Let’s look at the following statements in Java.
System.out.println(new DecimalFormat("0").format(2.4)); //returns 2
System.out.println(new DecimalFormat("0").format(2.5)); //returns 2 <---Concentrate here
System.out.println(Math.round(2.5)); //returns 3
System.out.println(new DecimalFormat("0").format(2.6)); //returns 3
System.out.println(new DecimalFormat("0").format(3.5)); //returns 4
In the above statements, all other cases are obvious except the following.
System.out.println(new DecimalFormat("0").format(2.5));
It should return 3 but it returns 2. How?
Advertisement
Answer
This is intentional behavior. From the documentation:
Rounding
DecimalFormat uses half-even rounding (see ROUND_HALF_EVEN) for formatting.
About ROUND_HALF_EVEN:
Rounding mode to round towards the “nearest neighbor” unless both neighbors are equidistant, in which case, round towards the even neighbor. Behaves as for ROUND_HALF_UP if the digit to the left of the discarded fraction is odd; behaves as for ROUND_HALF_DOWN if it’s even. Note that this is the rounding mode that minimizes cumulative error when applied repeatedly over a sequence of calculations.
This is also known as banker’s rounding.
Math.Round on the other hand uses the following formula, which is “normal” rounding:
(long)Math.floor(a + 0.5d)