@SamiKorhonen said this in a comment:
Multiply by two, round and finally divide by two
So this is that code:
public static double roundToHalf(double d) {
return Math.round(d * 2) / 2.0;
}
public static void main(String[] args) {
double d1 = roundToHalf(1.1);
double d2 = roundToHalf(1.3);
double d3 = roundToHalf(2.5);
double d4 = roundToHalf(3.223920);
double d5 = roundToHalf(3);
System.out.println(d1);
System.out.println(d2);
System.out.println(d3);
System.out.println(d4);
System.out.println(d5);
}
Output:
1.0
1.5
2.5
3.0
3.0