Not really unterstanding the treetsByAuthor method

    // Want to create a Map of Authors to their Treets 
    // Start by creating an empty Map based on HashMap. 
    // The key is the author name (string) and value will be the list of Treets
    Map<String, List<Treet>> treetsByAuthor = new HashMap<String, List<Treet>>();
    // Loop through all treets....
    for (Treet treet : treets) {
      // Use the current treet's author (the key) to look up in our Map the list of treets
      // already parsed and stored in this author's treet list (the value). Assign the
      // pointer to this list to the variable 'authoredTreets'
      List<Treet> authoredTreets = treetsByAuthor.get(treet.getAuthor());
      // The author of this tweet may not have been seen yet and hence has no Map 
      // entry yet. If not yet seen, the pointer will have a 'null' value
      if (authoredTreets == null ) {
        // Since it's 'null', we need to create a new entry in the Map
        // Create a new empty list to hold this author's treets 
        authorTreets = new ArrayList<Treet>();
        // Add new Map entry with key=this-treets-author, and value=new-empty-treets-list
        treetsByAuthor.put(treet.getAuthor(), authoredTreets);
      }
      // Now we definitely have a Map entry for this author that is either the pre-existing 
      // tweet list or our newly created empty list. In either case, 'authoredTreets' points 
      // to the list reference. Add this treet to this author's list of treets
      authoredTreets.add(treet)
    }