Write a simple command-line calculator with an exception handler that deals with nonnumeric operands. Your program should display a message that informs the user of the wrong operand type before exiting. It should also ask the user to re-input the number to finish the calculation.

Here is the JAVA program:

import java.util.InputMismatchException;  // exception that is thrown when input does not match the expected type

import java.util.Scanner;  //used to take input from user

public class Main {

public static void main(String[] args) { //start of main function

   Scanner input = new Scanner(System.in);  // creates Scanner class object

   double num1=0.0;  // double type operand 1

   double num2=0.0;  // double type operand 2

   System.out.println("Please enter 2 operands");  //prompts user to enter two operands

   try {  //defines a chunk of code to be tested for non numeric operand error

      num1 = input.nextDouble();  //reads operand 1 value

      num2 = input.nextDouble(); //reads operand 2 value

   } catch (InputMismatchException  ex) {  //code chunk executed when exception occurs in program

     System.out.println("wrong operand type, re-input " + input.next() + " to finish the calculation.");  //displays wrong operand type if user enters non numeric operands

     System.exit(1);  }//exits the program    

   System.out.print("Enter an operator (+, -, *, /): ");  //prompts user to enter an operator

       char operator = input.next().charAt(0);  //reads the input operator

       double result;  // to store the result of the operation

       switch(operator)         {  //decided operation on basis of input operator

           case '+':  // if user enters + operator

               result = num1 + num2;  //adds two operands

               break;

            case '-':  // if user enters - operator

               result = num1 - num2;  //subtracts two operands

               break;  

           case '*':  // if user enters * operator

               result = num1 * num2;  //multiplies two operands

               break;  

           case '/':  // if user enters / operator

               result = num1 / num2;  //divided two operands

               break;  

           default:  //if user enters wrong operator

               System.out.printf("wrong operator!");  //displays wrong operator

               return;          }  

       System.out.printf("%.1f %c %.1f = %.1f", num1, operator, num2, result); }} //displays the result of the operation

Explanation:

The program is well explained in the comments mentioned with each line of program. The user is prompted to enter the values of two operands num1 and num2. The try block is used to test for non numeric operand error and throws InputMismatchException when any of the input operands (num1 or num2) is non numeric. InputMismatchException is thrown by Scanner class. This informs that the token retrieved (which is the num1 or num2 operand) does not match the expected type (which is type numeric). When this error occurs in the program, catch block is used to execute a code chunk, which is the print statement in this program. The print statement informs the user of the wrong operand type before exiting. It also asks the user to re-input the number to finish the calculation and the wrong input type is also mentioned in the output that is to be re-input.

If you want the program to take the input again from user after throwing this exception then you can alter the above program by using two try catch blocks separately for the two operands:

//for operand 1 i.e. num1

System.out.println("Please enter operand 1: ");

   try {

      num1 = input.nextDouble();        

   } catch (InputMismatchException  ex) {

     System.out.println("wrong operand type, re-input " + input.next() + " to finish the calculation.");

      num1 = input.nextDouble();  }  //reads input of operand 1 again from user

//for operand 2 i.e. num2

          System.out.println("Please enter operand 2: ");  

      try {

      num2 = input.nextDouble();  

   } catch (InputMismatchException  ex) {

     System.out.println("wrong operand type, re-input " + input.next() + " to finish the calculation.");

     num2 = input.nextDouble();  } //reads input of operand 1 again from user

The program and its output is attached.