I have written a JAVA program that takes input from the user and check if the user has entered the correct thing or not. I have taken the input from the Scanner class. If the user enters an invalid character like a String, I want to display 'Invalid Input'.
public class Main {
public static void main(String[] args) {
Scanner takeInteger = new Scanner(System.in);
System.out.println("Enter a number");
int enteredNumber = takeInteger.nextInt();
}
}
Just ask the Scanner whether the next input is a valid int value, e.g.
Scanner takeInteger = new Scanner(System.in);
System.out.println("Enter a number");
while(!takeInteger.hasNextInt()) {
System.out.println("Invalid Input: " + takeInteger.next());
}
int enteredNumber = takeInteger.nextInt();
This will retry the operation until the user entered a number. If you just want a single attempt, use something like
Scanner takeInteger = new Scanner(System.in);
System.out.println("Enter a number");
if(!takeInteger.hasNextInt()) {
System.out.println("Invalid Input: " + takeInteger.next());
}
else {
int enteredNumber = takeInteger.nextInt();
// ... proceed with the input
}
You will get an Exception that is InputMismatchException when an invalid input is passed.(i.e except integer value),you can use a try-catch block to hold the exception and inform the user about the invalid input. Try block , Catch block
import java.util.*;
Scanner takeInteger = new Scanner(System.in);
System.out.println("Enter a number");
try{
int enteredNumber = takeInteger.nextInt();
}
catch(InputMismatchException e) {
System.out.println("Enter a valid input");
}
You can use Exception handling for the same.
public class Main {
public static void main(String[] args) {
Scanner takeInteger = new Scanner(System.in);
System.out.println("Enter a number");
try {
int enteredNumber = takeInteger.nextInt();
}
catch (Exception e) {
System.out.println("Invalid Input");
}
}
}
You can add a try-catch block in your program to check if the user's input is a number or not.
Scanner takeInteger = new Scanner(System.in);
System.out.println("Enter a number");
String input = takeInteger.next();
int enteredNumber;
try
{
enteredNumber = Integer.parseInt(input); // Input is a number
}
catch(NumberFormatException ex)
{
System.out.println("Wrong Input!"); // Invalid input
}
You need to call the .nextLine method of the Scanner class and then parse to the desired type.
Example:
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.println("Enter a number");
String line = sc.nextLine();
try {
int enteredNumber = Integer.parseInt(line);
System.out.println("You have entered: " + enteredNumber);
} catch (Exception e) {
System.out.println("Invalid Input");
}
}
}
Result with a number:
Enter a number
12
You have entered: 12
Result a text:
Enter a number
abcd
Invalid Input
Related
This question already has answers here:
Validating input using java.util.Scanner [duplicate]
(6 answers)
Closed 1 year ago.
As the title says, i'm attempting to make my program re-ask for user input if the given input is invalid (In this case, invalid input is any input that is not an integer)
I've already tried this, but it does not work:
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
System.out.println("Please input your age");
if (scanner.hasNextInt()) {
int age = scanner.nextInt();
System.out.println("Your age is: " + age);
System.out.println();
} else {
System.out.println("This input is not an integer - Please try again!");
int age = scanner.nextInt();
System.out.println("Your age is: " + age);
System.out.println();
}
}
}
I'm aiming for this to be done with while loop and scanner
My current code:
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
System.out.println("Please input your age");
if (scanner.hasNextInt()) {
int age = scanner.nextInt();
System.out.println("Your age is: " + age);
}
}
}
Any reply on this post is greatly appreciated.
You can use while-loop:
Scanner scanner = new Scanner(System.in);
boolean ageGiven = false;
while (!ageGiven) {
System.out.println("Please input your age");
String next = scanner.next();
try {
int age = Integer.parseInt(next);
System.out.println("Your age is: " + age);
ageGiven = true;
} catch (NumberFormatException e) {
System.out.println("This input is not an integer - Please try again!");
}
}
I think you should put while instead of if statement. Break after having correct input. Try out once.
As you suggested, a while loop is more appropriate to this.
Something like this should work:
Scanner scanner = new Scanner(System.in);
System.out.println("Please input your age");
boolean isValid = false;
int age;
while (!isValid) {
if (scanner.hasNextInt()) {
age = scanner.nextInt();
isValid = true;
}
else {
System.out.println("Invalid input. Please type a number");
}
}
System.out.println("Your age is: " + age);
In this case, it would loop while the IsValid boolean is false. And that variable would be set to true, once the user inputs a valid age.
Edit: Changed the code to check if the input is an integer.
I'm trying to make a program where a user needs to input a random integer. If the user inputs a String I want an error message to pop out: "This is not a number" and after that restart the program until the user inputs a number. I got this so far and I'm stuck. I just get an error message if I input a string and program crashes.
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int number = 0;
do {
System.out.println("Input a number!");
number = scanner.nextInt();
if (!scanner.hasNextInt()) {
System.err.println("This is not a number");
}
} while (!scanner.hasNextInt());
System.out.println("You entered: " + number);
}
You're getting an InputMisMatchException because if you input a string into a scanner.nextInt(), it will immediately give an error and stop the program before it does anything else, so it won't reach your if statement. One way to get around this issue is to instead receive user input as a string, try to parse it for an int, and end the loop if it doesn't throw an exception. This is my implementation:
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
String input = "";
int number = 0;
boolean end = true;
do {
System.out.println("Input a number!");
input = scanner.nextLine();
try {
number = Integer.parseInt(input);
end = true;
} catch(Exception e) {
System.err.println("This is not a number");
end = false;
}
} while (!end);
System.out.println("You entered: " + number);
}
See if the below code can help achieve what you want to do.
public class Solution {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
String number;
do {
System.out.println("Input a number!");
number = scanner.next();
} while (!isNumeric(number));
System.out.println("You entered: " + number);
}
public static boolean isNumeric(final String str) {
// null or empty
if (str == null || str.length() == 0) {
return false;
}
return str.chars().allMatch(Character::isDigit);
}
}
I am new in java. I have written a program in which the user chooses how many numbers he wants to add. If the user enters a string it will throw an exception and the programs tell the user to enter all details again.
My problem is i want the program to ask the user to re enter details from that number which he entered wrong.
Eg: the user chooses to add 4 numbers but he enters the third number as string, the program should ask the user to re-enter from the third number and not the entire details again.
My code is as follow:
import java.io.*;
import java.util.*;
class Add
{
public static void main(String args[]) throws Exception
{
boolean loop=true;
while(loop)
try
{
String yn;
do
{
Scanner s=new Scanner(System.in);
System.out.println("Enter how many numbers to add: ");
int num=Integer.parseInt(s.next());
int a,sum=0;
for(int i=1;i<=num;i++)
{
System.out.println("Enter number["+i+"]: ");
a=Integer.parseInt(s.next());
sum=sum+a;
}
System.out.println("The Sum is:"+sum);
System.out.println("Do you want to continue?(Y/N):");
yn=s.next();
} while(yn.equals("y")||yn.equals("Y"));
}
catch(Exception e)
{
System.out.println("Try Again\n");
}
}
}
You can resolve this, by placing the "retry" one level deeper in the code:
import java.io.*;
import java.util.*;
class Add {
public static void main(String args[]) throws Exception {
boolean loop=true;
Scanner s=new Scanner(System.in);
while(loop) {
try {
String yn;
do {
loop = true;
System.out.println("Enter how many numbers to add: ");
int num=Integer.parseInt(s.next());
int a,sum=0;
for(int i=1;i<=num;) {
try {
System.out.println("Enter number["+i+"]: ");
a=Integer.parseInt(s.next());
sum=sum+a;
i++;
}
catch(Exception e) {
System.out.println("Invalid input. Try Again.\n");
}
}
System.out.println("The Sum is:"+sum);
System.out.println("Do you want to continue?(Y/N):");
yn=s.next();
loop = yn.equals("y")||yn.equals("Y");
} while(loop);
}
catch(Exception e) {
System.out.println("Number of elements invalid. Try Again.\n");
}
}
}
}
As #Zhuinden indicates, one better uses Scanner.nextInt, since it is more generic...
Below code should help
int i =1;
while(i <=num){
System.out.println("Enter number["+i+"]: ");
try{
a=Integer.parseInt(s.next());
sum=sum+a;
i++;
}catch(NumberFormatException ex){
System.out.println("Please enter a valid interger");
}
}
Use additional method that reads single entry until it is valid number:
private static int readNumber(Scanner s) {
Integer value = null;
while (value == null) {
try {
value = Integer.parseInt(s.next());
} catch (NumberFormatException e) {
System.out.println("bad format, try again...");
}
}
return value;
}
Then you can use this method whenever you can read valid number:
System.out.println("Enter how many numbers to add: ");
int num = readNumber(s);
...
System.out.println("Enter number[" + i + "]: ");
a = readNumber(s);
....
Method getNumber(s) guarantee to return only when user give correct integer.
works according to what you want, but not ethical
import java.io.*;
import java.util.*;
class Add
{
public static void main(String args[]) throws Exception
{
boolean loop=true;int pos=0;int num=0,sum=0;
while(loop)
try
{
String yn;
do
{sum=0;num=0;
Scanner s=new Scanner(System.in);
System.out.println("Enter how many numbers to add: ");
num=Integer.parseInt(s.next());
int a;
for(int i=1;i<=num;i++)
{
System.out.println("Enter number["+i+"]: ");
a=Integer.parseInt(s.next());
sum=sum+a;pos=i;
}
System.out.println("The Sum is:"+sum);
System.out.println("Do you want to continue?(Y/N):");
yn=s.next();
} while(yn.equals("y")||yn.equals("Y"));
}
catch(Exception e)
{
Scanner s=new Scanner(System.in);
int a=0;
for(int i=pos+1;i<=num;i++)
{ System.out.println("Enter number["+i+"]: ");
a=Integer.parseInt(s.next());
sum=sum+a;}
System.out.println("The Sum is:"+sum);
}
}
}
I need int num to only accept numbers. If I input letters I get an error. Is there a way to immediately flag letters, or do I have to take num in as a string and run loops?
import java.util.Scanner;
public class Test
{
public static void main(String[] args)
{
Scanner input = new Scanner(System.in);
System.out.println("Input a number.");
int num = input.nextInt();
}
}
You must use Scanner.hasNextInt():
It Returns true if the next token in this scanner's input can be interpreted as an int value in the default radix using the nextInt() method. The scanner does not advance past any input.
public static void main(String[] args)
{
System.out.println("Input a number.");
Scanner sc = new Scanner(System.in);
System.out.print("Enter number 1: ");
while (!sc.hasNextInt()) sc.next();
int num = sc.nextInt();
System.out.println(num);
}
You probably want to do something like this:
import java.util.InputMismatchException
import java.util.Scanner;
public class Test {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.println("Input an integer.");
int num = 0; // or any other default value
try {
num = input.nextInt();
} catch (InputMismatchException e) {
System.out.println("You should've entered an integer like I told you. Fool.");
} finally {
input.close();
}
}
}
If the user enters something that is not an integer, the code within the catch block will be executed.
I want the user to enter a number which is scanned by the following code:
scanner.nextInt();
If a user enters a string instead, the program throws InputMismatchException, which is obvious. I want to catch the exception in such a way that the program prompts the user to enter an input until the user enters an integer value.
Scanner scanner = new Scanner(System.in);
while(true) {
try {
System.out.println("Please enter a number: ");
int input = scanner.nextInt();
System.out.println(input);
//statements
break;
}
catch(InputMismatchException | NumberFormatException ex ) {
continue;
}
}
This code creates an infinite loop if a string is entered.
The answer to my problem is as follows:
Scanner scanner = new Scanner(System.in);
while(true) {
try {
System.out.println("Please enter a number: ");
int input = scanner.nextInt();
System.out.println(input);
//statements
break;
}
catch(InputMismatchException | NumberFormatException ex ) {
scanner.next();//new piece of code which parses the wrong input and clears the //scanner for new input
continue;
}
}
Put Scanner scanner = new Scanner(System.in); within your while loop.
Scanner scanner;
while(true) {
try {
System.out.println("Please enter a number: ");
scanner = new Scanner(System.in);
int input = scanner.nextInt();
System.out.println(input);
//statements
break;
}
catch(InputMismatchException | NumberFormatException ex ) {
System.out.println("I said a number...");
}
}
How about this?
while(true) {
try {
System.out.println("Please enter a number: ");
Scanner scanner = new Scanner(System.in);
int input = scanner.nextInt();
System.out.println("\n\nEntered number is : " + input);
break;
} catch(InputMismatchException | NumberFormatException ex ) {
System.out.println("\n\nInput was not a number. Please enter number again : ");
} catch(Exception e ) {
System.out.println("\n\nException caught :: " + e);
}
}
I have also removed continue syntax as those are not needed.