Question

I want to parse an integer accurately, one that has been potentially formatted according to the current locale. If I didn’t parse the integer accurately, I want to know it. So I use:

String string = "1111122222333334444455555";
Locale locale = Locale.getDefault();
NumberFormat numberFormat = NumberFormat.getIntegerInstance(locale);
numberFormat.setParseIntegerOnly();
Number number = numberFormat.parse(string);

Obviously "1111122222333334444455555" represents a big number, bigger than a Long can handle. So NumberFormat gives me… a Double??

I guess I would have expected to receive a BigInteger rather than a Double, especially since I asked for an integer-specific number formatter. But never mind that; the bigger problem is that the double value I get back is 1.1111222223333344E24! This is not equal to 1111122222333334444455555!!

If NumberFormat gives me a parsed value that does not equal that stored in the input string, how do I detect that?

Put another way: “How can I know if the Double value I get back from NumberFormat is exactly equivalent to the integral value represented in the original string?”

Answer 1

The javadocs for parse() state that it will return a Long if possible, otherwise it will return a Double. So just check that the return value is a Long.

“Returns a Long if possible (e.g., within the range [Long.MIN_VALUE, Long.MAX_VALUE] and with no decimals), otherwise a Double.”

“How can I know if the Double value I get back from NumberFormat is exactly equivalent to the integral value represented in the original string?”

If it returns a Double, then it is not exactly equivalent to your integral value because a Double cannot accurately represent values at that magnitude. Concrete example:

  Number a = numberFormat.parse("-9223372036854775809"); // Integer.MIN_VALUE - 1
  Number b = numberFormat.parse("-9223372036854775810"); // Integer.MIN_VALUE - 2
  System.out.println((a.equals(b))); // prints "true"
  Number c = numberFormat.parse("-9223372036854776800");
  System.out.println((a.equals(c))); // prints "true"