Why does my sign in function only work for one user in my CSV file with usernames and passwords?

I have a csv file that holds all the customers and admins of my car dealership and one of the pieces of information is a username and password. I have a sign in function where the user inputs their username and password and I use a for loop to see if the user is typing the correct credentials. I tried signing into one of my accounts which worked. However, I tried to sign into any other account and it doesnt work the same way. Below is the sign in function.

private static void signIn(CarDealership carDealership, Scanner scnr, ArrayList<Customer> customerList, Car carInUse) //method used to sign in
{
String userNameAttempt;
String passWordAttempt;
try {
System.out.println("Enter username:");
userNameAttempt = scnr.next();
for (Customer customerU : carDealership.getCustomerList()) {
if (customerU.getUserName().equals(userNameAttempt)) {
System.out.println("Enter password:");
passWordAttempt = scnr.next();
if (customerU.getPassWord().equals(passWordAttempt)) {
System.out.println("Welcome " + customerU.getFullName());
if (customerU instanceof Admin) {
Customer customerInUse = customerU;
adminMainMenu(carDealership, scnr, customerInUse, customerList, carInUse);
}
Customer customerInUse = customerU;
customerMainMenu(carDealership, scnr, customerInUse, customerList, carInUse);
}
throw new IllegalArgumentException("password is incorrect. ");
}
throw new IllegalArgumentException("username does not exist. ");
}
} catch (IllegalArgumentException ex) {
System.out.println(ex.getMessage() + "Would you like to retry?(1) register yourself?(2) or go back?(3) ");
int input = scnr.nextInt();
switch (input) {
case 1 -> {
System.out.println("-----------------------------------");
signIn(carDealership, scnr, customerList, carInUse);
break;
}
case 2 -> {
System.out.println("-----------------------------------");
registerCustomer(carDealership, scnr, customerList, carInUse);
break;
}
case 3 -> {
System.out.println("-----------------------------------");
beginningSite(carDealership, scnr, customerList, carInUse);
break;
}
default -> {
}
}
}
}

I will say that maybe the reason one of the customers works while the others doesnt is because its the first user in the csv file. How do I make it do the for loop checks every user then throws the exceptions?

I did try moving the throws higher but it just made the password throw work before even asking the user for their password.

> How do I make it do the for loop checks every user then throws the exceptions?

Maybe something like:

//  Find the Customer
Customer customerInUse = null;
for (Customer customerU : carDealership.getCustomerList()) {
if (customerU.getUserName().equals(userNameAttempt)) {
customerInUse = customerU;
break;
}
}
if (customerInUse == null)
// throw the Exception
//  prompt and validate the password
System.out.println("Enter password:");
passWordAttempt = scnr.next();
if (!customerInUse.getPassWord().equals(passWordAttempt))
// throw the Exception
//  valid sign in, display the appropriate menu
if (customerInUse instanceof Admin)
adminMainMenu(carDealership, scnr, customerInUse, customerList, carInUse);
else    
customerMainMenu(carDealership, scnr, customerInUse, customerList, carInUse);

There are a few thing to consider with your code.

Here is the customerU for-loop.

for (Customer customerU : carDealership.getCustomerList()) {
    if (customerU.getUserName().equals(userNameAttempt)) {
        System.out.println("Enter password:");
        passWordAttempt = scnr.next();
        if (customerU.getPassWord().equals(passWordAttempt)) {
            System.out.println("Welcome " + customerU.getFullName());
            if (customerU instanceof Admin) {
                Customer customerInUse = customerU;
                adminMainMenu(carDealership, scnr, customerInUse, customerList, carInUse);
            }
            Customer customerInUse = customerU;
            customerMainMenu(carDealership, scnr, customerInUse, customerList, carInUse);
        }
        throw new IllegalArgumentException("password is incorrect. ");
    }
    throw new IllegalArgumentException("username does not exist. ");
}

Your first if-conitional statement is the following.

if (customerU.getUserName().equals(userNameAttempt))

Which, if not met, will skip directly to your throw for IllegalArgumentException.
And, at that rate, even if the if-conditional was met, upon exit of the code block, the exception would be thrown.

throw new IllegalArgumentException("username does not exist. ");

Thus, if the first Customer within carDealership.getCustomerList() does not match userNameAttempt, the exception will be thrown.

I recommend re-ordering your code, as follows.

We'll need to capture whether the userNameAttempt was found or not.
So, I've added a boolean value, named "found", that we'll assign as true when the if-conditional is met.

I then placed it's accompanying IllegalArgumentException outside of the for-loop, under the condition that found == false— which can also be written as, !found.

Furthermore, I inverted the if-conditional for the passWordAttempt evaluation.
Thus, having it throw the exception if the password is not a match.
Therefore, any code after would be under the condition that the password was a match.

I then placed a break keyword within the conditional scope, to exit the for-loop—since subsequent iterations would be erroneous.

And, finally, you need to create a distinction between whether customerU is an instance of Admin or not.
So, the if-conditional here now includes an else block.

Feel free to provide feedback, as I cannot run the code.

boolean found = false;
for (Customer customerU : carDealership.getCustomerList()) {
    if (customerU.getUserName().equals(userNameAttempt)) {
        found = true;
        System.out.println("Enter password:");
        passWordAttempt = scnr.next();
        if (!customerU.getPassWord().equals(passWordAttempt))
            throw new IllegalArgumentException("password is incorrect. ");
        System.out.println("Welcome " + customerU.getFullName());
        Customer customerInUse = customerU;
        if (customerU instanceof Admin) {
            adminMainMenu(carDealership, scnr, customerInUse, customerList, carInUse);
        } else {
            customerMainMenu(carDealership, scnr, customerInUse, customerList, carInUse);
        }
        break;
    }
}
if (!found) throw new IllegalArgumentException("username does not exist. ");