Develop Reference

How to loop through until user inputs a valid integer?

I'm new to java and am trying to validate numbers being entered into the console. I want an integer, but I know if a letter is entered for example an error would occur, so I thought I'd use try & catch. This works if try and do it first time round, but I want to loop through until the user inputs a valid integer. Got this working, but when I get a valid number and print out the number I get a list of all attempts.... Hope this makes sense
import java.util.Scanner;
public class ConvertStringInt {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
int i = 0;
System.out.print("Enter a whole number: ");
try {
i = sc.nextInt();
} catch (Exception e){
System.out.println("Error");
main(args);
}
System.out.println(i);
}
}
Here is the output from the console....
Enter a whole number: a
Error
Enter a whole number: d
Error
Enter a whole number: 2.0
Error Enter a whole number: 1
1
0
0
0
Could somebody please explain this?
Thanks in advance
Neil

You are seeing outputs for each input because you call main recursively. If you immediately type a correct int, the flow is this:
main
ask for input -> int
print i (1)
But in your case the input is not an int. This is what happens: you type the first input, it fails. You do not print yet because you first call main again, asking for the next input. Only when you get a correct int you print, and then finish and allow the previous main-execution to finish by printing, which then allows the previous... and so on:
main(args)
ask for input -> a !int
main(args)
ask for input -> d !int
main(args)
ask for input -> 2.0 !int
main(args)
ask for input -> 1 int
print 1 (1)
print 0 (2.0)
print 0 (d)
print 0 (a)
Look at Ravi's answer for a proper way to repeatedly ask for input without using try/catch (which is discouraged).

You could check for integer token in loop
while (!sc.hasNextInt()) // loop until next token is integer
{
// do something or print error
System.out.println(sc.next() +"is not number");
}
i = sc.nextInt();

What do I need to do in order to write my input on a new line in Java?

So after I input the numbers needed for the while instruction, it should display
the result and then the "Enter your name here:" text.
Afterwards I should be able to input text, but apperantly I can't, it just displays the "Hello" message and jumps to the next instruction.
However, if I write the text right after the series of numbers, it works just fine. How can I make the program let me input text after the "Enter your name here" message?
import java.util.Scanner;
public class apples {
public static void main(String[] args) {
Scanner cristi = new Scanner(System.in);
int age= 21, k=10, nr=1, sum=0, val=0;
k=cristi.nextInt();
while(nr<=k){
val=cristi.nextInt();
sum=sum+val;
nr++;
}
sum=sum/k;
System.out.println(sum);
System.out.println(age>50 ? "You are old!" : "You are young!");
tuna tunaObject= new tuna();
System.out.println("Enter the name here: ");
String a= cristi.nextLine();
tunaObject.simpleMessage(a);
System.out.println("Enter first name: ");
String temp= cristi.nextLine();
tunaObject.setName(temp);
tunaObject.afisaj();
}
}

This is an issue about buffers and Scanner. To resolve your issue, just do cristi.nextLine() after each cristi.nextInt()so you clear the buffer.
For more info, you can look at this thread

Add the line:
cristi.nextLine();
after the while loop to solve your problem.
Edit: #Xatyrian answer provides more context.

Java scanner expecting more inputs than needed

Im having some trouble with scanning user input in one of my first java programs. When I compile and run this, I am immediately prompted for input (i.e the command line stops and blinks). When I enter anything, the first line is printed, asking me to enter an integer. Then the second line is printed and I'm prompted to enter another value.
The output from this program is the first two values that I input. This is hard to explain, but it basically asks for 3 input values and only uses two.
import java.util.Scanner;
public class objects
{
public static void main(String[] args)
{
Scanner sc = new Scanner(System.in);
System.out.println("Enter an integer please...");
int input = sc.nextInt();
System.out.println("Enter your name please...");
String name = sc.nextLine();
System.out.println("The read values: " + input + ", " + name);
sc.close();
}
}

Put a System.out.flush() command after your println statements if you're reading from the console directly afterward

just use this:
Scanner sc = new Scanner(System.in);
System.out.print ("Enter your name please... ");
String name = sc.nextLine();
System.out.print ("Enter an integer please... ");
int input = sc.nextInt();
System.out.println ("The read values: " + input + ", " + name);
i just moved the integer below the name and it sorta fixed it. hahaha

When you introduce a number you press enter key, nextInt() uses the number but the enter (\n) remains buffered. After this if you call again nextInt(), Java tries to convert \n into a number giving you a NumberFormatException, but if you invoke nextLine() they read the enter as empty string
Here you have a better explanation and one solution
Can't use Scanner.nextInt() and Scanner.nextLine() together

It seems this is an error to do with my installation of VirtualBox. No matter what I try, the problem persists. Even if i try to only read ONE integer, it will ask me to input two values.
Thanks for everyone who tried to help, I learned a lot just trying to debug this.

ArrayIndexOutOfBoundsException on a Split String in a array

So I am doing some problems on the UVa online problem judge, but on a relativity easy problem, I keep on getting a ArrayIndexOutOfBoundsException. To understand the code, here is the problem.
import java.util.Arrays;
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
int t = scan.nextInt();
int sum = 0;
for (int i = 0; i <= t; i++){
String d = scan.nextLine();
if (d.equals("report")) {
System.out.println(sum);
} else {
String[] parts = d.split(" ");
int z = Integer.parseInt(parts[1]);
sum+=z;
}
}
}
}
The error message is:
reportException in thread "main" java.lang.ArrayIndexOutOfBoundsException: 1
at Main.main(Main.java:16)
And I am using the sample input given.
Edit:
I have already tried added println statements in the code and figured out that the number is not being read. I am trying to understand why.

OK, after some messing around on my machine I think I found what might be at least part of the problem. The issue is that I'm not sure what the precise input is, so I'm going off of what I could get working on my machine.
So you start up your program, and it waits for a prompt at this line:
int t = scan.nextInt();
You enter your integer, and the program moves on as expected:
Input: 100 // Then press enter to continue
The input is parsed, and now t is set to 100.
Then when your program enters your for loop, it comes across this line:
String d = scan.nextLine();
Yet for some reason the program doesn't wait for input! (Or at least it didn't on my machine)
I believe the issue lies here:
Input: 100 // Then press enter to continue
^^^^^^^^^^^
What I think is happening is that your input is really
Input: 100\n
^^
That character (\r\n on Windows) is what's input when you hit enter. It's a newline character that tells the console to go to the next line.
So as a result, what I think happens is this:
Input: 100\n
Scanner parses 100, leaving the \n in the input stream
Then at the nextLine() call, the scanner sees \n on the input stream, which denotes end of line, so it thinks you already input the entire line! Because what it thought was your input was only the newline character, it returns an empty string, because your "input" was an empty string and the newline character. Your program then goes to split the newline character by spaces, rightly returns an array with a single element, and then your program promptly crashes when accessing an out-of-bounds index.
What might work better is reading an entire line first and parsing the integer so your scanner doesn't get ahead of itself, like this:
int t = Integer.parseInt(scan.nextLine());
Just as a warning: This is what I've been able to come up with based on using OP's code as-is on my machine. I was unable to get a situation where the only element in parts was "donate". I will update further as I get more info.

The error message means the array parts's length less than 2, sometimes.
It means the variable d does not always contain the string BLANK SPACE, " ", what you split by.
try this code:
import java.util.Arrays;
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
int t = scan.nextInt();
int sum = 0;
for (int i = 0; i <= t; i++){
String d = scan.nextLine();
if (d.equals("report")) {
System.out.println(sum);
} else {
String[] parts = d.split(" ");
/*
* Add IF statement,
*/
if (parts.length() > 1) {
int z = Integer.parseInt(parts[1]);
sum+=z;
}
}
}
}
}

Problem in looping when using method in Java

I'm doing a simple program regarding methods.
But I have one problem. Everything is already working except when looping.
When I choose to loop again. The program skips on inputting the name. And proceeds directly to the year and section.
Here's the code:
public static void main(String[] args) {
do{
System.out.println("Input info:");
name=stringGetter("Name: ");
yearandsec=stringGetter("Year and section: ");
sex_code=charGetter("Sex code: " + "\n" + "[M]" + "\n" + "[F]:");
scode=intGetter("Scholarship code: ");
ccode=intGetter("Course code: ");
units=intGetter("Units: ");
fee_per_unit=doubleGetter("Fee per unit: ");
misc=doubleGetter("Miscellaneous: ");
display();
switches(scode, units, fee_per_unit, misc);
System.out.println("Another?");
dec=rew.nextInt();
}while(dec==1);
}
Here's the method getting the value for name together with the year and section:
public static String stringGetter(String ny){
String sget;
System.out.println(ny);
sget=rew.nextLine();
return sget;
}
I'm really annoyed with this problem, and I don't have any idea on how to fix this. Please help. thanks

Here is a simpler and more complete program that reproduces the error:
public static Scanner rew = new Scanner(System.in);
public static void main(String[] args) {
int dec;
do {
System.out.println("Input info:");
String name=stringGetter("Name: ");
String yearandsec=stringGetter("Year and section: ");
dec=rew.nextInt();
} while(dec==1);
}
public static String stringGetter(String ny){
System.out.println(ny);
return rew.nextLine();
}
The problem is that after calling nextInt() the call to nextLine() reads up to the new line after the int (giving a blank line), not up to the next new line.
If you change dec to a String and change dec=rew.nextInt(); to dec=rew.nextLine(); then it will work fine. Here is a complete example that you can copy and paste into a blank file to see that it works correctly:
import java.util.*;
public class Program
{
public static Scanner rew = new Scanner(System.in);
public static void main(String[] args) {
String dec;
do {
System.out.println("Input info:");
String name = stringGetter("Name: ");
String yearandsec = stringGetter("Year and section: ");
dec = stringGetter("Enter 1 to continue: ");
} while(dec.equals("1"));
}
public static String stringGetter(String ny){
System.out.println(ny);
return rew.nextLine();
}
}
You may also want to consider adding proper parsing and validation to your program. Currently your program will behave in an undesirable way if the user enters invalid data.

The line:
dec = rew.nextInt();
Is reading an int value from the input stream and is not processing the newline character, then when you come back to point where you get the name at which point a new line is still in the Reader's buffer and gets consumed by the stringGetter returning an empty value for name.
Change the line to do something like:
do {
//....
s = stringGetter("Another (y/n)? ");
} while ("y".equals(s));

Well you haven't told us what "rew" is, nor what rew.nextInt() does. Is it possible that rew.nextInt() is waiting for the user to hit return, but only actually consuming one character of the input - so that the next call to rew.nextLine() (for the name) just immediately takes the rest of that line? I suspect that's what's happening because you're using System.in - usually reading from System.in only gives any input when you hit return.
(It's possible that this is also only a problem on Windows - I wonder whether it consumes the "\r" from System.in as the delimiter, leaving "\n" still in the buffer. Not sure.)
To test this, try typing in "1 Jon" when you're being asked whether or not to continue - I think it will then use "Jon" as the next name.
Essentially, I think using Scanner.nextInt() is going to have issues when the next call is to Scanner.nextString(). You might be better off using a BufferedReader and calling readLine() repeatedly, then parsing the data yourself.