Java double precision sum trouble

Question

I would like to know why I get this error. (this is Display log of Eclipse debug)

var
     (double) 2.8
tot.getIva()
     (java.lang.Double) 0.17
var+tot.get()
     (double) 2.9699999999999998

I can not understand why I did not get simply 2.97!

Read this Q&A - http://stackoverflow.com/questions/2100490/floating-point-inaccuracy-examples — Stephen C, May 28 '12 at 14:56
Or this Q&A - http://stackoverflow.com/questions/5257166/java-floats-and-doubles-how-to-avoid-that-0-0-0-1-0-1-0-9000001 — Stephen C, May 28 '12 at 14:58
Also, semi-duplicate of http://stackoverflow.com/questions/5423545/sum-of-decimal-number-in-java — Jim Kiley, May 28 '12 at 14:59

score 15 · Accepted Answer · answered May 28 '12 at 14:55

If you wanted 2.97, you should have used BigDecimal.

doubles are stored as fractions in binary, not decimal. So 3.75, for example, is just stored as 2^1 + 2^0 + 2^(-1) + 2^(-2).

2.8 and 0.17 cannot be represented exactly as binary fractions, so there's going to be some rounding error.

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score 2 · Answer 2 · answered May 28 '12 at 14:55

2

This is due to the precision of floating point types in java (float and double). If you need indefinite precision you should try using BigDecimal instead of double.

answered May 28 '12 at 14:55

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