Java: Rounding error from Double to BigDecimal

Today’s amusement in my job:

Rounding error from Double to BigDecimal, see java docs for public BigDecimal(double val).

Java creates a BigDecimal which is exactly equal to 0.1 (an unscaled value of 1, with a scale of 1), but it is actually equal to 0.1000000000000000055511151231257827021181583404541015625


Workaround 1: Convert Double to String before converting to BigDecimal, as suggested in the java docs for BigDecimal

BigDecimal b = new BigDecimal(Double.toString(double));


Workaround 2: Round!
Double d = ...; // your (rounded) value
BigDecimal b = new BigDecimal(d).setScale(2, BigDecimal.ROUND_HALF_UP);


Another problem with Double in Java:

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3 Responses to Java: Rounding error from Double to BigDecimal

  1. Hans says:

    Round error ist kein error, sondern so dokumentiertes Verhalten:
    “Translates a double into a BigDecimal which is the exact decimal representation of the double’s binary floating-point value.” (Javadoc)
    Ps: Dein Tip zur JPA-Annotationen hat mir sehr geholfen. Danke

    • Hans says:

      Es gibt einen netten Java-Puzzler zu genau dem Problem.

      (kannst zu 7 Minuten vorspulen, aber der ganze Vortrag ist super)

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