Java编程：输入无效密码时如何终止程序或重新输入？

你的核心问题是无论密码是否有效，程序都会执行登录详情打印逻辑，以下提供两种符合你需求的修复方案：

方案一：密码无效时直接终止程序 #

将打印登录详情的代码移到密码有效的分支中，无效时输出错误提示后终止程序：

final int MAX_LENGTH = 8;
final int MIN_UPPERCASE = 1;
final int MIN_DIGITS = 1;
final int MIN_SPECIAL = 1;

Scanner sc = new Scanner(System.in);

// 收集用户基本信息
System.out.println("Enter Name: ");
String name = sc.nextLine();
int spacePosition = name.indexOf(" ");

System.out.println("\nEnter Birthday (MM DD YYYY): ");
String birthday = sc.nextLine();

// 密码验证逻辑
System.out.println("\nEnter password: ");
String password = sc.nextLine();

int uppercaseCounter = 0;
int digitCounter = 0;
int specialCounter = 0;
int spaceCounter = 0;

// 遍历密码字符统计各类字符数量
for (int i = 0; i < password.length(); i++) {
    char c = password.charAt(i);
    if (Character.isUpperCase(c)) {
        uppercaseCounter++;
    }
    if (c == ' ') {
        spaceCounter++;
    }
    if (c == '$' || c == '#' || c == '?' || c == '!' || c == '_' || c == '=' || c == '%' || c == '.') {
        specialCounter++;
    } else if (Character.isDigit(c)) {
        digitCounter++;
    }
}

// 判断密码是否有效
boolean isPasswordValid = password.length() >= MAX_LENGTH 
        && uppercaseCounter >= MIN_UPPERCASE 
        && specialCounter >= MIN_SPECIAL 
        && digitCounter >= MIN_DIGITS
        && spaceCounter == 0; // 新增空格校验，原代码未在有效判断中添加

if (isPasswordValid) {
    // 密码有效，打印登录详情
    System.out.println("\nYour login details");
    System.out.println("Name    : " + name);
    System.out.println("Birthday: " + birthday);
    System.out.println("Username: " + name.substring(0, 2) + name.substring(spacePosition + 1) + birthday.substring(3, 5));
    System.out.println("Password: " + password);
} else {
    // 输出错误提示
    if (password.length() < MAX_LENGTH) {
        System.out.println("Enter at least 8 characters");
    }
    if (spaceCounter > 0) {
        System.out.println("Password contains whitespaces");
    }
    if (uppercaseCounter < MIN_UPPERCASE) {
        System.out.println("Enter at least 1 Uppercase Character");
    }
    if (digitCounter < MIN_DIGITS) {
        System.out.println("Enter at least 1 digit");
    }
    if (specialCounter < MIN_SPECIAL) {
        System.out.println("Enter at least 1 special character");
    }
    // 终止程序
    System.exit(1);
}

sc.close();

方案二：循环要求用户输入直到密码有效 #

如果希望用户输入无效密码后重新输入，而非直接终止，可将密码输入和验证逻辑放到循环中，每次循环重置计数器：

final int MAX_LENGTH = 8;
final int MIN_UPPERCASE = 1;
final int MIN_DIGITS = 1;
final int MIN_SPECIAL = 1;

Scanner sc = new Scanner(System.in);

// 收集用户基本信息
System.out.println("Enter Name: ");
String name = sc.nextLine();
int spacePosition = name.indexOf(" ");

System.out.println("\nEnter Birthday (MM DD YYYY): ");
String birthday = sc.nextLine();

String password = "";
boolean isPasswordValid = false;

// 循环直到输入有效密码
while (!isPasswordValid) {
    System.out.println("\nEnter password: ");
    password = sc.nextLine();

    // 每次循环重置计数器，避免上次输入的计数影响
    int uppercaseCounter = 0;
    int digitCounter = 0;
    int specialCounter = 0;
    int spaceCounter = 0;

    // 遍历密码字符统计各类字符数量
    for (int i = 0; i < password.length(); i++) {
        char c = password.charAt(i);
        if (Character.isUpperCase(c)) {
            uppercaseCounter++;
        }
        if (c == ' ') {
            spaceCounter++;
        }
        if (c == '$' || c == '#' || c == '?' || c == '!' || c == '_' || c == '=' || c == '%' || c == '.') {
            specialCounter++;
        } else if (Character.isDigit(c)) {
            digitCounter++;
        }
    }

    // 判断密码是否有效
    isPasswordValid = password.length() >= MAX_LENGTH 
            && uppercaseCounter >= MIN_UPPERCASE 
            && specialCounter >= MIN_SPECIAL 
            && digitCounter >= MIN_DIGITS
            && spaceCounter == 0;

    if (!isPasswordValid) {
        // 输出错误提示
        if (password.length() < MAX_LENGTH) {
            System.out.println("Enter at least 8 characters");
        }
        if (spaceCounter > 0) {
            System.out.println("Password contains whitespaces");
        }
        if (uppercaseCounter < MIN_UPPERCASE) {
            System.out.println("Enter at least 1 Uppercase Character");
        }
        if (digitCounter < MIN_DIGITS) {
            System.out.println("Enter at least 1 digit");
        }
        if (specialCounter < MIN_SPECIAL) {
            System.out.println("Enter at least 1 special character");
        }
        System.out.println("Please try again.");
    }
}

// 密码有效，打印登录详情
System.out.println("\nYour login details");
System.out.println("Name    : " + name);
System.out.println("Birthday: " + birthday);
System.out.println("Username: " + name.substring(0, 2) + name.substring(spacePosition + 1) + birthday.substring(3, 5));
System.out.println("Password: " + password);

sc.close();

额外修复说明 #

1. 移除了重复创建的Scanner input，只用一个sc即可处理所有输入
2. 修正了原代码中密码有效判断的漏洞：新增spaceCounter == 0校验，避免含空格的密码被判定为有效
3. 调整了变量命名（如MAX改为MAX_LENGTH），提升代码可读性
4. 修正了原代码中name.substring(spacePosition + 3)的索引越界风险，改为spacePosition + 1（适配常规的"名 姓"格式）

内容的提问来源于stack exchange，提问作者Amimir