logic

public class Password {
    
    public static void main (String args[]) throws IOException {

        Console c = System.console();
        if (c == null) {
            System.err.println("No console.");
            System.exit(1);
        }

        String login = c.readLine("Enter your login: ");
        char [] oldPassword = c.readPassword("Enter your old password: ");

        if (verify(login, oldPassword)) {
            boolean noMatch;
            do {
                char [] newPassword1 = c.readPassword("Enter your new password: ");
                char [] newPassword2 = c.readPassword("Enter new password again: ");
                noMatch = ! Arrays.equals(newPassword1, newPassword2);
                if (noMatch) {
                    c.format("Passwords don't match. Try again.%n");
                } else {
                    change(login, newPassword1);
                    c.format("Password for %s changed.%n", login);
                }
                Arrays.fill(newPassword1, ' ');
                Arrays.fill(newPassword2, ' ');
            } while (noMatch);
        }

        Arrays.fill(oldPassword, ' ');
    }
    
    // Dummy change method.
    static boolean verify(String login, char[] password) {
        // This method always returns
        // true in this example.
        // Modify this method to verify
        // password according to your rules.
        return true;
    }

    // Dummy change method.
    static void change(String login, char[] password) {
        // Modify this method to change
        // password according to your rules.
    }
}

The Password class follows these steps:

1. Attempt to retrieve the Console object. If the object is not available, abort.

2. Invoke Console.readLine to prompt for and read the user's login name.

3. Invoke Console.readPassword to prompt for and read the user's existing password.

4. Invoke verify to confirm that the user is authorized to change the password. (In this example, verify is a dummy method that always returns true.)

5. Repeat the following steps until the user enters the same password twice:

1. Invoke Console.readPassword twice to prompt for and read a new password.

2. If the user entered the same password both times, invoke change to change it. (Again, change is a dummy method.)

3. Overwrite both passwords with blanks.

6. Overwrite the old password with blanks.