Validate input data
Validate data
Prevent exceptions from being thrown

How to prevent exceptions from being thrown

The has methods of the Scanner class let you check whether additional data is available at the console and whether that data can be converted to a specific data type.
You can use the Scanner has methods to prevent an exception from being thrown when one of the next methods is called.
You can use the nextLine method to retrieve and discard any additional data that the user enters on a line that isn't required.

Methods of the Scanner class you can use to validate data

Method	Description
hasNext()	Returns true if the scanner contains another token.
hasNextInt()	Returns true if the scanner contains another token that can be converted to an int value.
hasNextDouble()	Returns true if the scanner contains another token that can be converted to a double value.
nextLine()	Returns any remaining input on the current line as a String object and advances the scanner to the next line.

Code that prevents an InputMismatchException


double subtotal = 0.0;
System.out.print("Enter subtotal:   ");
if (sc.hasNextDouble())
    subtotal = sc.nextDouble();
else
{
    sc.nextLine();    // discard the entire line
    System.out.println(
        "Error! Invalid number. Try again.\n");
    continue;         // jump to the top of the loop
}

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"))
        {
		    	// get the invoice subtotal from the user
				double subtotal = 0.0;
				System.out.print("Enter subtotal:   ");
				if (sc.hasNextDouble())
				    subtotal = sc.nextDouble();
				else
				{
				    sc.nextLine();    // discard the entire line
				    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();
		}
    }
}

Code that prevents a NullPointerException

if (customerType != null)
{
    if (customerType.equals("R"))
         discountPercent = .4;
}

Previous | Prevent exceptions from being thrown | Validate a single entry | Generic methods to validate an entry | Next