Java无法检查密码字符串是否有效

Question

So I'm trying to get this to take user input as a string and check the password to make sure of these two things:

1. The password is a minimum of 8 characters.
2. The password only contains letters and numbers.

Now the problem is this: Checking the password for a minimum of 8 characters works, but checking it to make sure it only contains letters and numbers does not work. It simply terminates without giving a single message if the minimum amount of numbers/letters are entered. However if it sees a character that is not a letter or number, it will print out this :

Please enter a password: ###

Password can only contain letters and numbers.

Password can only contain letters and numbers.

Password can only contain letters and numbers.

Password accepted!

What it should output is this:

Please enter a password: ###

Password can only contain letters and numbers.

or

Please enter a password: test1234

Password accepted!

    password.java    
    package Password;
    import java.util.Scanner;

    public class Password {

    public static void main(String[]args)
    {

    Scanner input = new Scanner (System.in);
    boolean valid = true;
    System.out.println("Please enter a password:");
    String password = input.nextLine();
    int i = 0;
    //declares i as the counter varible to control the loop and initializes it to 0

    if((password.length( ) < 8 )) //check the passwords length and make sure it's a minimum of 8 characters
    {
    System.out.println("Password must have at least 8 characters.");
    valid = false;
    }
    //loops the code below it for the length of i until the password length is reached
    while(i < password.length())
    {
    if ((password.charAt(i)>='a' && password.charAt(i)<='z') ||  (password.charAt(i)>='A' && password.charAt(i)<='Z') ||(password.charAt(i)>='0' && password.charAt(i)<='9'))
    //loop through all the characters in the string entered and make sure they only consist of letters and numbers
       valid = true;
    else
    {
        System.out.println("Password can only contain letters and numbers.");  
        valid = false;
    }
    i++;
    //add an iteration to the loop
    }

    if(!valid == true)
    System.out.println("Password accepted!");
    }
    }

Any help at all with this would be great.

Answer 1

You could simplify the code a bit, first start with valid by checking the password.length() ; then test each character in the password (stopping if any are invalid). Then finally check if the password was valid before displaying your accepted message. Like,

Scanner input = new Scanner(System.in);
System.out.println("Please enter a password:");
String password = input.nextLine();
boolean valid = password.length() >= 8;

if (!valid) {
    System.out.println("Password must have at least 8 characters.");
} else {
    for (char ch : password.toCharArray()) {
        if (!Character.isLetter(ch) && !Character.isDigit(ch)) {
            System.out.println("Password can only contain letters and numbers.");
            valid = false;
            break;
        }
    }
}
if (valid) {
    System.out.println("Password accepted!");           
}

Answer 2

the main error in this check code is the while loop,when you see a wrong character there is no need to continue the loop,simply you do this kind of checks in that way :

String toCheck;       //the string to check some criteria 
boolean valid = true; // we assume that nothing wrong happen till now

for(int i=0;i<toCheck.length();i++){ //loop over the characters
    if(/*condition of wrong case*/){ 
        valid = false;               //mark that something wrong happen
        break;                       //exit the loop no need to continue
    }
 }

 if(valid){
    //nothing wrong happen
 } else {
    //something wrong happen
 }