Java BigDecimal memory usage?

Question

Is there a guideline for estimating the amount of memory consumed by a BigDecimal?

Looking for something similar to these guidelines for estimating String memory usage.

Answer 1

If you dig into the internals of BigDecimal you'll see that it uses a compact representation if the significand is <= Long.MAX_VALUE. Hence, the memory usage can vary depending on the actual values you're representing.

Answer 2

If you look at the fields in the source for BigDecimal there is:

BigDecimal:
  long intCompact +8 bytes
  int precision +4 bytes
  int scale +4 bytes
  String stringCache +?
  BigInteger intVal +?

BigInteger:
  int bitCount +4 bytes
  int bitLength +4 bytes
  int firstNonzeroIntNum +4 bytes
  int lowestSetBit +4 bytes
  int signum +4 bytes
  int[] mag +?

The comment for stringCache is "Used to store the canonical string representation, if computed.", so assuming that you don't call toString we will leave that as zero bytes, so in total there are (8+4+4)=16 bytes + BigInteger in BigDecimal and 4+4+4+4+4=20 bytes + mag for BigInteger. So a total of 36 bytes plus the magnitude. As far as I can tell magnitude is always the minimum number of bits necessary to represent the full integer, so for a number n it will need log2(n) bits, which can be converted to ints. So in general you should be using about:

36 + Cieling(log2(n)/8.0) bytes

(note this doesn't include any of the other object descriptor overhead as your example link for strings does, but it should give you a good general idea.)