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setScale(int newScale, RoundingMode roundingMode) returns a BigDecimal whose scale is the specified value, and whose unscaled value is determined by multiplying or dividing this BigDecimal's unscaled value by the appropriate power of ten to maintain its overall value.
You can use setScale() to reduce the number of fractional digits to zero. Assuming value holds the value to be rounded: BigDecimal scaled = value. setScale(0, RoundingMode.
compareTo(BigDecimal val) compares the BigDecimal Object with the specified BigDecimal value. Two BigDecimal objects that are equal in value but have a different scale (like 2.0 and 2.00) are considered equal by this method.
The enum RoundingMode provides eight rounding modes: CEILING – rounds towards positive infinity. FLOOR – rounds towards negative infinity. UP – rounds away from zero. DOWN – rounds towards zero.
One important point that is alluded to but not directly addressed is the difference between "precision" and "scale" and how they are used in the two statements. "precision" is the total number of significant digits in a number. "scale" is the number of digits to the right of the decimal point.
The MathContext constructor only accepts precision and RoundingMode as arguments, and therefore scale is never specified in the first statement.
setScale() obviously accepts scale as an argument, as well as RoundingMode, however precision is never specified in the second statement.
If you move the decimal point one place to the right, the difference will become clear:
// 1.
new BigDecimal("35.3456").round(new MathContext(4, RoundingMode.HALF_UP));
//result = 35.35
// 2.
new BigDecimal("35.3456").setScale(4, RoundingMode.HALF_UP);
// result = 35.3456
There is indeed a big difference, which you should keep in mind. setScale really set the scale of your number whereas round does round your number to the specified digits BUT it "starts from the leftmost digit of exact result" as mentioned within the jdk. So regarding your sample the results are the same, but try 0.0034 instead. Here's my note about that on my blog:
http://araklefeistel.blogspot.com/2011/06/javamathbigdecimal-difference-between.html
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