Java RoundingMode HALF_UP doesn't round up as expected

Question

So I have this code

public String getRounded(double amount) {
    DecimalFormat df = new DecimalFormat("0.00");
    df.setRoundingMode(RoundingMode.HALF_UP);
    System.out.println(df.format(amount));
    System.out.println(amount);
    return df.format(amount);
}

I've added those two lines to see the difference and this is an example of what they display.

The output is this for example:

1.25

1.255

Why does this happen? Isn't the point of HALF_UP to round up? I've tried with CEILING and it does round up, although that's not what I want in this case.

Thank you

score 7 · Answer 1 · answered Dec 12 '17 at 21:03

7

double is lying to you. System.out.println is lying to you.

amount is not exactly 1.255. It is actually 1.25499999999999989341858963598497211933135986328125, which is less than the exact real number 1.255.

Rounding that with HALF_UP returns, correctly, 1.25.

If you care about fine distinctions like this, you will need to use BigDecimal from the beginning.

answered Dec 12 '17 at 21:03

Louis Wasserman

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Worth noting that `System.out.println(new BigDecimal(amount))` will tell the truth. – Roman Dec 12 '17 at 21:14
That makes sense. Thank you – Dec 12 '17 at 23:27

Java RoundingMode HALF_UP doesn't round up as expected

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