losing precision converting from java BigDecimal to double

Question

I am working with an application that is based entirely on doubles, and am having trouble in one utility method that parses a string into a double. I\'ve found a fix where

Answer 1

The problem is that a double can hold 15 digits, while a BigDecimal can hold an arbitrary number. When you call toDouble(), it attempts to apply a rounding mode to remove the excess digits. However, since you have a lot of 9's in the output, that means that they keep getting rounded up to 0, with a carry to the next-highest digit.

To keep as much precision as you can, you need to change the BigDecimal's rounding mode so that it truncates:

BigDecimal bd1 = new BigDecimal("12345.1234599999998");
System.out.println(bd1.doubleValue());

BigDecimal bd2 = new BigDecimal("12345.1234599999998", new MathContext(15, RoundingMode.FLOOR));
System.out.println(bd2.doubleValue());