Java: Is (int) double reliable?

Question

When I perform simple math in java with doubles and other number data types, the double values seem to randomly vary a bit from the supposed result, which might be 5,59999999997 or 6,0000000002 or something. When I cast to int, the double value is obviously rounded down to the next whole number. Does this mean the double could be both 5 or 6? Or does that "5,999999999997" still count as 6 though which would be depending on the binary float value? If not, is there a way to let the negative vary be rounded up, but not lower values from 5,5 to 5,999999999996?

• I mean, I dont really want to round the value as described in my last sentence. I'd like to always round down to the next whole number, but I don't want to cause an extra decrement due to wrong double math results.

Answer 1

Converting a double to an int always rounds down. You can round to the nearest whole integer via Math.round(double). The double is varying from what you expect because of floating point error.

Answer 2

If you want to round, you can use the round() method.

 double d = 6 +/- some small error
 long l = Math.round(d);

Or you can add 0.5 for positive numbers

 long l = (long) (d + 0.5);

or

 long l = (long) (d + (d < 0 ? -0.5 : 0.5));

Answer 3

I'm not sure I understand the question. Usually when you cast a double to int you add 0.5 to have a nice round.

Answer 4

From the Java Language Specification:

The Java programming language uses round toward zero when converting a floating value to an integer (§5.1.3), which acts, in this case, as though the number were truncated, discarding the mantissa bits. Rounding toward zero chooses at its result the format's value closest to and no greater in magnitude than the infinitely precise result.

So 5,999999999997 when casted to an int will 5 and 6,0000000002 will be 6. If I understand what you are asking with having negative versions of the values (e.g. -5.97), I fail to see how Math.round() does not suffice you. -6,0000000002 will be rounded to -6 as will -5,999999999997 and every other value above (but not including) -5.5.