The Evil BigDecimal Constructor

One of the first things that most people learn as software developers, quite probably by accident, is that thanks to the binary representation that computers use for numbers, floating point operations are really kind of screwed up. So Java programmers learn early that if the calculation involves floating point numbers, it’s better to use java.math.BigDecimal to preserve all those fractions exact value to get the correct calculation result.

Which is correct. Except that some developers mistakenly use the BigDecimal(double) constructor, like the snippets below:

js> importPackage(java.math);
js> var oneTenth = new BigDecimal(0.1);
js> var oneMillion = new BigDecimal(1000000);
js> oneTenth.multiply(oneMillion)
100000.0000000000055511151231257827021181583404541015625000000


Which brings them back to square one, really. The reason is that when you call BigDecimal(double), the BigDecimal instance that you get back is the exact representation of that double value you’re passing in. In other words, you’re just reproducing the inaccuracy that you get when you try to represent 0.1 in binary. Pity.

To get what you really want, remember to use the other constructor, which accept a String. This is more like it:

js> var oneTenth = new BigDecimal("0.1");
js> var oneMillion = new BigDecimal(1000000);
js> oneTenth.multiply(oneMillion)
100000.0


Note that we can still pass 1000000 to BigDecimal, because we can represent integers in binary exactly. Likewise, those fractions that can be represented exactly in binary (such as 0.5, 0.25, 0.125, 0.0625, and so on) are fine:

js> var oneSixtyFourth = new BigDecimal(0.015625);
js> oneSixtyFourth.multiply(oneMillion)
15625.000000


But why bother wasting your brain cycles to keep track of this? It’s much easier to just remember this simple rule: when you want exact calculation results the way you want it, use BigDecimal(String). When in doubt, use BigDecimal(String). In fact, I can’t think of a case where one has to use BigDecimal(double). If you know of a legitimate case for using it, I’ll be grateful if you leave a comment about it.