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I am Seeing some strange behavior from BigDecimal When I do division using mathContext the output is different than when I do the division by directly providing the scale and rounding mode Here is an example that I think should provide the same output

public static void main(String...args){
    MathContext mc = new MathContext(3,RoundingMode.HALF_UP);
    BigDecimal four = new BigDecimal(4);
    BigDecimal three = new BigDecimal(3);
    System.out.println(four.divide(three,3,RoundingMode.HALF_UP));
    System.out.println(four.divide(three,mc));
}

Output:

1.333
1.33

It appears that the scale is treated differently when using MathContext. Or I dont understand when to use which.

thermite
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  • Java's lack of named-parameters strikes hard here. The first param of `MathContext` is significant digits, starting from the left/most. Whereas the first param of `.setScale` is digits-to-the-right-of-the-decimal-point which almost always what _I_ want when I think of "rounding." – MarkHu Sep 23 '21 at 00:36

1 Answers1

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The divide method of BigDecimal lets you specify the scale of the result, which loosely speaking is number of decimal places. scale = 3 means that a number will be expressed with 3 decimal places. A negative scale indicates the number of insignificant zeroes at the end of a whole number - so for example to round to the nearest 1000, you can specify scale = -3. 

four.divide(three,3,RoundingMode.HALF_UP);  // scale = 3, so round to 3 decimal places

But a MathContext is different. It lets you specify precision - that is, the number of significant digits. This is different from scale.

MathContext mc = new MathContext(3,RoundingMode.HALF_UP);
four.divide(three, mc); // precision = 3, so round to 3 significant figures
Dawood ibn Kareem
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