We Keep Coding

How to validate for a non-integer being entered in an integer data type? [duplicate]

This question already has answers here:
Validating input using java.util.Scanner [duplicate]
(6 answers)
Closed 3 years ago.
I have to make a store that has items for purchase. After choosing an item, I prompt the user to enter the quantity of the item they would like to buy.
// 'input' is my Scanner object
int quantity;
quantity = input.nextInt();
If the user enters a non-integer (i.e. decimal, char...), it breaks the program.
Is there a way I can validate for this non-integer input?
Thank you

Sure, accept a String value instead of an int, check to see if you can parse that String value to an int, if you can, then do so. If not, sent a message stating that then entered value must be an number.
This could be done in a while loop.
import java.util.Scanner;
public class ScannerInputInt {
public static void main(String... args) {
Scanner in = new Scanner(System.in);
Integer input = null;
do {
System.out.println("Please enter number: ");
String s = in.nextLine();
try {
input = Integer.parseInt(s);
} catch (NumberFormatException e) {
System.out.println("ERROR: " + s + " is not a number.");
}
} while (input == null);
}
}

If you don't wanna use Exceptions method. You can try this.
This piece of code will continue to ask user input until user has entered correct input.
System.out.print("Enter quantity: ");
Scanner input = new Scanner(System.in);
boolean isInt = input.hasNextInt(); // Check if input is int
while (isInt == false) { // If it is not int
input.nextLine(); // Discarding the line with wrong input
System.out.print("Please Enter correct input: "); // Asking user again
isInt = input.hasNextInt(); // If this is true it exits the loop otherwise it loops again
}
int quantity = input.nextInt(); // If it is int. It reads the input
System.out.println("Quantity: " + quantity);
input.close();
Output:
Enter quantity: 12.2
Please Enter correct input: 12.6
Please Enter correct input: s
Please Enter correct input: s6
Please Enter correct input: as
Please Enter correct input: 2
Quantity: 2
I think it is slightly better approach because I think controlling flow of your program with Exceptions is bad practice and should be avoiding when there are other things that you can use.

When user presses "enter" key or space enter then error message pops up

I would like to print an error message when the user presses enter or space enter instead of a string. I have tried isEquals("") and isEmpty() but haven't found anything that works yet.
Here's my code:
import java.util.Scanner;
public class check{
public static void main(String args[]){
System.out.println("Enter a number: ");
Scanner keyboard = new Scanner(System.in);
String input = keyboard.next();
if(input.equals("")){
System.out.println("Empty");
} else {
System.out.println("number inputed");
}
}
}

One way to do this, change keyboard.next() to keyboard.nextLine(), use trim() to remove unnecessary spaces, check with isEmpty().
String input = keyboard.nextLine().trim();
if (input.isEmpty()) {
// error message
} else {
// good to go
}

import java.util.Scanner;
public class check{
public static void main(String args[]){
System.out.println("Enter a number: ");
Scanner keyboard = new Scanner(System.in);
String input = keyboard.nextLine();
if(input.trim().equals("")){
System.out.println("Empty");
} else {
System.out.println("number inputed");
}
}
}

Strangely, I don't get an error when running your code. However, I noticed that your code simply doesn't react to an empty input (just pressing enter). If you want to check for that, you can use keyboard.nextLine().
Judging by the rest of your code, it seems like you want the user to input only a number. An easy way to check if the user entered an integer if you're using Scanner is keyboard.hasNextInt().
Meaning you can do something like this:
if(keyboard.hasNextInt()) {
int yourNumber = keyboard.nextInt();
System.out.println("Your number is: " + your Number);
}
else {
System.out.println("Please enter a valid integer");
}

To check whether the string input is empty, you can use the String.isEmpty() method. Look below:
String input = keyboard.nextLine();
if(!input.isEmpty()) {
//the input is not empty!
}
else {
//the input is empty!
}
Note, however, that since you want to receive numbers as inputs you should not retrieve them as strings. Below is an example where the program retrieves a double from the user. Scanner provides many methods to validate the user's input. In this case, I'm using hasNextDouble() to check whether the input is a number.
Scanner scanner = new Scanner(System.in);
System.out.println("Enter a number:");
while(!scanner.hasNextDouble()) {
System.out.println("That's not a number!");
scanner.next();
}
double numberInput = scanner.nextDouble();
System.out.println("The entered number was " + numberInput);

I made a sample program similar to yours and used nextLine() instead of next(). When user enters space and clicks enter he will print "space" else "a number".

Is there another way to use hasNextInt() in Java without putting everything in the if-statement?

My objective is to make sure the user inputs an int. Else, exit the program. Then I do some coding that requires that int.
Code Snippet :
Scanner input = new Scanner(System.in);
if (input.hasNextInt()) {
//check if user enters an int
int userinput = input.nextInt();
// assign that int input to variable userinput
// over 100+ lines of code using nextInt var "userinput"
} else {
System.exit(1);
// user did not enter an int
}
Is there a better way to check for whether a user has entered an int and then use that int that doesn't require my entire program to be coded into that if-statement (because nextInt's scope is limited to that if-statement)?
It feels messy to me to put everything into one if-statement.
I wouldn't be allowed to use separate objects/classes since it's early in the semester for my class. This all goes in the main method, and I'm just using simple if-statements/scanner inputs.
Thanks

Definitely! Just negate the if statement and early exit:
Scanner input = new Scanner(System.in);
if (!input.hasNextInt()) {
System.exit(1);
}
// "else"
doMagicalThings(input.nextInt());
Oh, I guess also to note: replace the 100 lines of code with a method call and break it up a bit. That'd be good to do in addition to the above.

Here is a simple example of using hasNextInt () to validate a positive integer input
Scanner input = new Scanner(System.in);
int number;
do {
System.out.println("Input Number ");
while (!input.hasNextInt()) {
System.out.println(" not a number!");
input.next();
}
number = input.nextInt();
} while (number <= 0);
System.out.println("Númber válid " + number);

scanner java validation and multiple instances

I am new to java and doing an assignment.
I have to request 3 inputs from the user and I have validation.
If I do it with only one instance of the scanner I get all messed up.
If I use three instances with a bit of workaround my code works.
Only I guess this is not best practice.
I have been reading a bit the manual regarding the scanner, but cannot understand the problem
Thanks
enter code here
Scanner input=new Scanner(System.in);
Scanner input2=new Scanner(System.in);
int input_integer=0;
double input_double=0.0;
String input_string="";
double value=0;
System.out.print("\n Please enter a number: ");
while(!input.hasNextInt()){
System.out.println("***** Error: the char inserted is not a number! *****");
String input_wrong=input.next();
System.out.print("\n Please enter a number: ");
}
input_integer=input.nextInt();
System.out.print("\n Please enter a double: ");
while(!input.hasNextDouble()){
System.out.println("***** Error: the char inserted is not a double! *****");
String input_wrong=input.next();
System.out.print("\n Please enter an double: ");
}
input_double=input.nextDouble();
System.out.print("\nPlease enter a string: ");
input_string=input.nextLine();
So I had two create 3 scanner instances and also to use a string to assign the wrong input in the while cycle to the able to prompt again.
Any suggestion?
I am sure there is a better way but I would try to understand..
Thanks!

I'm not exactly sure I understand what problem you're having, but scanner has some strange behaviors which are not immediately obvious. For instance, if you type "1234bubble" then press enter, then nextInt() will return 1234 and the next nextLine() will say "bubble". That is usually not desired behavior for inputs like this because "1234bubble" is not an integer and should have failed when the user pressed enter.
For that reason, I typically only use the function nextLine(). Then, I just process the data manually using functions like Integer.parseInt(..). That way, I can guarantee that I'm processing the whole line in a clear and obvious manner, unlike other techniques which create confusing code.
Here's how I would have written your program:
import java.io.IOException;
import java.util.Random;
import java.util.Scanner;
public class Main
{
static Random rand = new Random();
public static void main(String[] args) throws IOException
{
Scanner input = new Scanner(System.in);
int input_integer = 0;
double input_double = 0.0;
String input_string = "";
double value = 0;
while (true)
{
System.out.print("Please enter an integer: ");
// Get the entire next line of text
String text = input.nextLine();
try
{
// Try to turn the line into an integer
input_integer = Integer.parseInt(text);
// Turning it into an int succeeded!
// Leave the while loop
break;
} catch (NumberFormatException e)
{
// Turning it into an int failed.
System.out.println("***** Error: the text inserted is not an integer! *****");
}
}
while (true)
{
System.out.print("Please enter a double: ");
// Get the entire next line of text
String text = input.nextLine();
try
{
// Try to turn the line into a double
input_double = Double.parseDouble(text);
// Turning it into an double succeeded!
// Leave the while loop
break;
} catch (NumberFormatException e)
{
// Turning it into an double failed.
System.out.println("***** Error: the text inserted is not a double! *****");
}
}
System.out.print("Please enter a string: ");
input_string = input.nextLine();
// This is done automatically when the program stops, but it's
// a good habit to get into for longer running programs.
input.close();
}
}

In java Scanner.next()

public class Calculator
{
public static void main(String[] args)
{
boolean isValid = false;
Scanner myScanner = new Scanner(System.in);
String customerType = null;
System.out.print("Customer Type? (C/R) ");
customerType = myScanner.next();
while (isValid == false)
{
System.out.print("Enter Subtotal: ");
if (myScanner.hasNextDouble())
{
double sobTotal = myScanner.nextDouble();
isValid = true;
}
else
{
System.out
.println("Hay! Entry error please enter a valid number");
}
myScanner.nextLine();
}
}
}
Hi, im new to java, as usual im trying out a few things in the Scanner Class.
is there a way to see the input of the scanner?
Cuz i have a problem here with the code above as you'll see. this is the output of my console window after i entered wrong data. instead of numbers i entered KKK so can somebody explain me why i got this error message 2 times?
"this is the console"
Customer Type? (C/R) R
Enter Subtotal: KKK
Hay! Entry error please enter a valid number
Enter Subtotal: Hay! Entry error please enter a valid number
Enter Subtotal:

Change your code as follows.
boolean isValid = false;
Scanner myScanner = new Scanner(System.in);
String customerType = null;
System.out.print("Customer Type? (C/R) ");
customerType = myScanner.next();
while (!isValid)
{
System.out.print("Enter Subtotal: ");
if (myScanner.hasNext())
{
try
{
double sobTotal =Double.parseDouble(myScanner.next());
isValid = true;
}
catch(Exception e)
{
System.out.println("Hay! Entry error please enter a valid number");
}
}
}
When you use Scanner.nextDouble(), it does not consume the new line (or other delimiter) itself so the next token returned will typically be an empty string.

The problem is you're calling scanner.nextdouble() which will work fine if and when there is a double in the input. When there is not a double in the input your input that was "KKK" is ignored by the call to nextDouble() and your error message is displayed then when you call nextLine() that same input "KKK" is still there waiting wo when you loop back throught the while the "KKK" is then passed back to your program and since it is still not a double you get the repeated error message.
Try this:
boolean isValid = false;
Scanner myScanner = new Scanner(System.in).useDelimiter("\\n");
String customerType = null;
System.out.print("Customer Type? (C/R) ");
customerType = myScanner.next();
while (!isValid)
{
System.out.print("Enter Subtotal: ");
if (myScanner.hasNextDouble())
{
double sobTotal = myScanner.nextDouble();
isValid = true;
}
else
{
System.out.println("Hay! Entry error please enter a valid number");
if(myScanner.hasNext()){
myScanner.next();
}
}
}
That will consume the invalid input of "KKK" and let the program continue properly.

I think the real problem is that you have some obfuscation in your loop structure that makes it hard to understand the in and out conditions of the scanner with each iteration of the loop.
Heres what is really happening. We enter the loop with one token in the Scanner which is not a double. We ask is there a double next in the scanner with hasNextDouble(), which since the one token can not be converted to a double, this returns false. We report an error and isValid remains false. After the branch we call scanner.next() and consumer our non double token. We renter the loop since isValid remains false. We again ask if there is a double next in the Scanner. Now the scanner is empty, since we consumed out token, and this call returns false, so we report another error. Now we call scanner.next() again. Since the scanner is now empty, and scanner.next() implements synchronous message passing (I/O), we now block until the scanner recieves a new token, so your program will stall until you input a new token. Try puting in more into your console. You would probably repeat the loop and see another error.
What you want to accomplish is with each loop, consume a token. If that token is a double, accept the input and continue, if not report an error and repeat. Try something like this instead, it should present a more clear picture of what the scanner is doing every loop.
public class Calculator
{
public static void main(String[] args)
{
Scanner scan = new Scanner(System.in);
String customerType = null;
System.out.print("Customer Type? (C/R) ");
customerType = scan.next();
Double test = null;
do{
try{
test = scan.nextDouble();
}catch (InputMismatchException ime){ //will be thrown if the token we input is not a double
System.err.println("Error Message");
}
}while(test == null);
}
}
Notice i use the wrapper class Double not the primative double. This is so that I can set the double to null, since Double is an Object and not a primitive.