public static String join(String separator, List<String> topicsArray) {

    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < topicsArray.size(); i++) {

      if (sb.length() > 0) {
        sb.append(",");
      }
      sb.append(topicsArray.get(i));
    }
    return sb.toString();
  }
public static String join ( String separator , List < String > topicsArray ) { StringBuilder sb = new StringBuilder ( ) ; for ( int i = 0 ; i < topicsArray . size ( ) ; i ++ ) { if ( sb . length ( ) > 0 ) { sb . append ( "," ) ; } sb . append ( topicsArray . get ( i ) ) ; } return sb . toString ( ) ; }
Joins a list of Strings

@param separator
@param topicsArray
@return