Java TreeMap: Retrieving multiple values from a single key

Question

I'm trying to retrieve multiple values from a single key in a TreeMap. The idea is that each key will link to multiple values and should be searchable. Right now the trouble I'm running into is that when I am only able to get one value back from the key.

The key is a String and the value is a custom object, called Song. Song contains multiple elements. The goal is to extract the lyrics, word by word, from each song and use each word as a key. The key then links to each Song (the value) that contains the key.

I've searched StackOverFlow and the web in general for tips, and I've seen a few, but nothing that directly addresses my stumbling block. One idea I saw was to change the value into some kind of array or list. I may try that tomorrow when my brain is refreshed.

Anyway, thanks in advance for any tips and advice. And yes, this is homework. No, I did not tag is because I was informed that the homework tag is no longer commonly in use.

Code:

public class SearchByLyricsWords {
   private static Song[] songs;

   private static TreeMap<String, Song> lyricsTreeMap = new TreeMap<String, Song>();

   private static TreeSet<String> wordsToIgnoreTree = new TreeSet<String>();
   private static File wordsToIgnoreInput = new File("ignore.txt");
   private static String wordsToIgnoreString;
   private static String[] wordsToIgnoreArray;

   private Song[] searchResults;  // holds the results of the search
   private ArrayList<Song> searchList = new ArrayList<Song>();  

public SearchByLyricsWords(SongCollection sc) throws FileNotFoundException {

  // Create a string out of the ignore.txt file
  Scanner scanInputFile = new Scanner(wordsToIgnoreInput);
  String ignoreToken = scanInputFile.next();
  ignoreToken.toLowerCase();
  wordsToIgnoreString = ignoreToken + " ";

  while (scanInputFile.hasNext()) {
     ignoreToken = scanInputFile.next();
     wordsToIgnoreString = wordsToIgnoreString + ignoreToken + " ";
  }

  // Split the string created from ignore.txt 
  wordsToIgnoreArray = wordsToIgnoreString.split("[^a-zA-Z]+");

  // Fill a TreeSet from the wordsToIgnoreArray
  for (int i = 0; i < wordsToIgnoreArray.length; i++) {
     ignoreToken = wordsToIgnoreArray[i];
     wordsToIgnoreTree.add(ignoreToken);
  }

  // Fill TreeMap with lyrics words as the key, Song objects as the value
  songs = sc.getAllSongs();

  for (int j = 0; j < songs.length; j++) {
     Song currentSong = songs[j];
     String lyrics = currentSong.getLyrics();         
     TreeSet<String> lyricsFound = new TreeSet<String>();

     String lyricsToken;
     String[] songLyricsArray;
     songLyricsArray = lyrics.split("[^a-zA-Z]+");

     for (int k = 0; k < songLyricsArray.length; k++) {
        lyricsToken = songLyricsArray[k];

        if (lyricsToken.length() <= 1) {
           continue;
        }

        lyricsFound.add(lyricsToken);
     }

     lyricsFound.removeAll(wordsToIgnoreTree);

     Iterator<String> iterator = lyricsFound.iterator();

     while(iterator.hasNext()) {
        String currentWord = (String)iterator.next();
        lyricsTreeMap.put(currentWord, currentSong);
     }

     //System.out.println(lyricsTreeMap); // testing only
  }
}


public Song[] search(String lyricsWords) {

  lyricsWords = lyricsWords.toLowerCase();
  TreeSet<String> searchTree = new TreeSet<String>();
  String searchToken;
  String[] lyricsWordsSearch = lyricsWords.split("[^a-zA-Z]+");

  for (int l = 0; l < lyricsWordsSearch.length; l++) {
     searchToken = lyricsWordsSearch[l];

     if (searchToken.length() <= 1) {
        continue;            
     }
     searchTree.add(searchToken);
  }
  searchTree.removeAll(wordsToIgnoreTree);

  Iterator<String> searchIterator = searchTree.iterator();

  while(searchIterator.hasNext()) {
     String currentSearchWord = (String)searchIterator.next();
     Collection<Song> lyricsTreeCollection = lyricsTreeMap.values();

     while (lyricsTreeMap.containsKey(currentSearchWord) == true) {

        Iterator collectionIterator = lyricsTreeCollection.iterator();

        while(collectionIterator.hasNext() && collectionIterator.next() == currentSearchWord) {

           Song searchSong = lyricsTreeMap.get(currentSearchWord);
           searchList.add(searchSong);          
        }
     }           
  }
  searchResults = searchList.toArray(new Song[searchList.size()]);

  Arrays.sort(searchResults);

  return searchResults;
}

Solution 1

TreeMap only keeps one value per key, as with all Map implementations:

A map cannot contain duplicate keys; each key can map to at most one value.

Your alternatives include

• use a TreeMap<String, List<Song>> instead, and manually deal with the List values and keeping them updated
• use e.g. a TreeMultimap from Guava, which (more or less) operates like a TreeMap<K, TreeSet<V>>, except a lot better. (Disclosure: I contribute to Guava.)

Solution 2

You need to use this format to initialize the map and set:

Map<String, TreeSet<Song>>=new Map<String,treeSet<Song>>();

Than you will need a multiple for loops to loop first thru the song set and than get the Map keys to insert into your Map. Like so:

    for(Song temp:songs){
       for(String word:temp.getLyrics().toLowerCase().split("[^a-zA-Z]+"){
             if(lyricsTreeMap.containsKey(word)){
                  lyricsTreeMap.get(word).add(temp);
    }

Solution 3

I think you shoule set your structure as

Map<String, Set<Song>>

Here Set is used as inner Collection class rather then List. Because it will automatically omit the redundent value for your scenario.