How can I tell if a Java integer is null?

Question

Greetings,

I'm trying to validate whether my integer is null. If it is, I need to prompt the user to enter a value. My background is Perl, so my first attempt looks like this:

int startIn = Integer.parseInt (startField.getText());

if (startIn) { 
    JOptionPane.showMessageDialog(null,
         "You must enter a number between 0-16.","Input Error",
         JOptionPane.ERROR_MESSAGE);       
}

This does not work, since Java is expecting boolean logic.

In Perl, I can use "exists" to check whether hash/array elements contain data with:

@items = ("one", "two", "three");
#@items = ();

if (exists($items[0])) {
    print "Something in \@items.\n";
}
else {
    print "Nothing in \@items!\n";
}

Is there a way to this in Java? Thank you for your help!

Jeremiah

P.S. Perl exists info.

Answer 1

parseInt() is just going to throw an exception if the parsing can't complete successfully. You can instead use Integers, the corresponding object type, which makes things a little bit cleaner. So you probably want something closer to:

Integer s = null;

try { 
  s = Integer.valueOf(startField.getText());
}
catch (NumberFormatException e) {
  // ...
}

if (s != null) { ... }

Beware if you do decide to use parseInt()! parseInt() doesn't support good internationalization, so you have to jump through even more hoops:

try {
    NumberFormat nf = NumberFormat.getIntegerInstance(locale);
    nf.setParseIntegerOnly(true);
    nf.setMaximumIntegerDigits(9); // Or whatever you'd like to max out at.

    // Start parsing from the beginning.
    ParsePosition p = new ParsePosition(0);

    int val = format.parse(str, p).intValue();
    if (p.getIndex() != str.length()) {
        // There's some stuff after all the digits are done being processed.
    }

    // Work with the processed value here.
} catch (java.text.ParseFormatException exc) {
    // Something blew up in the parsing.
}

Answer 2

ints are value types; they can never be null. Instead, if the parsing failed, parseInt will throw a NumberFormatException that you need to catch.

Answer 3

Try this:

Integer startIn = null;

try {
  startIn = Integer.valueOf(startField.getText());
} catch (NumberFormatException e) {
  .
  .
  .
}

if (startIn == null) {
  // Prompt for value...
}

Answer 4

I don't think you can use "exists" on an integer in Perl, only on collections. Can you give an example of what you mean in Perl which matches your example in Java.

Given an expression that specifies a hash element or array element, returns true if the specified element in the hash or array has ever been initialized, even if the corresponding value is undefined.

This indicates it only applies to hash or array elements!

Answer 5

There is no exists for a SCALAR in Perl, anyway. The Perl way is

defined( $x )

and the equivalent Java is

anInteger != null

Those are the equivalents.

exists $hash{key}

Is like the Java

map.containsKey( "key" )

From your example, I think you're looking for

if ( startIn != null ) { ...