java giving .9999999 instead of a rounded whole number [duplicate]

Question

This question already has answers here:

Floating point arithmetic not producing exact results [duplicate] (7 answers)

Closed 8 years ago.

Here is the extracted code :

long timeMs = 1473;
double timeS = (timeMs / 1000) + (timeMs% 1000) / 1000.0;
System.out.println(timeS);

And the output is:

1.4729999999999999

So basically, I was just trying to convert the time taken in seconds into milliseconds. After I saw this I thought my method is wrong, so I've tried other inputs, such as 1472, 1474, 1173, 3 etc which all gave the correct values(1.472, 1.474, 1.173, 0.003).

I think I've came across something similar to this a while ago in a book called Java Puzzlers, but have forgotten. Can anyone tell me why this is happening (and a proper term/error)?

Thanks.

Answer 1

Use this instead, but it's because of IEEE 754 rounding rules.

double timeS = (timeMs / 1000.0); //+ ((double) (timeMs % 1000) / 1000.0);

Answer 2

Use BigDecimal for more accurate rounding and scaling.

   long timeMs = 1473;
   double timeS = (timeMs / 1000d);
   BigDecimal usefulName = new BigDecimal(timeS).setScale(3, RoundingMode.HALF_UP);
   System.out.println(usefulName);