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.

Much the same with a bigger constant. Of course, like `String`, it is implementation dependent. — Tom Hawtin - tackline, Mar 23 '10 at 15:35
Did you read this one: http://www.javaworld.com/javaworld/javatips/jw-javatip130.html — PeterMmm, Mar 23 '10 at 15:47

score 26 · Accepted Answer · edited Jul 17 '19 at 12:26

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 says

Used to store the canonical string representation, if computed.

Assuming you don't call .toString(), it will remain zero bytes. Hence BigDecimal is (8+4+4)=16 bytes + BigInteger.

BigInteger itself is 4+4+4+4+4=20 bytes + mag.

20+16 gives total of 36 bytes plus the magnitude, which is always the minimum number of bits necessary to represent the full integer. For a number n it will need log2(n) bits, which can be converted to ints. You should be using about:

36 + Ceiling(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.)

Keep in mind that 'stringCache' and 'intVal' will have reference pointers (jvm impl dependent 4 or 8 bytes). Also remember the min 12byte object header overhead ('BigInteger' and 'BigDecimal', int[]) and finally 8byte align (round up) all object sizes. — Joseph Lust, Jan 25 '14 at 17:07

score 2 · Answer 2 · answered Mar 23 '10 at 15:42

2

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.

answered Mar 23 '10 at 15:42

Adamski

54,009
15
113
152

What is the compact representation? – Marcus Leon Mar 23 '10 at 15:45

Java BigDecimal memory usage?

2 Answers2

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