Anjanesh,
The code is trying to reuse the variable 's' and not an object (a reference actually). It is trying to declare a variable 's' again as an Integer when it has already been declared a few lines above as a String reference variable.
Variable names in the same scope should be unique. You cannot have two variables with the same name in the same scope.
However, if you want to use the same reference variable to point to a different object, you can do so.
HTH
String s = new String("a-string");
System.out.println(s);
s = null;
s = new String("b-string");
System.out.println(s);