Java BigDecimal Possible Overflow Bug

Question

I was testing boundary conditions on some code involving a BigDecimal, and I noticed that when a BigDecimal is initialized with the String "1e2147483647" it behaves unexpectedly. It seems to have a value between 0 and 1e-2147483647. When I try calling intValue(), I get a NegativeArraySizeException. I should note that 2147483647 is the max value of an integer on my system. Am I doing something wrong, or is this a problem with BigDecimal?

BigDecimal test = new BigDecimal("1e2147483647");

test.compareTo(new BigDecimal(0));  //Returns 1
test.compareTo(new BigDecimal("1e-2147483647"));  //Returns -1
test.intValue();  //Throws NegativeArraySizeException

Thanks, I hadn't seen that question. I was just surprised it didn't throw a NumberFormatException from the constructor like it does for a number one digit larger. — DJMatch3000
– DJMatch3000, Commented Jul 1, 2015 at 20:02
This is more a suggestion than knowing, but 1e-2147483647 is a pretty large number. To be precise, log_2(10^2147483647) / 8 / 1024^3 = 0.83... should yield the minimal size (in Gigabytes) to represent such a large number as integer. Maybe this is some kind of memory allocatin problem? — Turing85
– Turing85, Commented Jul 1, 2015 at 20:03
@DJMatch3000: No, your input is valid and representable, though that's the maximum exponent you can represent for BigDecimal. Your bug is legit. — Louis Wasserman
– Louis Wasserman, Commented Jul 1, 2015 at 20:05

nhahtdh · Accepted Answer · 2015-07-02 06:09:01Z

87

No, you appear to have a legit bug. The bug presents in JDK7 but fixed in JDK8. Your values are correctly representable as BigDecimals, and should behave correctly, but don't.

Tracing through the source code of BigDecimal, on line 2585, this.precision() is 1, and this.scale is -2147483647. this.precision() - this.scale therefore overflows, and the following overflow is not handled correctly.

This bug has been fixed in JDK8 by doing the subtraction in long arithmetic.

edited Jul 2, 2015 at 6:09

nhahtdh

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answered Jul 1, 2015 at 19:56

Louis Wasserman

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Ky - Over a year ago

Does this in Android 17's Java (similar to JDK6)?

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