public String join(String separator){
  StringBuilder sb=new StringBuilder();
  boolean isFirst=true;
  for (  Object e : this) {
    if (isFirst)     isFirst=false;
 else     sb.append(separator);
    sb.append(e);
  }
  return sb.toString();
}

public String join ( String separator ) { StringBuilder sb = new StringBuilder ( ) ; boolean isFirst = true ; for ( Object e : this ) { if ( isFirst ) isFirst = false ; else sb . append ( separator ) ; sb . append ( e ) ; } return sb . toString ( ) ; }
put all elements into a string , the string is separated by the given separator
