why second parameter is this....

Map<String, List<Treet>> treetsByAuthor = new HashMap<>;

for (Treet treet : treets) {

       String currentAuthor = treet.getAuthor();

      // first we try to get treets by author from the Map above

      LIst<Treet> authoredTreets = treetsByAuthor.get(currentAuthor);

      // two scenarios are possible

      // 1. authoredTreets will be null, because in the Map there are no 
      // treets mapped to "currentAuthor"
      // That is why we do the following "if"
      if (authoredTreets == null) {
             // Create empty array with treets to put in our Map.
             authoredTreets = new ArrayList<Treet>();

            // Put that empty array for key "currentAuthor"
            //  { "currentAuthor" : new ArrayList<> }
           treetsByAuthor.put(currentAuthor, authoredTreets);

          // Logical step would be to write here
          //  authoredTreets.add(treet);
          // but because for both scenarios we do the same, we can add that later
         // see below
      }

      // 2. authoredTreets already have treets with currentAuthor
      // In this case we just add treets in a map for current author
      // And this code also works if we just created empty array for
     // current author
     authoredTreets.add(treet);

    // note: we don't need to write "put" second time, because authored treets is
    // pointing to the array that is in a Map, that is why by adding treet to
    // "authoredTreets" we add it to Map as well

}