"Normalizing" BigDecimal's hash code: howto?

Convert to Double and use the Double's hashCode, but base equality on the BigDecimal compareTo order.

Two numerically equivalent BigDecimals will map to the same Double, and get the same hashCode. Some BigDecimal values that are very slightly different will get the same hashcode because of double rounding, but most distinct values will get different hashcodes, which is all you need.