Stuck on treetsByAuthor

// Creates a new, empty Map called treetsByAuthor.
// The keys will be Strings, and the values will be Lists of Treets.
Map<String, List<Treet>> treetsByAuthor = new HashMap<String, List<Treet>>();

// Loops over every treet in the treets array.
for (Treet treet : treets) {

  // There's several things going on here:
    // treet.getAuthor() is using the method from the Treet class
    // to return the name of the treet's author (i.e. craigdennis);
    // treetsByAuthor.get() is attempting to 'get' a key/value pair
    // from the Map, where the key is the author (i.e. craigdennis)
    // and the value is that author's List of treets;
    // if this key/value pair exists, the List will be stored in authoredTreets
    // and the code will skip down to adding the new treet currently being looped over.
  List<Treet> authoredTreets = treetsByAuthor.get(treet.getAuthor());

  // If the key/value pair does not exist, because this author hasn't been looped over yet,
  // the previous line will have returned null, so this code block will run.
  if (authoredTreets == null) {

    // So this is what happens the first time an author's treet is iterated over.
    // First, a new empty List is created.
    authoredTreets = new ArrayList<Treet>();

    // Then, the Map adds a new key/value pair, where the key is the current author
    // and the value is the (currently empty) authoredTreets List we're about to add to.
    treetsByAuthor.put(treet.getAuthor(), authoredTreets);
  }

  // The current treet being looped over is added to the existing author list,
  // or the one newly created in the above 'if' block.
  authoredTreets.add(treet);
}