Determine if a String is an Integer in Java [duplicate]

Answer 1

You want to use the Integer.parseInt(String) method.

try{
  int num = Integer.parseInt(str);
  // is an integer!
} catch (NumberFormatException e) {
  // not an integer!
}

Answer 2

As an alternative approach to trying to parse the string and catching NumberFormatException, you could use a regex; e.g.

if (Pattern.compile("-?[0-9]+").matches(str)) {
    // its an integer
}

This is likely to be faster, especially if you precompile and reuse the regex.

However, the problem with this approach is that Integer.parseInt(str) will also fail if str represents a number that is outside range of legal int values. While it is possible to craft a regex that only matches integers in the range Integer.MIN_INT to Integer.MAX_INT, it is not a pretty sight. (And I am not going to try it ...)

On the other hand ... it may be acceptable to treat "not an integer" and "integer too large" separately for validation purposes.

Answer 3

Using regular expression is better.

str.matches("-?\\d+");


-?     --> negative sign, could have none or one
\\d+   --> one or more digits

It is not good to use NumberFormatException here if you can use if-statement instead.

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If you don't want leading zero's, you can just use the regular expression as follow:

str.matches("-?(0|[1-9]\\d*)");