第 4 章，问题 21（核对 SSN）（核对 SSN）

import java.util.*;

public class CheckSSNQuestion21 {
public static void main(String[] args) {
String ssnString;

System.out.print("Enter a SSN: ");
Scanner input = new Scanner(System.in);
ssnString = input.nextLine();

if(ssnString.length() == 11)
{
if(Character.isDigit(ssnString.charAt(0))
&& Character.isDigit(ssnString.charAt(1))
&& Character.isDigit(ssnString.charAt(2))
&& ssnString.charAt(3) == '-'
&& Character.isDigit(ssnString.charAt(4))
&& Character.isDigit(ssnString.charAt(5))
&& ssnString.charAt(6) == '-'
&& Character.isDigit(ssnString.charAt(7))
&& Character.isDigit(ssnString.charAt(8))
&& Character.isDigit(ssnString.charAt(9))
&& Character.isDigit(ssnString.charAt(10)))

System.out.println(ssnString + " is a valid social security number");
else
System.out.println(ssnString + " is an invalid social security number");
}
else
System.out.println(ssnString + " is an invalid social security number");

input.close();
}
}