Catch blank variable int input in java

Question

I want my code to catch if the user didn't input anything. I can't use ab.equals(null), ab.isEmpty() because I convert the String to integer as you can see.

import javax.swing.JOptionPane;
public class Mango {


    public static void main (String[] args){

        String ab = JOptionPane.showInputDialog("Enter Anything");
        double sum = 0;
        double x = Integer.parseInt(ab);

        if(x <= 0){

            System.out.println("Empty!");

        }else{

            sum+=x; 

        }
    }

MadProgrammer · Accepted Answer · 2014-12-28 07:42:23Z

3

Check the return value BEFORE you try converting it...

String ab = JOptionPane.showInputDialog("Enter Anything");
if (ab != null && !ab.trim().isEmpty()) {
    // Try and convert the value...
} else {
    // Bad input
}

Example

double sum = 0;
String ab = JOptionPane.showInputDialog("Enter Anything");
if (ab != null && !ab.trim().isEmpty()) {
    double x = Integer.parseInt(ab);
    sum += x;
} else {
    System.out.println("\"" + ab + "\" is not a valid input value");
}

edited Dec 28, 2014 at 7:42

answered Dec 28, 2014 at 7:28

MadProgrammer

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4 Comments

SMA Over a year ago

You might need to use try catch as well ?

MadProgrammer Over a year ago

@almasshaikh That depends, NumberFormatException is a RuntimeException and you should avoid using try-catch blocks to control logic flow. Not saying you should never do it, you just need to provide the use-case justificaiton for it...

SMA Over a year ago

It's edge cases that OP wants to cover and hence i asked for the same :)

MadProgrammer Over a year ago

@almasshaikh This is tre, as I said, you need to make the justification on each case, you could, for example use a regular expression to test for a numeric value before doing the conversion

Eran · Accepted Answer · 2014-12-28 07:27:47Z

2

You have to catch NumberFormatException, since that's the exception that would be thrown if you call parseInt on an empty String:

double x = -1;
try {
    x = Integer.parseInt(ab);
}
catch (NumberFormatException exc) {
    x = -1;
}

You can check if the String is empty prior to calling parseInt, but then you might still get this exception if the user enters some non-numeric characters or a too large number.

answered Dec 28, 2014 at 7:27

Eran

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3 Comments

MadProgrammer Over a year ago

This is a personal nit pick, so I don't think the answer is "wrong", but you should generally try an avoid using try-catch for branching logic

Eran Over a year ago

@MadProgrammer To avoid having to catch the exception entirely, you'll need to implement some of the logic that parseInt already contains - check that all characters of the String are digits, and that the number is not too large (or too small) to fit in an int.

MadProgrammer Over a year ago

That's true. Considering NumberFormatException is a RuntimeException, it also makes it more dangerous to trap it. As a general rule of thumb, it's unadvisable to use try-catch blocks to control program logic, not saying your NEVER should do it, just that's it's unadvisable. You could equally use a regular expression, for example, to do a quick check on the validity of the String

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