Updating a java map

Hi Bruce. Null is not the same as the collection being empty -- it literally means that it doesn't exist! If it's null, you're not redefining it, as it wasn't defined to begin with! I don't see any code so I'm not entirely sure on the implementation of the method you're describing for your second question, however. Please post the code in question and I'm sure we'll figure it out!

If you don't update the map every time you get a new song, how could this possibly work? This code is in the Java Data Structures course.

private Map<String, List<Song>> byArtist() { Map<String,List<Song>> byArtist = new HashMap<>(); for (Song song : mSongs) { // read the map to get list of songs by artist that are already in map List<Song> artistSongs = byArtist.get(song.getArtist()); if (artistSongs == null) { artistSongs = new ArrayList<>(); byArtist.put(song.getArtist(), artistSongs); } artistSongs.add(song); } return byArtist; }

It doesn't make any sense that the only time you do a put is when the list is empty. How are songs being added to the map?

It's better to think of it as a Map of Lists (of Songs) then a Map of Songs. So you don't add the Songs to the Map directly. You add the List to the Map. Each List is not an independent copy of what you put. It is it's own object you can change by adding, removing, etc, . You then add the Songs to the List. When you want to retrieve the Songs you've added you retrieve the List (of Songs) from the Map and then get the Songs from the List.

Let's say I start with an empty map that has artist as the key and a list of the artist's songs as the value.

Let's say I have 10 songs by the same artist that I want to add to the map by adding the key of artist and the value of the list.

The first time I read the map there is no list for the artist so I create a new list and I write the list to the map.

Every time I process a song I need to update the list that goes with the artist and need to do a put to the map with that list. So I would have to do at least 10 puts for the map to get all of the songs into the map. If all I do is add a song to a list and not to the map, when the method returns the map, how could it be usable?

Bruce,

You said, "It doesn't make any sense that the only time you do a put is when the list is empty. How are songs being added to the map?"

Sorry for the confusion Bruce. Lets see if we can clear this up for you.

    1  private Map<String, List<Song>> byArtist() {
    2       Map<String, List<Song>> byArtist = new TreeMap<>();
    3       for (Song song : mSongs) {
    4           List<Song> artistSongs = byArtist.get(song.getArtist());
    5           if (artistSongs == null) {
    6               artistSongs = new ArrayList<>();
    7               byArtist.put(song.getArtist(), artistSongs);
    8           }
    9           artistSongs.add(song);
   10      }
   11       return byArtist;
   12 }

On line 4 we declare a variable of type List<Song> named artistSongs and assign it to either null or a reference to a list. We do this by calling the byArtist map's get method and passing in a String that we acquire from calling song.getArtist(). You already seem to have a good grasp of how this is done. However, I think where the confusion stems from is the fact that you can get a reference to the list as a value that is within the map. It does not remove the list from the map and the reference is the location of the list in memory.

Therefore, if we get a reference because the list exists, we skip the if block and just add the song to the list. Since artistSongs holds the reference to the list within the map, we can update it directly and not need to use the put method.

I hope this clears it up. Please let us know.

Regards,

Chris

Adding the Song to the List changes the object inside the Map. It may be better to say the Map refers to the List and any changes to the List are reflected when you retrieve the List from the Map, which is the same object you put into the Map and latter manipulated.

So if you do a get for a map and and update the value, it automatically updates the map even if you don't do a put? I didn't hear that feature mentioned in the courseware.