Question

Is it possible to check for a NumberFormatException for input string: ""?

I tried to write my program so that if the user didn’t put in any values an error message would come out instead of NumberFormatException:

if(pasientNavnFelt.getText() == "" || pasientNrFeIt.getText() == "")
{
  utskriftsområde.setText("ERROR, insert values");
}

if(pasientNavnFelt.getText() != "" || pasientNrFeIt.getText() != "")
{ 
  // rest of code here if the program had values in it
}

I also tried with null:

if(pasientNavnFelt.getText() == null || pasientNrFeIt.getText() == null)
{
  utskriftsområde.setText("ERROR, insert values");
}

if(pasientNavnFelt.getText() != null || pasientNrFeIt.getText() != null)
{ 
  // rest of code here if the program had values in it
}

I still get:

Exception in thread "AWT-EventQueue-0" java.lang.NumberFormatException: For input string: ""

The program works fine if it has values.

Answer 1

Never compare Strings with ==. == checks that the two objects are the same, and not that the two objects have the same characters. Use equals() to compare strings.

That said, to validate that a string is a valid Integer, you indeed need to catch the exception:

try {
    int i = Integer.parseInt(s);
    // s is a valid integer
}
catch (NumberFormatException e) {
    // s is not a valid integer
}

This is basic Java stuff. Read the Java tutorial over exceptions.