floating point - 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!

Answer 1

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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