code: public static String join(String separator, String... strings) {
  		StringBuilder builder = new StringBuilder();
  		if(strings != null) {
  			boolean first = true;
  			for(String string : strings) {
  				if(string != null) {				
  					if(!first) {
  						builder.append(separator);					
  					}
  					builder.append(string);
  					first = false;
  				}
  			}
  			if(builder.length() > 0) {
  				return builder.toString();
  			}
  		}
  		return null;
  	}
docs: Joins the given set of strings using the provided separator.
  
  @param separator
  a character sequence used to separate strings.
  @param strings
  the set of strings to be joined.
  @return
  a string containing the list of input strings, or null if no valid input
  was provided.