Improve performance on BigDecimal to double conversion

Question

This is the Jdk7-b147 version of BigDecimal.doubleValue()

public double doubleValue(){
  if (scale == 0 && intCompact != INFLATED)
    return (double)intCompact;
  // Somewhat inefficient, but guaranteed to work.
  return Double.parseDouble(this.toString());
}

They admit that this way is inefficient! Is there a better/faster way than to use this method?

Answer 1

There isn't a much better way to convert a BigDecimal to a double. This is because the algorithms to convert foo * 10^bar to baz * 2^quux efficiently, while keeping very specific rounding semantics, are extremely nasty and unpleasant -- see sun.misc.FloatingDecimal for details, or read this paper.

BigInteger.doubleValue(), on the other hand, does have lots of opportunities for optimization, since it needn't deal with decimal fractions, but only integers. I have a JDK patch pending that optimizes BigInteger.doubleValue() by slightly more than two orders of magnitude, though it's still awaiting review.

Update: The fix was added in OpenJDK 8, made available to the general public on March 18, 2014.