Strange behavior of Math.round() method

Question

I noticed an incomprehensible thing for me in static method round() in class Math:

Math.round(0.4999999999999999);  // is 0
Math.round(0.49999999999999999); // is 1

Why?

@dystroy: This doesn't look like a bug to me. It looks like the OP is really rounding 0.5. (Note that the final digit is 9, not 4.) — Jon Skeet, Feb 16 '13 at 18:59
It's not a duplicate and it's not a bug. It's how floating point numbers are behaving. — Tomasz Nurkiewicz, Feb 16 '13 at 19:01

score 14 · Accepted Answer · answered Feb 16 '13 at 18:58

0.49999999999999999 has too many significant digits and one double variable cannot store them all. So implicit rounding happens during compilation. By the time you are calling Math.round(), the argument is already 0.5 (check yourself: 0.49999999999999999 == 0.5 yields true).

Strange behavior of Math.round() method

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