Modify this program to Catch the error if the user enters an operator where a number should be. import textio.TextIO; /** * This program evaluates simple expressions such as 2 + 2 * and 34.2 * 7.81, consisting of a number, an operator, * and another number. The operators +, -, *, / are allowed. * The program will read and evaluate expressions until * the user inputs a line that starts with the number 0. */ public class SimpleCalculator { public static void main(String[] args) { double firstNum; // First number in the expression. double secondNum; // Second number in the expression. char operator; // The operator in the expression. double value; // The value of the expression. System.out.println("Enter expressions such as 2 + 2 or 34.2 * 7.81"); System.out.println("using any of the operators +, -, *, /."); System.out.println("To end, enter a 0."); System.out.println(); while (true) { /* Get user's input, ending program if first number is 0. */ System.out.print("? "); firstNum = TextIO.getDouble(); if (firstNum == 0) break; operator = TextIO.getChar(); secondNum = TextIO.getlnDouble(); /* Compute the value of the expression. */ switch (operator) { case '+': value = firstNum + secondNum; break; case '-': value = firstNum - secondNum; break; case '*': value = firstNum * secondNum; break; case '/': value = firstNum / secondNum; break; default: System.out.println("Unknown operator: " + operator); continue; // Back to start of loop! } // end switch /* Display the value. */ System.out.println("Value is " + value); System.out.println(); } // end while System.out.println("Good bye"); } // end main() } // end class SimpleCalculator