关于else语句（java）

Question

So first, the question as follows: Write a program that prompts the user to enter a Social Security number in the format DDD-DD-DDDD, where D is a digit. Your program should check whether the input is valid. Here are sample runs:

Enter a SSN: 232-23-5435

232-23-5435 is a valid Social Security Number. 

Another test:

Enter a SSN: 23-23-5435

23-23-5435 is an invalid Social Security Number.

My code as follows:

import java.util.Scanner;

public class SSN {

  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    System.out.print("Please enter a Social Security Number in the format DDD-DD-DDDD: ");
    String ssn = input.nextLine();

    boolean correct = true;

    if (ssn.length() != 11) {
      correct = false;
    } else {
      if (!('0' <= ssn.charAt(0) && ssn.charAt(0) <= '9')) {
        correct = false;
      }
      if (!('0' <= ssn.charAt(1) && ssn.charAt(1) <= '9')) {
        correct = false;
      }
      if (!('0' <= ssn.charAt(2) && ssn.charAt(2) <= '9')) {
        correct = false;
      }
      if (ssn.charAt(3) != '-') {
        correct = false;
      }
      if (!('0' <= ssn.charAt(4) && ssn.charAt(4) <= '9')) {
        correct = false;
      }
      if (!('0' <= ssn.charAt(5) && ssn.charAt(5) <= '9')) {
        correct = false;
      }
      if (ssn.charAt(6) != '-') {
        correct = false;
      }
      if (!('0' <= ssn.charAt(7) && ssn.charAt(7) <= '9')) {
        correct = false;
      }
      if (!('0' <= ssn.charAt(8) && ssn.charAt(8) <= '9')) {
        correct = false;
      }
      if (!('0' <= ssn.charAt(9) && ssn.charAt(9) <= '9')) {
        correct = false;
      }
      if (!('0' <= ssn.charAt(10) && ssn.charAt(10) <= '9')) {
        correct = false;
      }
    }

    if (correct == true) {
      System.out.println(ssn + " is a valid Social Security Number.");
    } else {
      System.out.println(ssn + " is an ivalid Social Security Number.");
    }
  }
}

This is a correct answer after asking my professer, though.

However, I still don't understand why I need the else{} between the bunch if statements.

If I take the else{} away, it works for correct SSNs, but when i enter something invalid, returns:

Please enter a Social Security Number in the format DDD-DD-DDDD: sd
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 2
    at java.lang.String.charAt(Unknown Source)
    at SSN.main(SSN.java:22)

What's the matter? Why do i need a else there? Doesn't the program just check every if statements and give the output in the end by checking if correct is correct of false?

I mean, even without the else, even though there are more calculations, but shouldn't it work the same?

I'm confused what went wrong, please help, thanks!

Answer 1

First if checks if input has correct lenght. If not it won't check other conditions.

If you remove else , then all conditions will be checked. So if you have 5 length input you get StringIndexOutOfBoundsException while accessing sixth char.

You can use Pattern.compile(regex) and then check if regex matches your input instead of writing such a unclean code ifs

Answer 2

Because you are trying to read a character that doesn't exists. If your input String is shorter than 9 characters the following code will throw an StringIndexOutOfBoundsException

ssn.charAt(8)