Java: Collections Exercise 4 - Word Translator (Generics)

    Map<String, ArrayList<String>> m_dictionary = new HashMap<String, ArrayList<String>>();
    
    /** Adds a new definition for a word. 
     *  If this is the first time the word is being defined, 
     *     it is entered into a map as the key, and an ArrayList is entered
     *     as the value.
     *  If the word already has a definition, it adds this new definition
     *     to the existing ArrayList of definitions.
     *@param word Word in the dictionary.
     *@param definition A definition for word.
     */
    public void AddWord(String word, String definition) {
        //... Get the existing list of definitions for this word.
        ArrayList<String> defs = m_dictionary.get(word);
        
        //... If there wasn't already an entry for it, create one.
        if (defs == null) {
            defs = new ArrayList<String>();
            m_dictionary.put(word, defs);
        }
        //... Add the target word to the list of target words.
        defs.add(definition);
    }
    
    /** Returns an iterator which will go over the collection of definitions
     *  for the given word.
     *@param word The word whose definitions are being requested.
     *@return An iterator over all defintions associated with <i>word</i>.
     *        If the word is not in the dictionary, null will be returned.
     */
    public Iterator<String> iterator(String word) {
        ArrayList<String> defs = m_dictionary.get(word);
        if (defs == null) return null;
        
        return defs.iterator();
    }