public void nodeAdder(DefaultMutableTreeNode top) {
        Set genreSet = multiMap.keySet(); // get key set
        Iterator keyIterator = genreSet.iterator(); // create iterator for set
        
        while (keyIterator.hasNext()) {
            String key = (String)keyIterator.next(); // cast to string
            DefaultMutableTreeNode genreNode = new DefaultMutableTreeNode(key);
            top.add(genreNode); // add key to parent node
            
            Collection values = (Collection)multiMap.get(key); // return values from key
            Iterator valuesIterator = values.iterator(); // create iterator
            
            while (valuesIterator.hasNext()) {
                DefaultMutableTreeNode bookNode = 
                        new DefaultMutableTreeNode(valuesIterator.next());
                genreNode.add(bookNode); // add value to node
                
            } // end while
        } // end while
    } // end method nodeAdder