2.1 Example - Playlist

PlaylistItem is the object of interest with fields: name, time, artist, album etc:

A Playlist object will contain a collection of PlaylistItem objects with operations to play, add and remove items etc. Note the class diagram doesn’t capture the nature of the composition association i.e. we are using an array.

public class Playlist {
  // the playlist
  private PlaylistItem[] playlist;
  private int numberOfItems;
  private int currentlyPlayingIndex;
 
  // constant - the maximum number of items allowed
  private static final int MAX_ITEMS = 10;
  …
}

Note we are using an attribute to keep track of how many values are currently stored in the array.

The playlist contains the following methods:

Constructor:

public Playlist() {
  this.playlist = new PlaylistItem[MAX_ITEMS];
  this.numberOfItems = 0;
  this.currentlyPlayingIndex = -1;
 }

Getter:

public PlaylistItem[] getPlaylist() { return this.playlist; }

setter:

public void setPlaylist(PlaylistItem[] playlist) { this.playlist = playlist; }

addItem:

public String addItem(PlaylistItem newItem) {
  if (this.numberOfItems < MAX_ITEMS) {
      this.playlist[this.numberOfItems++] = newItem;
      return "Succesful Addition";
}
  return "Playlist Full - unsuccesful addition";        
}

Note we check to see if there is sufficient room before adding the new Object and incrementing the number of items.

removeItem:

public String removeItem(int itemNumber) {
   if (itemNumber <= numberOfItems) {            
       for (int i=itemNumber-1; i<numberOfItems-1; i++) {
            this.playlist[i] = this.playlist[i+1];
            this.playlist[i].setItemNumber(i+1);
}
   this.numberOfItems--;
   return "Succesful Removal";
}
   return "Item Does Not Exist - unsuccesful removal";        
}

Note we have to shuffle remaining items to ensure that objects are stored from index 0 onwards.

searchByArtist:

public Playlist searchByArtist(String artist) {
     Playlist searchResults = new Playlist();
     for (PlaylistItem p : this.playlist) {
          if (p.getArtist().equals(artist)) {
              searchResults.addItem(p);
          }                
     }
     return searchResults;        
}

Note the for-loop used to iterate over the collection. A new Playlist object is created containing only those PlaylistItems which match the search criteria.