = "";                        // Holds user's role text.

for (loginCounter = 1; loginCounter <= 3; ++loginCounter) {
        // Get User's Credentials.
        System.out.println("Enter Username: ");
        String username = input.next().toLowerCase();
        input.nextLine();                      // Allows User to enter password.
        System.out.println("Enter Password: ");
        String password = input.nextLine();

        String hash = sysLogin.convertToMd5(password);
        for (i = 0; i < users.length; ++i) {
            if (username.equals(users[i].getUsername())) {
                if (hash.equals(users[i].getHash())) {
                    userRole = users[i].getRole();
                    sysLogin.goodLogin();      // Prints Good Login message.
                    break;
                }
            } else {
                sysLogin.badLogin();           // Prints Bad Login message.
            }
        }
    }

发生了什么：

代码将读取username和password，验证它是一个良好的登录，然后再次返回询问用户名和密码

预计会发生什么：

一旦成功登录，它将userRole设置为当前用户的角色，显示goodLogin消息，然后退出循环

Answer 1

break只会跳出内部for循环，而不是外部循环，在内部for循环中，用户列表将被写入

试着做这样的事情

boolean login = false;
do {

    // Get User's Credentials.
    System.out.println("Enter Username: ");
    String username = input.next().toLowerCase();
    input.nextLine();                      // Allows User to enter password.
    System.out.println("Enter Password: ");
    String password = input.nextLine();

    // Convert password to MD5 hash.
    String hash = sysLogin.convertToMd5(password);
    for (i = 0; i < users.length; ++i) {
        if (username.equals(users[i].getUsername())) {
            if (hash.equals(users[i].getHash())) {

                login = true;

                userRole = users[i].getRole();
                sysLogin.goodLogin();      // Prints Good Login message.
                break;
            } else {

                sysLogin.badLogin();           // Prints Bad Login message.
            }
        }
    }
} while ( login == false && failCondition == false );

您需要将失败计数重新添加到循环中，但这会给您带来更预期的结果

Answer 2

break;语句打破了最内部的循环；最外层的循环将继续执行

在最外层的循环中，需要一个变量，比如bool success = false;。在断开最内部的循环之前，分配true，并在最外部的循环中进行检查，如：

if(success) break;

Answer 3

可以附加一个标签并断开该标签（Java example about labels in branches）：

final int NUM_USERS = 6;                     // Max Number of Users.
UserInfo[] users = new UserInfo[NUM_USERS];  // Array of Users.
int loginCounter = 0;                        // Counts bad login attempts.
int i = 0;                                   // Loop index variable.
String userRole = "";                        // Holds user's role text.
OUTER:
for (loginCounter = 1; loginCounter <= 3; ++loginCounter) {
        // Get User's Credentials.
        System.out.println("Enter Username: ");
        String username = input.next().toLowerCase();
        input.nextLine();                      // Allows User to enter password.
        System.out.println("Enter Password: ");
        String password = input.nextLine();

        // Convert password to MD5 hash.
        String hash = sysLogin.convertToMd5(password);
        for (i = 0; i < users.length; ++i) {
            if (username.equals(users[i].getUsername())) {
                if (hash.equals(users[i].getHash())) {
                    userRole = users[i].getRole();
                    sysLogin.goodLogin();      // Prints Good Login message.
                    break OUTER;
                }
            else {
                sysLogin.badLogin();           // Prints Bad Login message.
            }
            }
        }
    }