Question

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Java floating point arithmetic

Hi All,
I was just reading a book and i came across this example which gave different outputs. Can somebody explain to me why.

 public class Test {

    public static void main(String[] args) {
        double x = 0.3 - 0.2;
        double y = 0.2 - 0.1;
        System.out.println(x);
        System.out.println(y);

        System.out.println(y == x);
    }
}

Output is :
0.09999999999999998

0.1

false

Answer 1

Computers represent floating-point numbers as an integer times a power of 2. 0.1 is not exactly representable as a floating point number (in the same way you can’t write 1/3 in decimal) and you therefore get rounding errors. The best way to avoid this is to use integer types when you can and to use a tolerant comparison function when you can’t. Do not compare floating point numbers for exact equality unless you know what you are doing.