Rounding with DecimalFormat in Java

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?

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)