How does Double.intValue() work?

Question

This code

Double dbl = 254.9999999999999;
Integer integ = dbl.intValue();
System.out.println(integ);

shows 254, but one "9" more

Double dbl = 254.99999999999999;
Integer integ = dbl.intValue();
System.out.println(integ);

and it is already 255.. Why?

Answer 1

To know the exact value of your double, you can use a BigDecimal:

System.out.println(new BigDecimal(254.9999999999999));
System.out.println(new BigDecimal(254.99999999999999));

prints:

254.9999999999998863131622783839702606201171875
255

So this is simply due to (limitations of) double precision...

Answer 2

System.out.println(254.9999999999999 == 255.0);
System.out.println(254.99999999999999 == 255.0);

prints

false
true

That should give you a hint. The double type can represent up to 16 significant digits, and your literal has 17. Therefore the double value nearest to the second decimal literal is 255.0.

For the second line of code, Eclipse actually issues a warning:

Comparing identical expressions

because it is already the compiler which is aware of the fact that these are just two literals for the exact same double value.