Please explain the logic....

class A1 { }
 class B extends A1 { }
 public class ComingThru {
   static String s = "-";
   public static void main(String[] args) {
     A1[] aa = new A1[2];
     B[] ba = new B[2];
     sifter(aa);
     sifter(ba);
     sifter(7);
     System.out.println(s);
   }
   static void sifter(A1[]... a2)    { s += "1"; }
      static void sifter(B[]... b1)    { s += "2"; }
      static void sifter(B[] b1)       { s += "3"; }
      static void sifter(Object o)     { s += "4"; }
      }