嵌套“if”语句时，打破这个“for”循环

Question

I'm trying to use a break; statement to exit a for loop.

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.
            }
        }
    }

What is happening:

The code will read in the username and password , verify it is a good login, then return to asking for the username and password, again.

What is expected to happen:

Once it hits a successful login, it sets userRole to the current User's role, displayed the goodLogin message, then exits the loop.

Answer 1

the break will only break out of the inner for loop where you itterate the user list, and not the outer loop.

Try doing some thing like this

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 );

You would need to add back in the failure count to the loop, but this should give you a more expected result.

Answer 2

You can attach a label and break to that label ( 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.
            }
            }
        }
    }

Answer 3

The break; statement breaks the inner-most loop; the outer-most loop will continue executing.

In the outer-most loop, you would want to have a variable such as bool success = false; . Before breaking in the inner-most loop, assign true , and do a check in the outer-most loop like:

if(success) break;