Verify if String matches real number

I'm trying to verify if a String s match/is a real number. For that I created this method:

public static boolean Real(String s, int i) {

  boolean resp = false;
  //
  if ( i == s.length() ) {
     resp = true;
  } else if ( s.charAt(i) >= '0' && s.charAt(i) <= '9' ) {
     resp = Real(s, i + 1);
  } else {
     resp = false;
  }
  return resp;
}

public static boolean isReal(String s) {

  return Real(s, 0);
}

But obviously it works only for round numbers. Can anybody give me a tip on how to do this?

P.S: I can only use s.charAt(int) e length() Java functions.