Adding and subtracting doubles are giving strange results [duplicate]

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In Java, double values are IEEE floating point numbers. Unless they are a power of 2 (or sums of powers of 2, e.g. 1/8 + 1/4 = 3/8), they cannot be represented exactly, even if they have high precision. Some floating point operations will compound the round-off error present in these floating point numbers. In cases you’ve described above, the floating-point errors have become significant enough to show up in the output.

It doesn’t matter what the source of the number is, whether it’s parsing a string from a JTextField or specifying a double literal — the problem is inherit in floating-point representation.

Workarounds:

  • If you know you’ll only have so many decimal points, then use integer
    arithmetic, then convert to a decimal:

    (double) (51 + 1) / 10
(double) (48 - 4) / 10

  • Use BigDecimal

  • If you must use double, you can cut down on floating-point errors
    with the Kahan Summation Algorithm.

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