Validate input data
Handle exceptions
How to catch exceptions

The compiler needs to know that you know you're calling a risky method

If you wrap the risky code in something called a try/cath, the compiler will relax.

A try/catch block tells the compiler that you know an exceptional thing could happen in the method you're calling, and that you're prepared to handle it. That compiler doesn't care how you handle it; it cares only that you say you're taking care of it.

The syntax for the try statement

try { statements }
catch(ExceptionClass exceptionName) { statements }

How to catch exceptions

In a try statement (or try/catch statement):

you code any statements that may throw an exception in a try block
you can code a catch block that will handle any exceptions that may occur in the try block

When an exception occurs, any remaining statements in the try block are skipped and the statements in the catch block are executed.
Any variables or objects that are used in both the try/catch blocks must be created before the try/catch blocks so both the try/catch blocks can access them.
If you use a catch block to catch a specific type of exception, you must also import the package that contains that exception class.

Two ways to import the InputMismatchException class

import java.util.InputMismatchException;
import java.util.*;

A try statement that catches an InputMismatchException

double subtotal = 0.0;
try
{
    System.out.print("Enter subtotal:   ");
    subtotal = sc.nextDouble();
}
catch(InputMismatchException e)
{
    sc.next();     // discard the incorrectly entered double
    System.out.println("Error! Invalid number.
                       Try again.\n");
    continue;      // jump to the top of the loop
}

Using the try/catch

import java.util.Scanner;
import java.util.InputMismatchException;

public class InvoiceApp
{
    public static void main(String[] args)
    {
        // welcome the user to the program
        System.out.println("Welcome to the Invoice Total Calculator");
        System.out.println();  // print a blank line

		// create a Scanner object named sc
		Scanner sc = new Scanner(System.in);

        // perform invoice calculations until choice isn't equal to "y" or "Y"
        String choice = "y";
        while (choice.equalsIgnoreCase("y"))
        {
			double subtotal = 0.0;
			try
			{
		    	// get the invoice subtotal from the user
   				 System.out.print("Enter subtotal:   ");
    			subtotal = sc.nextDouble();
			}
				catch(InputMismatchException e)
			{
  				sc.next();     // discard the incorrectly entered double
 			   	System.out.println("Error! Invalid number.Try again.\n");
    			continue;      // jump to the top of the loop
			}

        	// calculate the discount amount and total
        	double discountPercent= 0.0;
        	if (subtotal >= 200)
        		discountPercent = .2;
        	else if (subtotal >= 100)
        		discountPercent = .1;
        	else
        		discountPercent = 0.0;
        	double discountAmount = subtotal * discountPercent;
        	double total = subtotal - discountAmount;

            // display the discount amount and total
        	String message = "Discount percent: " + discountPercent + "\n"
        	               + "Discount amount:  " + discountAmount + "\n"
                           + "Invoice total:    " + total + "\n";
        	System.out.println(message);

        	// see if the user wants to continue
        	System.out.print("Continue? (y/n): ");
        	choice = sc.next();
        	System.out.println();
		}
    }
}

If you can't recover from the exception, at least get a stack trace

import java.util.Scanner;
import java.util.InputMismatchException;

public class InvoiceApp
{
    public static void main(String[] args)
    {
        // welcome the user to the program
        System.out.println("Welcome to the Invoice Total Calculator");
        System.out.println();  // print a blank line

		// create a Scanner object named sc
		Scanner sc = new Scanner(System.in);

        // perform invoice calculations until choice isn't equal to "y" or "Y"
        String choice = "y";
        while (choice.equalsIgnoreCase("y"))
        {
			double subtotal = 0.0;
			try
			{
		    	// get the invoice subtotal from the user
   				 System.out.print("Enter subtotal:   ");
    			subtotal = sc.nextDouble();
			}
				catch(InputMismatchException e)
			{
  				sc.next();     // discard the incorrectly entered double
 			   	System.out.println("Error! Invalid number.Try again.\n");
    			e.printStackTrace();      // get a stack trace that all exceptions inherit
			}

        	// calculate the discount amount and total
        	double discountPercent= 0.0;
        	if (subtotal >= 200)
        		discountPercent = .2;
        	else if (subtotal >= 100)
        		discountPercent = .1;
        	else
        		discountPercent = 0.0;
        	double discountAmount = subtotal * discountPercent;
        	double total = subtotal - discountAmount;

            // display the discount amount and total
        	String message = "Discount percent: " + discountPercent + "\n"
        	               + "Discount amount:  " + discountAmount + "\n"
                           + "Invoice total:    " + total + "\n";
        	System.out.println(message);

        	// see if the user wants to continue
        	System.out.print("Continue? (y/n): ");
        	choice = sc.next();
        	System.out.println();
		}
    }
}