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Java: 234

class C{public static void main(String[] a){int x,y,s=0,r=Integer.parseInt(a[0]);for(y=1-r;y<r;y+=2){for(x=1-r;x<r;++x){boolean b=x*x+y*y<=r*r;s+=b?1:0;System.out.print(b?'*':' ');}System.out.println();}System.out.println(s*2d/r/r);}}

Unminified:

class C{
 public static void main(String[] a){
 int x,y,s=0,r=Integer.parseInt(a[0]); 
 for(y=1-r;y<r;y+=2){
 for(x=1-r;x<r;++x) {
 boolean b=x*x+y*y<=r*r;
 s+=b?1:0;
 System.out.print(b?'*':' ');
 }
 System.out.println();
 }
 System.out.println(s*2d/r/r);
 }
}