In line 4 of this code, there is a resource leak warning. input=scan();
I want a method that takes an integer from the user. It will ask the user to re-enter an integer value if the value he previously entered is unacceptable, i.e. chars, spec. chars, etc. Also, I want to close the scanner before returning the int taken.
How do I fix this problem?
public static int readInt() {
Scanner input = scan();
while(!input.hasNextInt()) {
input = scan();
int n = input.nextInt();
while(n<0) {
System.out.println("Input should be a positive interger!");
n = readInt();
}
input.close();
return n;
}
return -1;
}
public static Scanner scan() {
System.out.print("Enter number: ");
Scanner input = new Scanner(System.in);
return input;
}
Use it with try catch block and close it in finally
finally {
input.close();
}
You should always close your Scanner when you're done with it:
You shouldn't create a new instance of Scanner every time you want to read something, you should create one instance and keep calling nextInt().
Also, I don't think your loop quite makes sense. I've rewritten the code slightly - if you want to keep asking the user for a number until n is positive, it makes sense to have n < 0 as your loop condition.
For example:
public static int readInt() {
Scanner input = new Scanner(System.in);
int n = -1;
while (n < 0) {
System.out.print("Enter number: ");
n = input.nextInt();
if (n < 0) {
System.out.println("Input should be a positive interger!");
}
}
input.close();
return n;
}
(also, the Scanner constructor throws a FileNotFoundException so you'd need to handle that in the code above - how did your example even compile?)
Related
For java practice, i am trying to create a program that reads integers from the keyboard until a negative one is entered.
and it prints the maximum and minimum of the integer ignoring the negative.
Is there a way to have continuous input in the same program once it runs? I have to keep running the program each time to enter a number.
Any help would be appreciated
public class CS {
public static void main(String []args) {
Scanner keys = new Scanner(System.in);
System.out.println("Enter a number: ");
int n = keys.nextInt();
while(true)
{
if(n>0)
{
System.out.println("Enter again: ");
n = keys.nextInt();
}
else
{
System.out.println("Number is negative! System Shutdown!");
System.exit(1);
}
}
}
}
Here is a part of my code - It works, but i think there is an easier way of doing what i want but not sure how!
import java.util.Scanner;
public class ABC {
public static void main(String []args) {
int num;
Scanner scanner = new Scanner(System.in);
System.out.println("Feed me with numbers!");
while((num = scanner.nextInt()) > 0) {
System.out.println("Keep Going!");
}
{
System.out.println("Number is negative! System Shutdown!");
System.exit(1);
}
}
}
You could do something like:
Scanner input = new Scanner(System.in);
int num;
while((num = input.nextInt()) >= 0) {
//do something
}
This will make num equal to the next integer, and check if it is greater than 0. If it's negative, it will fall out of the loop.
A simple loop can solve your problem.
Scanner s = new Scanner(System.in);
int num = 1;
while(num>0)
{
num = s.nextInt();
//Do whatever you want with the number
}
The above loop will run until a negative number is met.
I hope this helps you
I am attempting to make sure the user input int type only and make sure the integer inputted is greater than 0.
I was able to come up with the following to make sure the input is int type:
Scanner scan = new Scanner(System.in);
while(!scanner.hasNextInt())
{
scanner.next();
}
int input = scan.nextInt();
But how should I include a condition checking to make sure the integer is greater than 0 as well?
The problem with your current approach is you've already ready the value from the Scanner before it reaches int input = scan.nextInt();, meaning that by the time you use nextInt, there's nothing in the Scanner to be read and it will wait for the next input from user...
Instead, you could read the String from the Scanner using next, use Integer.parseInt to try and parse the result to an int and then check the result, for example...
Scanner scanner = new Scanner(System.in);
int intValue = -1;
do {
System.out.print("Please enter a integer value greater than 0: ");
String next = scanner.next();
try {
intValue = Integer.parseInt(next);
} catch (NumberFormatException exp) {
}
} while (intValue < 0);
System.out.println("You input " + intValue);
put an if statement inside your while loop like this
if(num <= 0){
System.out.println("Enter a number greater than zero");
}
else{
break;
}
You may use a condition in your code but not in the loop as.
`
Scanner scan = new Scanner(System.in);
abc:
while(!scanner.hasNextInt())
{
scanner.next();
}
int input = scan.nextInt();
if(input <= 0){
goto abc;
}
`
using .isDigit() method then checking to see if that number is greater than 0 if it is a digit
I am a newbie to Java, and as an exercise wanted to WAP a simple program to print required no. of '*' characters according to the user. But somehow, the output of this code always remains similar:
package stars;
public class Stars {
public static void main(String[] args) {
int no_stars=0;
try {
System.out.print("Enter the number of stars:");
no_stars = (int)System.in.read();
} catch ( Exception e) {
System.out.println("Error! Invalid argument!");
System.out.println();
}
printstars(no_stars);
}
public static void printstars(int n){
int i;
for(i=0;i<=n;i++)
{
System.out.println('*');
}
}
}
If I replace '*' with i, I can see that it loops upto 50/52/54, even though i run the loop no_stars times.
What seems to be the problem here?
You need to parse the number received from System.in.read() or alternatively read it as an integer, currently you just cast it, so if you enter 5, it passes 0x35 times (which is the value of the character '5')
You can do, for example:
Scanner scan = new Scanner( System.in );
printstars( scan.nextInt() );
Because you are reading ASCII code of the character from input here:
no_stars = (int)System.in.read();
It should be
no_stars = Integer.parseInt(Console.readLine());
There are two mistakes in your code.
First
System.in.read()
Is reading a byte, not a integer, so, it is parsing the integer and getting the first byte of it.
Second
for (i = 0; i <= n; i++) {
will print always one star more than requested.
So, it should be changed to
for (i = 0; i < n; i++) {
Suggestion: You could use Scanner to read your integer, for example
Scanner scanner = new Scanner(System.in);
no_stars = scanner.nextInt();
no_stars = (int)System.in.read();
This is using the ASCII value of whatever character the user enters. Try this instead:
no_stars = System.in.read() - '0';
Or, remove the no_stars variable all together,
printstars(System.in.read() - '0');
Also, in your for-loop, the condition should be i < n, in order to perform the correct number of iterations. And there's no need to declare i outside of the loop, you can just do for (int i = 0; i < n; i++).
Here is the corrected program for you: (the main problem was this line //no_stars = (int)System.in.read();)
public static void main(String[] args) {
int no_stars=0;
try{
System.out.print("Enter the number of stars:");
Scanner sc=new Scanner(System.in);
String name=sc.nextLine();
no_stars = Integer.parseInt(name);
//no_stars = (int)System.in.read();
}
catch ( Exception e) {
System.out.println("Error! Invalid argument!");
System.out.println();
}
printstars(no_stars);
}
public static void printstars(int n)
{System.out.println(n);
int i;
for(i=0;i<=n;i++)
{
System.out.println('*');
}
}
So far I have this:
public double checkValueWithin(int min, int max) {
double num;
Scanner reader = new Scanner(System.in);
num = reader.nextDouble();
while (num < min || num > max) {
System.out.print("Invalid. Re-enter number: ");
num = reader.nextDouble();
}
return num;
}
and this:
public void askForMarks() {
double marks[] = new double[student];
int index = 0;
Scanner reader = new Scanner(System.in);
while (index < student) {
System.out.print("Please enter a mark (0..30): ");
marks[index] = (double) checkValueWithin(0, 30);
index++;
}
}
When I test this, it can't take double number and I got this message:
Exception in thread "main" java.util.InputMismatchException
at java.util.Scanner.throwFor(Scanner.java:909)
at java.util.Scanner.next(Scanner.java:1530)
at java.util.Scanner.nextDouble(Scanner.java:2456)
at MarkingSystem.checkValueWithin(MarkingSystem.java:25)
at MarkingSystem.askForMarks(MarkingSystem.java:44)
at World.main(World.java:6)
Java Result: 1
How do I fix this?
Instead of using a dot, like: 1.2, try to input like this: 1,2.
Here you can see the nature of Scanner:
double nextDouble()
Returns the next token as a double. If the next token is not a float or
is out of range, InputMismatchException is thrown.
Try to catch the exception
try {
// ...
} catch (InputMismatchException e) {
System.out.print(e.getMessage()); //try to find out specific reason.
}
UPDATE
CASE 1
I tried your code and there is nothing wrong with it. Your are getting that error because you must have entered String value. When I entered a numeric value, it runs without any errors. But once I entered String it throw the same Exception which you have mentioned in your question.
CASE 2
You have entered something, which is out of range as I have mentioned above.
I'm really wondering what you could have tried to enter. In my system, it is running perfectly without changing a single line of code. Just copy as it is and try to compile and run it.
import java.util.*;
public class Test {
public static void main(String... args) {
new Test().askForMarks(5);
}
public void askForMarks(int student) {
double marks[] = new double[student];
int index = 0;
Scanner reader = new Scanner(System.in);
while (index < student) {
System.out.print("Please enter a mark (0..30): ");
marks[index] = (double) checkValueWithin(0, 30);
index++;
}
}
public double checkValueWithin(int min, int max) {
double num;
Scanner reader = new Scanner(System.in);
num = reader.nextDouble();
while (num < min || num > max) {
System.out.print("Invalid. Re-enter number: ");
num = reader.nextDouble();
}
return num;
}
}
As you said, you have tried to enter 1.0, 2.8 and etc. Please try with this code.
Note : Please enter number one by one, on separate lines. I mean, enter 2.7, press enter and then enter second number (e.g. 6.7).
I encountered the same problem.
Strange, but the reason was that the object Scanner interprets fractions depending on localization of system.
If the current localization uses a comma to separate parts of the fractions, the fraction with the dot will turn into type String.
Hence the error ...
Since you have the manual user input loop, after the scanner has read your first input it will pass the carriage/return into the next line which will also be read; of course, that is not what you wanted.
You can try this
try {
// ...
} catch (InputMismatchException e) {
reader.next();
}
or alternatively, you can consume that carriage return before reading your next double input by calling
reader.next()
Are you providing write input to the console ?
Scanner reader = new Scanner(System.in);
num = reader.nextDouble();
This is return double if you just enter number like 456.
In case you enter a string or character instead,it will throw java.util.InputMismatchException when it tries to do num = reader.nextDouble() .
For a program I am writing, I need to ask a user for an integer between 1 and 8. I've tried multiple (cleaner) ways of doing this but none of them worked, so I'm left with this:
int x = 0;
while (x < 1 || x > 8)
{
System.out.print("Please enter integer (1-8): ");
try
{
x = Integer.parseInt(inputScanner.next());
}
catch(NumberFormatException e)
{
x = 0;
}
}
Where inputScanner is a Scanner. Surely there is a better way?
Scanner does regular expressions, right? Why not check if it matches "^[1-8]$" first?
Using the nextInt() is already an improvement compare to simply using the next() method. And before that, you can use the hasNextInt() to avoid haing all this bunch of useless exceptions.
Resulting in something like this:
int x = 0;
do {
System.out.print("Please...");
if(scanner.hasNextInt()) x = scanner.nextInt();
else scanner.next();
} while (x < 1 || x > 8);
I had to do a graphic interface calculator (works only with Integers), and the problem was, that
the Tests didn't allow any Exceptions to be thrown if the input wasn't
Integer. So I couldn't use
try { int x = Integer.parseInt(input)} catch (Exception e) {dosomethingelse}
Because Java programs generally treat an input to a JTextField as a String
I used this:
if (input.matches("[1-9][0-9]*"){ // String.matches() returns boolean
goodforyou
} else {
dosomethingelse
}
// this checks if the input's (type String) character sequence matches
// the given parameter. The [1-9] means that the first char is a Digit
// between 1 and 9 (because the input should be an Integer can't be 0)
// the * after [0-9] means that after the first char there can be 0 - infinity
// characters between digits 0-9
hope this helps :)
You could try something like this:
Scanner cin = new Scanner(System.in);
int s = 0;
boolean v = false;
while(!v){
System.out.print("Input an integer >= 1: ");
try {
s = cin.nextInt();
if(s >= 1) v = true;
else System.out.println("Please input an integer value >= 1.");
}
catch(InputMismatchException e) {
System.out.println("Caught: InputMismatchException -- Please input an integer value >= 1. ");
cin.next();
}
}
Apache Commons is your friend. See NumberUtils.toInt(String, int)
String input;
int number;
while (inputScanner.hasNextLine())
{
input = inputScanner.nextLine();
if (input.equals("quit")) { System.exit(0); }
else
{
//If you don't want to put your code in here, make an event handler
//that gets called from this spot with the input passed in
try
{
number = Integer.parseInt(input);
if ((number < 1) || (number > 8))
{ System.out.print("Please choose 1-8: "); }
else { /* Do stuff */ }
}
catch (NumberFormatException e) { number = 0; }
}
}
I always like to pull in the full string so you can be sure that the user pushed the Enter button. If you just use inputScanner.nextInt() you can put two ints on a line and it will pull in one, then the other.
Example code:
int x;
Scanner in = new Scanner(System.in);
System.out.println("Enter integer value: ");
x = in.nextInt();
An array can also be used to store the integer.