Email validation using java program

Question

I am writing a java program on email validation.

The code must not contain:

• built-in functions (aside from String methods and the like)
• regular expressions

The code must contain:

• loops
• collections

I understand that email validation is harder without the use of regular expressions, but this question has been asked in an interview.

Are there any possibilities to write such a code or any alternate methods?

Answer 1

Use something like this :

public class EmailValidation {
    public static void main(String[] args) {
        String email = "SAMPLE_EMAIL";

        String[] deniedIdCharList = { ",", ";", "'", "\"", "[", "]", "|", "\\",
                 "=",  "!", "#", "$", "%", "^", "&", "*", "(", ")",
                "/", "`", "~", ":", "<", ">", "?", "{", "}" };

        int atLoc = email.indexOf("@");
        if (atLoc == -1) {
            System.out.println("fail");
        } else {
            String id = email.substring(0, atLoc);
            String domain = email.substring(atLoc + 1, email.length());

            if (domain.indexOf("@") != -1) {
                System.out.println("fail");
            }

            else {

                for (String deny : deniedIdCharList) {
                    if (id.indexOf(deny) != -1) {
                        System.out.println("fails");
                    }
                    if (domain.indexOf(deny) != -1) {
                        System.out.println("fails");
                    }

                }
                if (id.length() == 0 || domain.length() == 0) {
                    System.out.println("fails");
                }

                int dotIndex = domain.indexOf(".");
                String host = domain.substring(0, dotIndex);
                String extn = domain.substring(dotIndex + 1);
                if (host.length() == 0) {
                    System.out.println("fail");
                }
                if ((extn.length() != 2 && extn.length() != 3 && extn.length() != 5)) {
                    System.out.println("fail");
                }
                if (extn.length() == 5 && extn.indexOf(".") == -1) {
                    System.out.println("fail");
                }

            }

        }

    }
}

This worked for most standard checks I subjected it to. The code can be improved (A LOT) in terms of efficiency, however my guess is this is more from a "Can it be done" or Academic point of view rather than usage perspective. If you plan to use this methodology I advise strongly against it and refer to the answer provided by @vikeng21