Java算数题运行为什么得0.0

public class Pi2 {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
int n = input.nextInt();
System.out.print(calcPi(n));
}

private static double calcPi(int n) {
    // TODO Auto-generated method stub
    double Pi = 1;
    for(int i=1;i<=n;i++) {
            Pi=Pi*(2*i/(2*i-1))*(2*i/(2*i+1));
    }
    return Pi*2;
}

}

 Pi=Pi*(2.0*i/(2*i-1))*(2.0*i/(2*i+1)); 

改为：

  Pi=Pi*(2.0*i/(2*i-1))*(2.0*i/(2*i+1));

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int n = input.nextInt();
        System.out.print(calcPi(n));

    }
    private static double calcPi(int n) {
        // TODO Auto-generated method stub
        double Pi = 1;
        for(int i=1;i<=n;i++) {
            Pi=Pi*(2.0*i/(2*i-1))*(2.0*i/(2*i+1));
        }
        return Pi*2;
    }