floating point - Round a double in Java

Question

I have found this great solution for rounding:

static Double round(Double d, int precise) {
    BigDecimal bigDecimal = new BigDecimal(d);
    bigDecimal = bigDecimal.setScale(precise, RoundingMode.HALF_UP);
    return bigDecimal.doubleValue();
}

However, the results are confusing:

System.out.println(round(2.655d,2)); // -> 2.65
System.out.println(round(1.655d,2)); // -> 1.66

Why is it giving this output? I'm using jre 1.7.0_45.

Answer 1

You have to replace

BigDecimal bigDecimal = new BigDecimal(d);

with

BigDecimal bigDecimal = BigDecimal.valueOf(d);

and you will get the expected results:

2.66
1.66

Explanation from Java API:

BigDecimal.valueOf(double val) - uses the double's canonical string representation provided by the Double.toString() method. This is preferred way to convert a double (or float) into a BigDecimal.

new BigDecimal(double val) - uses the exact decimal representation of the double's binary floating-point value and thus results of this constructor can be somewhat unpredictable.