[Codebug]

I was reading through my newest java book in the section about dialog boxes and reading input from the keyboard. It showed how to use the JOptionPane class to take a user inputted number like 72 and convert it to an actual integer. I understand that whatever a user enters at a JOption prompt is converted to a string. To convert a number entered in the prompt, you must use the Integer.parseInt method. My question is, if I want to write a program that asks the user to enter a number, any integer value, how can I prevent them from entering anything but an integer value. For example, if the user is prompted to enter an integer value and they enter "dog", I want the program to print an error message and reject what they entered. I've thought about this for a while and haven't come to any solid conclusions. I've come up with a sort of pseudo code but I'm having trouble translating it to java code:

if(user enters appropriate value)
{
perform operation
}

else
{
print an error message and reject user input
}

I hope I've made some sense and you guys can understand what I'm asking.

Thanks for the help.

*Edited: topic title edited

This post has been edited by pbl: 19 December 2009 - 10:16 PM

[Calix_]

try something like this maybe:

int1 = getScannerInput.anInt();

if
int1 = (int || double)

System.out.println("This works");

else

System.out.println("Enter a number");

This post has been edited by Calix_: 19 December 2009 - 06:17 PM

[macosxnerd101]

To do something like this, you will want to use a try-catch block, which are used for error handling. In the try-block, you place code which may throw an error. In the catch-block, you put in code to handle the error in the event that it occurs; and if the error doesn't occur, then this code isn't executed.

Since the Scanner nextInt() method throws an InputMismatchException, you can use a try-catch block here. Like so:

int x = 0;
try{
   x = scannerName.nextInt(); //try to get input
}
catch(InputMismatchException e){
   System.out.println("Invalid input");
}

Using the try-catch block in combination with a loop, you can actually force the input prompt to occur repeatedly until the user enters a valid input.

[Codebug]

Calix_, on 19 Dec, 2009 - 05:13 PM, said:

try something like this maybe:

int1 = getScannerInput.anInt();

if
int1 = (int || double)

System.out.println("This works");

else

System.out.println("Enter a number");

Are you sure that will work? I didn't think that you could use int or double in an argument as the compiler would think it was a variable or something else. I could be wrong.

macosxnerd101, on 19 Dec, 2009 - 05:35 PM, said:

To do something like this, you will want to use a try-catch block, which are used for error handling. In the try-block, you place code which may throw an error. In the catch-block, you put in code to handle the error in the event that it occurs; and if the error doesn't occur, then this code isn't executed.

Since the Scanner nextInt() method throws an InputMismatchException, you can use a try-catch block here. Like so:

int x = 0;
try{
   x = scannerName.nextInt(); //try to get input
}
catch(InputMismatchException e){
   System.out.println("Invalid input");
}

Using the try-catch block in combination with a loop, you can actually force the input prompt to occur repeatedly until the user enters a valid input.

Thanks for that info. Really helpful. Could you explain how I could do this with the JOptionPane prompt?

[macosxnerd101]

Codebug, on 19 Dec, 2009 - 09:36 PM, said:

Calix_, on 19 Dec, 2009 - 05:13 PM, said:

try something like this maybe:

int1 = getScannerInput.anInt();

if
int1 = (int || double)

System.out.println("This works");

else

System.out.println("Enter a number");

Are you sure that will work? I didn't think that you could use int or double in an argument as the compiler would think it was a variable or something else. I could be wrong.

You're right- that actually won't work. You're better off using the try-catch as I showed above.

@Calix: You can't test for type using the assignment operator. Java has the instanceof keyword that allows you to test for reference types, but not primitives if that helps.

[pbl]

Don't really understand where you are going at
If you are going to use JOptionPane, as your topic title suggests, what are you doing with Scanner ?

[Codebug]

pbl, on 19 Dec, 2009 - 06:19 PM, said:

Don't really understand where you are going at
If you are going to use JOptionPane, as your topic title suggests, what are you doing with Scanner ?

I wasn't doing anything with Scanner, macosxnerd101 was talking about it.

[pbl]

macosxnerd101, on 19 Dec, 2009 - 05:35 PM, said:

To do something like this, you will want to use a try-catch block, which are used for error handling. In the try-block, you place code which may throw an error. In the catch-block, you put in code to handle the error in the event that it occurs; and if the error doesn't occur, then this code isn't executed.

Since the Scanner nextInt() method throws an InputMismatchException, you can use a try-catch block here. Like so:

int x = 0;
try{
   x = scannerName.nextInt(); //try to get input
}
catch(InputMismatchException e){
   System.out.println("Invalid input");
}

Using the try-catch block in combination with a loop, you can actually force the input prompt to occur repeatedly until the user enters a valid input.

Macosxnerd we are talking about JOptionPane
what are you doing with Scanner ?

[macosxnerd101]

I skimmed over the OP and saw Calix talking about Scanner, so I thought we were talking about Scanner (and that the title was poorly worded). Anyways, I should have read the OP more thoroughly. Sorry about the confusion.

Anyways, same concept but slightly different steps. The parseInt() method, by the way, throws a NumberFormatException instead of the InputMismatchException from Scanner.

String input = JOptionPane.showInputDialog("enter input");
int x = 0;
try{x = Integer.parseInt(input);}
catch(NumberFormatException e){JOptionPane.showMessageDialog(null, "Invalid input");}

Sorry again about the confusion. My mistake completely.

This post has been edited by macosxnerd101: 19 December 2009 - 08:40 PM

[Codebug]

macosxnerd101, on 19 Dec, 2009 - 07:38 PM, said:

I skimmed over the OP and saw Calix talking about Scanner, so I thought we were talking about Scanner (and that the title was poorly worded). Anyways, I should have read the OP more thoroughly. Sorry about the confusion.

Anyways, same concept but slightly different steps. The parseInt() method, by the way, throws a NumberFormatException instead of the InputMismatchException from Scanner.

String input = JOptionPane.showInputDialog("enter input");
int x = 0;
try{x = Integer.parseInt(input);}
catch(NumberFormatException e){JOptionPane.showMessageDialog(null, "Invalid input");}

Sorry again about the confusion. My mistake completely.

Its quite alright. No worries. You have been a huge help. Thanks!

p.s. For future reference on my part, how could I more clearly and concisely word my titles? I thought that my title was fairly descriptive but nonetheless, I want to make sure everyone can understand what I'm asking.

[macosxnerd101]

For a new member, this was a pretty good title. If it was me, I would have named it something like "input validation." However, the miscommunication occurred when Calix started talking about Scanner off the wall, which threw me off. Again, I apologize for the confusion, and I'm glad I could be of assistance.

[pbl]

macosxnerd101, on 19 Dec, 2009 - 09:00 PM, said:

For a new member, this was a pretty good title. If it was me, I would have named it something like "input validation." However, the miscommunication occurred when Calix started talking about Scanner off the wall, which threw me off. Again, I apologize for the confusion, and I'm glad I could be of assistance.

Done my friend
And thanks for your quick fix