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Loop when incorrect input is given?

So basically I've been trying to get this small simple code to work but I'm running into the problem of making a loop. What I want to happen is basically this: User enters an Integer, if its not an integer it will display an error and ask for an Integer until and Integer is given. I'm having a difficult time setting up a loop cause I don't quite know what to do. Im pretty new and dumb so this is probably really easy but I'm kind of an idiot and suck at this but I'm learning.
Here's what I have.
import java.util.Scanner;
public class Loop{
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
System.out.println("Enter an Integer: ");
if (scan.hasNextInt()) {
int Index = scan.nextInt();
scan.nextLine();
System.out.println("Index = " + Index);
}
else if (scan.hasNextDouble()) {
System.out.println("Error: Index is Double not Integer.");
}
else {
System.out.println("Error: Index is not Integer.");
}
}
}

You can use while loop for that.
while (true) {
Scanner scan = new Scanner(System.in);
System.out.println("Enter an Integer: ");
if (scan.hasNextInt()) {
int Index = scan.nextInt();
scan.nextLine();
System.out.println("Index = " + Index);
break;
} else if (scan.hasNextDouble()) {
System.out.println("Error: Index is Double not Integer.");
} else {
System.out.println("Error: Index is not Integer.");
}
}

You need to use a loop (for or while) to your code. You can do it like this.
import java.util.Scanner;
public class Loop {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
while (true) {
System.out.println("Enter an Integer: ");
if (scan.hasNextInt()) {
int Index = scan.nextInt();
scan.nextLine();
System.out.println("Index = " + Index);
} else if (scan.hasNextDouble()) {
System.out.println("Error: Index is Double not Integer.");
} else {
System.out.println("Error: Index is not Integer.");
}
// add same condition to break the loop
}
// close the scanner
scan.close()
}
}

Never ending for loop with user input

I am new to java and have just learned how to use user input. I have a for loop that goes through 10 times with user input to ask for a number. If the number is invalid, it should print "Invalid number" and not count towards the increasing for loop. Instead, it just loops forever saying 'Invalid number'.
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
int sum = 0;
Scanner scanner = new Scanner(System.in);
for(int i = 1; i<=10; i++){
System.out.println("Enter number #" + i + " ");
boolean validInt = scanner.hasNextInt();
if(validInt){
int num = scanner.nextInt();
sum += num;
} else{
System.out.println("Invalid Number");
i--;
}
}
System.out.println("Sum was " + sum);
scanner.close();
}
}

The problem is you are updating the iterator i at 2 places.
The better way is to update it according to the condition.
I would also suggest you to make use of wrapper classes for safe integer conversions and handle the exceptions properly like done in the following code:
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
int sum = 0;
Scanner scanner = new Scanner(System.in);
for(int i = 1; i<=10; ){
System.out.println("Enter number #" + i + " ");
String input = scanner.nextLine();
try{
int num = Integer.parseInt(input);
sum += num;
i++; // If input is a valid integer, then only update i
}catch(NumberFormatException e){
System.out.println("Invalid Number");
}
}
System.out.println("Sum was " + sum);
scanner.close();
}
}

I think you also can tweak the code using hasNextInt() directly in the while loop.
while (scanner.hasNextInt()) {
int num = scanner.nextInt();
sum += num;
}

I needed to add a
scanner.nextLine();
After the if and else statement to clear the scanner in both situations.

Ask to insert numbers only

So I'm trying to make a simple calculator.
How do I make when I enter the first number, it works but if I insert "abc" it will give me an error.
How I make it in order when you write "abc" to say " please enter a number "
import java.util.Scanner;
public class calculator
{
public static void main(String[] args0) {
Scanner test = new Scanner(System.in);
int x;
int y;
String c;
System.out.println("Insert a number ");
x = test.nextInt();
System.out.println("insert a value e.g * / + -");
c = test.next();
System.out.println("Insert another number");
y = test.nextInt();
if ( c.equals("*")) {
System.out.println("the total is " + x*y);
}
if (c.equals("+")) {
System.out.println("the total is " + (x+y));
}
if (c.equals("-")) {
System.out.println("the total is "+ (x-y));
}
if (c.equals("/")) {
System.out.println("the total is "+ (x/y));
}
}
}

You can verify the input until be a int using a scanner property Scanner.hasNextInt()
Scanner scanner = new Scanner(System.in);
System.out.print("Enter number 1: ");
while (!scanner.hasNextInt()) scanner.next();
Example:
public static void main(String[] args0) {
Scanner test = new Scanner(System.in);
int x;
int y;
String c;
System.out.println("Insert a number ");
while (!test .hasNextInt()) test .next(); // Scanner Validation
int x = test .nextInt();
}
JavaDoc of Scanner

The error you get is an exception. You can actually "catch" your exceptions, so that when they appear, your program doesn't break, and you can do what is in place for that error (output a "Please, insert only numeric values" feedback?)
You can find some info on try-catch blocks here try-catch blocks

Try this:
public static void main(String[] args) {
// TODO Auto-generated method stub
Scanner test = new Scanner(System.in);
int x;
int y;
String c;
try {
System.out.println("Insert a number ");
x = test.nextInt();
System.out.println("insert a value e.g * / + -");
c = test.next();
System.out.println("Insert another number");
y = test.nextInt();
if (c.equals("*")) {
System.out.println("the total is " + x*y);
}
if (c.equals("+")) {
System.out.println("the total is " + (x+y));
}
if (c.equals("-")) {
System.out.println("the total is "+ (x-y));
}
if (c.equals("/")) {
System.out.println("the total is "+ (x/y));
}
} catch(InputMismatchException e) {
System.out.println("Please enter correct values.");
}
}
Modifications:
The error you are getting is known as RunTime Error or Exceptions due to wrong input type. In order to handle RunTime Exceptions, You need to use try and catch block.
try and catch blocks are used to handle RunTime Exceptions. If any error or exception occurs within try block then It will be thrown to catch block to be handled instead of terminating your program.

Try this:
boolean success = false;
while (!success) {
try {
y = test.nextInt();
success = true;
} catch (InputMismatchException e) {
test.nextLine();
System.out.println("Please enter a number.");
}
}

If you're willing to accept doubles instead of ints, java doubles have a built in method isNaN(), where NaN stands for Not a Number.
if (Double.isNaN(doubleValue)) {
...
}

Catching IllegalArgumentException?

I am having a little bit of a problem here. I am trying to figure out how to catch the IllegalArgumentException. For my program, if the user enters a negative integer, the program should catch the IllegalArgumentException and ask the user if he/she wants to try again. But when the exception is thrown, it doesn't give that option. It just terminates. I tried to use the try and catch method but it doesn't work for me. How do I catch this particular exception to continue to run instead of terminating?
public static void main(String[] args) throws IllegalArgumentException
{
String keepGoing = "y";
Scanner scan = new Scanner(System.in);
while(keepGoing.equals("y") || keepGoing.equals("Y"))
{
System.out.println("Enter an integer: ");
int val = scan.nextInt();
if (val < 0)
{
throw new IllegalArgumentException
("value must be non-negative");
}
System.out.println("Factorial (" + val + ") = "+ MathUtils.factorial(val));
System.out.println("Another factorial? (y/n)");
keepGoing = scan.next();
}
}
}
and
public class MathUtils
{
public static int factorial(int n)
{
int fac = 1;
for(int i = n; i > 0; i--)
{
fac *= i;
}
return fac;
}
}

You need to add the try catch block inside the loop to continue the working for the loop. Once it hits the illegal argument exception catch it in catch block and ask if the user wants to continue
import java.util.Scanner;
public class Test {
public static void main(String[] args)
{
String keepGoing = "y";
populate(keepGoing);
}
static void populate( String keepGoing){
Scanner scan = new Scanner(System.in);
while(keepGoing.equalsIgnoreCase("y")){
try{
System.out.println("Enter an integer: ");
int val = scan.nextInt();
if (val < 0)
{
throw new IllegalArgumentException
("value must be non-negative");
}
System.out.println("Factorial (" + val + ") = "+ MathUtils.factorial(val));
System.out.println("Another factorial? (y/n)");
keepGoing = scan.next();
}
catch(IllegalArgumentException i){
System.out.println("Negative encouneterd. Want to Continue");
keepGoing = scan.next();
if(keepGoing.equalsIgnoreCase("Y")){
populate(keepGoing);
}
}
}
}
}
Hope this helps.
Happy Learning :)

I don't think you want your main() method to be throwing an exception. Typically, this is the kind of thing that you'd put in try and catch blocks.
Honestly though, for this sort of thing an if/else would work better. (Unless you're just doing this as a toy example, to learn exceptions.)
Make another method called getNumber() that throws the IllegalArgumentException, that returns an int. Then put it inside the try/catch in the main().

public static void main(String[] args)
{
String keepGoing = "y";
Scanner scan = new Scanner(System.in);
while(keepGoing.equals("y") || keepGoing.equals("Y"))
{
int val = 0;
boolean flag=true;
while(flag){
try{
System.out.println("Enter an integer: ");
val = scan.nextInt();
if (val < 0)
{
throw new IllegalArgumentException
("value must be non-negative");
}
flag = false;
} catch(IllegalArgumentException e){
System.out.println("value must be non-negative");
}
}
System.out.println("Factorial (" + val + ") = "+ MathUtils.factorial(val));
System.out.println("Another factorial? (y/n)");
keepGoing = scan.next();
}
}
}

I would suggest you add a test on the negative value and display your message on the spot, then use an else block. Also, you could use String.equalsIgnoreCase() in your loop test like
String keepGoing = "y";
Scanner scan = new Scanner(System.in);
while (keepGoing.equalsIgnoreCase("y")) {
System.out.println("Enter an integer: ");
int val = scan.nextInt();
if (val < 0) {
System.out.println("value must be non-negative");
} else { // <-- skip negative value
System.out.println("Factorial (" + val + ") = "
+ MathUtils.factorial(val));
}
System.out.println("Another factorial? (y/n)");
keepGoing = scan.next();
}
Also, an int factorial(int) method can only the first 12 correct values. You could use a long or a BigInteger like
public static BigInteger factorial(int n) {
BigInteger fac = BigInteger.ONE;
for (int i = n; i > 1; i--) {
fac = fac.multiply(BigInteger.valueOf(i));
}
return fac;
}

Similar to some other answers, I would say that your main() method should not throw an exception to display an error message to the user, because that is not the purpose of exception handling mechanisms in Java. Exception handling is designed to enable methods to signal that something happened that should not have happened, so the methods that call those methods will know that they need to deal with them. In general, exceptions are caught, not thrown, by main() methods and other user interface methods.
I would make the method factorial() throw the IllegalArgumentException, rather than your main() method in your program class. Your main() method should use try and catch to handle this exception. With this design, if someone else wanted to use your MathUtils class, they would know that your factorial() method throws an IllegalArgumentException (especially if you document your code with javadoc), and would write their code to handle the exception. In the current situation, if someone tries to call MathUtils.factorial(-1), the return value would be 1 because the for loop inside factorial() would not execute at all (because i is initially set to -1, which is not greater than 0).
This is how I would revise your code:
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
String keepGoing = "y";
while(keepGoing.equalsIgnoreCase("y")) {
try { // This code might throw an exception
System.out.println("Enter an integer: ");
int val = scan.nextInt();
System.out.println("Factorial (" + val + ") = "+ MathUtils.factorial(val));
System.out.println("Another factorial? (y/n)");
keepGoing = scan.next();
} catch (IllegalArgumentException | InputMismatchException e) {
/* An InputMismatchException is thrown if the input is not an integer.
See the documentation for Scanner method nextInt() for more details.
*/
System.out.println("You must enter a non-negative integer.");
System.out.println("Try again? (y/n)");
keepGoing = scan.next();
}
}
}
}
and
public class MathUtils throws IllegalArgumentException {
public static int factorial(int n) {
if (fac < 0) {
throw new IllegalArgumentException("value must be non-negative");
}
int fac = 1;
for(int i = n; i > 0; i--) {
fac *= i;
}
return fac;
}
}

Array not Recognized

My program is to enter 10 numbers and add them together then display the result. Then I am to divide the smaller number by the larger number and display the result. Users cannot enter characters or zero.
I am new and have been working on this for DAYS. I do not see my mistake.
Now my problem is the Variable i isn't being recognized.
I introduced an Exception (try..catch) and it wouldn't read. I tried moving things all over (I'm new, I'm guessing and seeing what does what..) I did something wrong and probably something stupidly small. I need some help and fresh eyes.
I also need to end the program when the user enters 9999. Any idea where that would go? 'Cause I'm about to break out into tears.
public static void main(String[] args) throws NumberFormatException {
Scanner in = new Scanner(System.in);
Scanner input = new Scanner(System.in);
double[ ] digit = new double[11];
int sum = 0;
//Declare an array
System.out.print("Please Enter Ten Numbers:");
System.out.println();
try{
for (int i = 1; i < digit.length; i++)
System.out.print("Numbers " + i + ": ");
digit[i] = (double)in.nextInt(); //Array Not recognized here
sum += (int)digit[i];//Or recognized here
// Input the data into array from the user.
if(digit[i]==0.0)//None of these are recognized either, what did I do?
{
System.out.println("You can't enter zero. Try again");
--i; //nope
in.nextLine();//dispose of wrong number
}
}catch (NumberFormatException e){
System.out.println("You Can Only Enter Numbers!");
--i; //nope, not recognizing here either
in.nextLine();//dispose of wrong input
}
System.out.println("Total Values in Array:"+ sum);
// Calculate the sum and print the total
System.out.println();
System.out.println("Would you like to divide the values?");
System.out.println("Yes or No to Exit the Program");
String a = input.next();
if(a.equalsIgnoreCase("yes")){
double [] divisionResult = new double[digit.length / 2];
//Division array declared
for (int i = 1; i < digit.length; i += 2)
//These are all good and recognized. No problem with the division part
{
double result = digit[i];
if (result > digit[i + 1])
result = result / digit[i + 1];
else {
result = digit[i + 1] / result;
}
divisionResult [i / 2] = result;
System.out.println(result);
}
}
else if(a.equalsIgnoreCase("no")){
System.exit(0);
}
}
}
}

You for loop doesn't have braces around it. Only the first line below it is part of the loop.

You need braces around the contents of your for loop.

I have tried to modified the attached code snippet. Hope this might help you.
package com.so.general;
import java.util.Scanner;
public class AddNumbers
{
private static final int SIZE_OF_ARRAY = 10;
public static void main(String[] args)
{
int counter = 0;
double sumOfTenDigits = 0.0;
double[] digit = new double[SIZE_OF_ARRAY];
int listOfElements = digit.length;
System.out.print("Please Enter Ten Numbers:"+"\n");
Scanner readInputFromUser = new Scanner(System.in);
try
{
for (counter=0; counter<listOfElements; counter++)
{
System.out.print("Numbers " + (counter+1) + ": ");
digit[counter] = Double.parseDouble(readInputFromUser.next());
if(digit[counter] == 0.0)
{
System.out.println("Zero is not allowed. Please try again.");
System.out.print("Numbers " + (counter+1) + ": ");
digit[counter] = Double.parseDouble(readInputFromUser.next());
}
sumOfTenDigits += digit[counter];
}
}
catch(NumberFormatException numberFormatExcep)
{
System.err.println(" You have entered other than numbers. Only numbers are allowed. Terminating the program.");
numberFormatExcep.printStackTrace();
System.exit(0);
}
System.out.println("Addition is: "+ sumOfTenDigits);
System.out.println("Would you like to divide the values? Press Y to continue, any other character to terminate the program.");
Scanner continueWithDivide = new Scanner(System.in);
String userInput = continueWithDivide.nextLine();
closeScanner(readInputFromUser);
closeScanner(continueWithDivide);
if(readInputFromUser != null)
{
readInputFromUser.close();
}
// Always use static string literals on the left hand side.
// This prevents Null pointer exception.
if("Y".equalsIgnoreCase(userInput))
{
double[] divisionResult = new double[listOfElements/2];
for(int i=0; i<listOfElements; i+=2)
{
double result = digit[i];
if (result > digit[i+1])
{
result = result/digit[i+1];
}
else
{
result = digit[i+1]/result;
}
divisionResult[i/2] = result;
System.out.println(result);
}
}
else
{
System.out.println("You have entered "+userInput+". Terminating the program.");
System.exit(0);
}
}
/**
* Closed the scanner
* #param scanner
*/
public static void closeScanner(Scanner scanner)
{
if(scanner != null)
{ try
{
scanner.close();
}
catch(IllegalStateException illegatStateExcep)
{
System.err.println("Scanner is already closed.");
illegatStateExcep.printStackTrace();
}
}
}
Please note the below points:
Always use proper indentation.
Always match { }
Even if your if or for is having only one statement, use { }.
For example:
for (int counter=1; i<digit.length; i++)
{
System.out.print("Numbers " + i + ": ");
}
Above three points will save a lot of your time if some thing goes wrong in your program.
Use proper name for the variables and method.
Always close the IO resources after the use.
For example:
if(scanner != null)
{
scanner.close();
}