How can I make sure that the variable "radius" is initialized before the for loop?

public class SphereVolume $\{$

public static void main$($String$[]$ args$)\{$

Scanner scan $=$ new Scanner$($System$.$in$)$;

double radius;

double singleVolume;

double totalVolume $=0$;

System.out.println$("$Enter the number of spheres"$)$;

int N $=$ scan.nextInt$()$;

for$($int i $=0$; i $<$ N; $++$i$)\{$

if $($radius $>0)\{$

System.out.println$("$Enter the radius of the sphere"$)$;

radius $=$ scan.nextDouble$()$;

singleVolume $=4./3*$Math$.$PI$*$Math$.$pow$($radius$,3)$;

totalVolume $+=$ singleVolume;

$\}$else$\{$

System.out.println$("$Please enter a positive value"$)$;

$\}$

$\}$

System.out.println$("$The total volume of the spheres is $"+$ totalVolume$)$;

$\}$

$\}$