overflow of java

largest number of java integer is about two billion. if variable integer over it, it get overflow.

public class MathInJava
{
    public double amdahlSpeedup(double s, double n)
    {
        return 1 / (s + (1 - s)/n);
    }
    public double crossRatio(double a, double b, double c, double d)
    {
        return ((a-c)*(b-d))/((b-c)*(a-d));
    }
    public double average(double a, double b, double c, double d)
    {
        // To do!
        return (a + b + c + d)/ 4.0;
    }
}

    public void reduce()
    {
        red = (red * 6 / 256) * 51;
        green = (green * 6 / 256) * 51;
        blue = (blue * 6 / 256) * 51;
    }