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.
Related
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".
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();
}
}
Ive got a problem.I 'm new to Java,I've started today:D) ..I've programmed before so I know it little bit,but I am new to Java. Here is my code: `
public class Tutorial {
public static void main(String[] args) {
double num1,num2;
String operacia;
Scanner in=new Scanner (System.in);
System.out.println("Write 2 numbers");
num1=in.nextDouble();
num2=in.nextDouble();
System.out.println("Choose the operation");
operacia=in.nextLine();
if (operacia.equals("+")){
System.out.println("Your result is "+(num1+num2)) ;
}
else if (operacia.equals("-")){
System.out.println("Your result is "+(num1-num2)) ;
}
else if (operacia.equals("/")){
System.out.println("Your result is "+(num1/num2)) ;
}
else if (operacia.equals("*")){
System.out.println("Your result is "+(num1*num2)) ;
}
}
}`
It wants from me 2 numbers,I write them and them it writes "Choose the operation" and its over.No more inputs.Thank you very much :)
Your problem is simple.
Just replace the code with next() instead of nextLine().Effectively, the line your code is returning is receiving is a blank line. Hence when it reaches the conditional statement it has an empty string and terminates.
next()
Finds and returns the next complete token from this scanner.
nextLine()
Advances this scanner past the current line and returns the input that was skipped.
Your code should be fixed by a simple change.
public static void main(String[] args) {
double num1,num2;
String operacia;
Scanner in=new Scanner (System.in);
System.out.println("Write 2 numbers");
num1=in.nextDouble();
num2=in.nextDouble();
System.out.println("Choose the operation");
operacia=in.next();
if (operacia.equals("+")){
System.out.println("Your result is "+(num1+num2)) ;
}
else if (operacia.equals("-")){
System.out.println("Your result is "+(num1-num2)) ;
}
else if (operacia.equals("/")){
System.out.println("Your result is "+(num1/num2)) ;
}
else if (operacia.equals("*")){
System.out.println("Your result is "+(num1*num2)) ;
}
}
Scanner#nextDouble() consumes only the next token as a double from the input. It does not consume the new line you typed using the Enter on the keyboard while entering the two numbers. When the execution reaches operacia=in.nextLine();, this new line is consumed, never allowing the user a chance to type the operating string.
To solve this, you need to read the whole line using Scanner#nextLine() and convert it to a double:
String input = in.nextLine();
num1 = Double.parseDouble(input);
input = in.nextLine();
num2 = Double.parseDouble(input);
I believe the in.nextLine(); operation is reading only to the end of the line where you input 2 numbers. If you want your program to only consider the next line, you have to clear the current one first.
Try this, it should work:
System.out.println("Choose the operation");
in.nextLine(); //clear the current line
operacia=in.nextLine();
Essentially the idea of this program is to test user input and throw exceptions that I've created when invalid data is entered. For example: name cannot be empty and must be all alpha characters (no special or numeric). I have embedded this in a do-while loop that will continue so long as q is not entered to quit. I'm reading in the user input via scanner line and then sending the string inputted to a function that validates whether it meets the criteria. If it does not, then the function throws my custom exceptions. It all works fine EXCEPT when the exception is thrown it still takes that string and puts it in the new Person object.
How do I throw the exception to the user but THEN require them to re-enter the name or age until it's entered correctly?
do{
Scanner input = new Scanner(System.in);
System.out.println("Enter person info or q to quit.");
System.out.print("Please enter the name of this person:");
String name = input.nextLine();
if(name.equalsIgnoreCase("q"))
{
break;
}
try{
isName(name);
}catch (InvalidNameException n){
System.out.println(n);
}
System.out.print("Please enter an age for this person:");
String age = input.nextLine();
try{
isValidAge(age);
}catch(InvalidAgeException a){
System.out.println(a);
}
public static void isName(String name) throws InvalidNameException
{
if(name.isEmpty())
{
throw new InvalidNameException("You did not enter a name.");
}
String[] namez = name.split(" ");
for(int i=0;i<namez.length;i++)
{
char[] charz = namez[i].toCharArray();
for (char n : charz)
{
if(!Character.isLetter(n))
{
throw new InvalidNameException("You have entered an invalid name.");
}
}
}
}
Put a continue; in your exception handling. It will break the loop an reenters it.
I would assume that the error lies within the compatibility of your isName() method and the method loop shown. It probably happens after it sets the name to a variable too. I cant tell you anything really specific because I cant see the isName method though.
The easiest way I know of doing this is to validate the obtained String using a regular expression. You can do something like this:
Scanner input = new Scanner(System.in);
System.out.print("Enter your name: ");
String name = input.nextLine();
String regex = "[A-Z a-z]+(\\s)[A-Z a-z]+";
System.out.println(name.matches(regex)? "matches": "does not match");
The expression regex is used to evaluate a sequence of alpha characters (no numbers or special characters) separated by a space. So, something like: "Joe Smith" will pass validation, but something like "Joe 123Klkjsd" will not.
You can take this code and test the input String in a while() loop:
while(!name.matches(regex))
{
// Prompt the user to re-enter a valid name and assign to name variable
}
Something like that should work.
It would be better to evaluate each variable within a do-while loop. Thus if there is an error in the variable age would not necessarily re-enter the name.
Scanner input = new Scanner(System.in);
String name;
String age;
System.out.println("Enter person info or q to quit.");
do{
System.out.print("Please enter the name of this person: ");
name = input.nextLine();
if(name.equalsIgnoreCase("q")) break;
try{
isName(name);
break;
}catch (InvalidNameException n){
System.out.println(n);
continue;
}
} while (true);
if(!name.equalsIgnoreCase("q"))
do{
System.out.print("Please enter an age for this person: ");
age = input.nextLine();
if(age.equalsIgnoreCase("q")) break;
try{
isValidAge(age);
System.out.printf("Nombre; %s\nEdad: %s",name,age);
break;
}catch (InvalidAgeException a){
System.out.println(a);
continue;
}
} while (true);
Basic problem here.. I will start off by asking that you please not respond with any code, as that likely will only confuse me further (programming noob). I am looking for a clear explanation on how to solve this issue that I'm having.
I have a scanner that reads input from the user. The user is prompted to enter an int value between 1 to 150 (whole numbers only). I obtain the value as follows:
Scanner scan = new Scanner(System.in);
int input = scan.nextInt();
And continue on with my program, and everything works fine.
Unfortunately, the code isn't exactly bulletproof, since any input that is not an integer can break it (letters, symbols, etc).
How can I make the code more robust, where it would verify that only an int was entered?
These are the results I'm hoping for:
Lets say the input was:
23 -> valid
fx -> display an error message, ask the user for input again (a while loop would do..)
7w -> error, again
3.7 -> error
$$ -> error
etc
Scanner.hasNextInt() returns true if the next token is a number, returns false otherwise.
In this example, I call hasNextInt(). If it returns true, I go past the while and set the input; if it returns false, then I discard the input (scanner.next();) and repeat.
Scanner scan = new Scanner(System.in);
while(!scan.hasNextInt()) {
scan.next();
}
int input = scan.nextInt();
Here's a simple example with prompts and comments.
Scanner scan = new Scanner(System.in);
System.out.print("Enter an integer: "); // Initial prompt for input
// Repeat until next item is an integer
while (!scan.hasNextInt())
{
scan.next(); // Read and discard offending non-int input
System.out.print("Please enter an integer: "); // Re-prompt
}
// At this point in the code, the user has entered an integer
int input = scan.nextInt(); // Get the integer
// And now you can use the input variable.
Use scan.hasNextInt() to make sure the next input is an int.
I have written an example that ensures that the program will continue only if a number and not an invalid value is entered. Do not worry, I added the desired explanation.
The program asks the user to input a number. A loop ensures that the processing will not go on until a valid number is entered. Before that I have defined a variable "inputAccepted" that has false as default value. If he enters a number, the variable "inputAccepted" is set to true and the program leaves the loop. But if he enters something else than a number, an exception is thrown right in this moment, and the line that sets the variable "inputAccepted" to true will not be executed. Instead a message will be printed out that tells the user that his input is not valid. Since "inputAccepted" could not be set to true, the loop will do the same stuff again until the string can be converted to a number.
You can test the program here.
import java.util.Scanner;
public class Test {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
boolean inputAccepted = false;
while (!inputAccepted) {
try {
System.out.print("Please enter a number: ");
Integer.valueOf(input.nextLine());
inputAccepted = true;
} catch (NumberFormatException e) {
System.out.println("Not a valid number.");
}
}
System.out.println("Thank you!");
}
}
Just get "anything" and parse it:
Scanner scan = new Scanner(System.in);
Integer number = null;
while (number == null) {
try {
number = Integer.parseInt(scan.next());
} catch (NumberParseException e) {
System.out.println("bad input: " + input);
}
}
Without any code and just in English, I'd say there's two things you have to test or look out for. First that the input is an int, second that the int is within the correct range.
In terms of pseudocode, the first thing to do is make sure it's an int. Declaring an int named "input", I would put a try / catch block, where you try to scan in the user input as an int, with parseInt(). If the try part fails, you know it's not an int and can return an error message.
Then, now that you know that "input" is an int, you can test whether it is less than 1 or more than 150, and return an error message if so!
public class Sample {
/**
* author CLRZ
*/
public static void main(String[] args) {
int a; // variable
Scanner in = new Scanner(System.in); // scans your input
System.out.println("Enter your number's choice:");
int sem1 = in.nextInt(); // reads next integer
if (sem1 == 1) // conditioned if your choice number is equal to 1
System.out.println("Hello World1"); // output wil be Hello World
int b;
System.out.println("Enter your number's choice:");
int sem2 = in.nextInt();
if (sem2 == 2)
System.out.println("Hello World2");
int c;
System.out.println("Enter your number's choice:");
int sem3 = in.nextInt();
if (sem3 == 3)
System.out.println("Hello World3");
}
}