CODEWITHSUNDEEP
javazeroparseintleading-zero
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Reputation: 706

Java ParseInt() - Catching Strings with a leading zero

Java's ParseInt method will happily parse decimal values supplied with a leading zero without throwing an exception, stripping the zero:

int value = Integer.parseInt("050", 10);

will result in the integer value 50.

But, I have an application requiring a string such as this to be rejected as invalid input. My solution to the problem so far has been to convert the parsed integer back to a string, and compare the lengths of original/parsed strings to see if any character has been stripped, eg:

String original = "050";
value  = Integer.parseInt( "050", 10);
String parsed = Integer.toString(value);
if (original.length() != parsed.length()) {
    System.exit(1);
}

Which works fine, but feels a little hacky. Are there better ways of detecting and handling a leading zero?

Upvotes: 5

Views: 27984

Answers (3)

Marco
Marco

Reputation: 330

As you get a string input you can use the method String.charAt(int) to check for explicit characters. However, when you are just using input.charAt(0) the user could avoid the leading zero with a space.

What I would suggest to do:

String decimalChars = "123456789";

boolean reject = true, hasZeros = false;
for (int i = 0; i < input.length; i++) {
    if (input.charAt(i) == '0') { // or other chars you don't want
        hasZeros = true;
        reject = false;
    } else if (decimalChars.contains(input.charAt(i)) {
           if (hasZeros) {
               reject = true;
            } else {
                reject = false;
            }
            break;
        }
    }
}

Using that you can do whatever you want to reject wrong inputs by checking reject. The code will break when it finds a character which is in the decimalChars array. reject will be true if it found a zero before, otherwise it'll be false. This code also allows spaces at the beginning and doesn't reject an input like 0000. Empty strings will be rejected as well, because reject initializes with true.

I hope this helps!

Upvotes: 0

SomeJavaGuy
SomeJavaGuy

Reputation: 7347

you could work with regex and check if it has a leading 0

you could just write

After seeing your comment about leading whitespaces beeing allowed you could use:

if(original.matches(".\\s0.*[1-9]")) // Wrong number with leading zeros

This way a 00000 would still be a zero, if it´s valid

Upvotes: 2

Eran
Eran

Reputation: 393801

Check if the first character is 0 :

if (original.charAt(0)=='0')

or

if (original.startsWith("0"))

If the String starts with a 0, you don't want to call parseInt at all.

I think that comparing a single character is more efficient than using regular expressions.

Upvotes: 12

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