how to check for invalid characters

public static String binaryStringToOctalString(String binString) {
  if (binString == null || binString.length() == 0) {
      return null;
  }
    if(binString.charAt(0)<'0' || binString.charAt(0)>'1'){// else check my 0 the char is valid
        return null;
    }
   int dec = Integer.parseInt(binString, 2);
  return Integer.toOctalString(dec);
 }