Which is better - Integer.parseInt() and catch exception or verify String with pattern before conversion? [duplicate]

Question

I have seen two styles for checking whether a variable is a valid integer in Java. One by doing an Integer.parseInt and catching any resulting exception. Another one is by using Pattern. Which of the following is better approach?

String countStr;
int count;
try {
    count = Integer.parseInt(countStr);
} catch (Exception e) {
    //return as the variable is not a proper integer.
    return;
}

or

String integerRegex = "([0-9]{0,9})";
if (countStr.isEmpty() || !Pattern.matches(integerRegex, countStr)) {
    //return as the variable is not a proper integer.
    return;
}

My question here is, is doing an Integer.parseInt() and catching an exception for validation a standard way to validate an int? I admit that my regex is not perfect. But is there any built-in methods available in Java for validation of int? Actually isn't it better to do some validation instead of simply catching the exception?

Answer 1

Using the approach above is better as it considers all types of possible errors and handles all cases. For instance what you have written will not parse negative numbers correctly.

It only makes sense to write your own verifier if you want to validate a given subset of all integers.

A general advice: don't re-invent the wheel unless you have strong reasons to do so.

Answer 2

There's a really good reason to not go with the second approach if you actually want to check if the given string can be represented as a 32bit integer and not just that it represents an integer in the mathematical sense.

I'm sure we all agree that 2147483648 (2**31 for those paying attention) is a perfectly fine integer, but it's only one of infinitely many numbers for which the two options will give different results. So if you want to check if you can represent a string as a 32bit integer use the parseInt method, if you just want to see if it's an integer go with the regex.

PS: That said don't catch Exception, but the correct NumberFormat exception instead..

Answer 3

These two function serve different purposes. If you just want to make sure that the string cotains a particular pattern, then use the second approach. If you need to convert it, then you should can parseInt() In this case it wouldn't make sense to check it and convert it as well.

However, if you have specific requirements for the number, then you may have to check it first regardless, because parseInt() may not always throw an exception if it can parse something which still doesn't fit your requirement.