Q:

using a loop to determine if an email address contains 2 characters

Check to see if the email address is valid.  A valid email address should contain BOTH the at symbols ( @ ) OR a dot ( . ).  If the user omits
either, your program must notify the user that the email provided was incorrect and prompt the
user to reenter the email address.  Continue to repeat this step until the user provides a valid email
address.  When the user enters a valid email address, continue with the rest of the program. 
- we are supposed to use a loop for this, but I'm not sure which to use or how to set it up.  I was thinking of using a while loop?  We cannot use the regex method I've seen people use on here as we haven't learned that yet. 
This is the code that we have to update
(we are now supposed to use a loop to continue asking the user 
for email address until both characters are used, 
instead of exiting the program):

if(emailAddress.contains("@")){ 
    //prompt user for major & classification code
    System.out.print("\nPleast enter two characters (Character #1: Major Code and Character #2: Classification Code): ");
    } 
else{ 
    //exit program if no '@' symbol in email address
    System.out.print("\nYou have entered an invalid email address."); 
    System.out.println("\n\nGoodbye!"); 
    System.exit(0);
}

A:

I would recommend using a while loop.  A for loop is best when you know how many times you need to repeat the operation, but a while loop is good for repeating an unknown amount of times.  It could run 0 to many times. 
As for the condition of your while loop, you just want to check if the email address is invalid.  So, check ! emailAddress.contains("@") to see if it does not include an @, ! emailAddress.contains(".")  if it does not include a .. 
You can use || to see if either condition is true. 
// Prompt the user
System.out.println("Please enter an email address");

String emailAddress =  // read in their email adress 

while(!emailAddress.contains("@") || ! emailAddress.contains(".")) { 
    System.out.println("You have entered an invalid email address, please enter another."); 

    emailAddress = // read in their email address again
}

// Email address now contains "@" and "."