Same decimal, different rounding results？ [duplicate]

Question

Two decimal places are kept, the fractional part is the same, and the result is inconsistent

jdk1.8.0_162

DecimalFormat df = new DecimalFormat("##.00");
df.setRoundingMode(RoundingMode.HALF_UP);
System.out.println("1.985 ≈ " + df.format(1.985));
System.out.println("23.985 ≈ " + df.format(23.985));

1.985 ≈ 1.99
23.985 ≈ 23.98

The output is as above, and should be the same as the fractional part.

Answer 1

This is how floating point types work. They can be an approximation. The approximations for your two numbers are not the same since they are not the same numbers. The one number might be 1.985000000000001 and the other number might be 23.98499999999999 internally. See also here.