getBestSellers

Bookstore.java

 public List<Book> getBestSellers(String subject) throws BestSellersNotFoundException {
    	Map<Book, Integer> map = new HashMap<Book, Integer>();   
    	
    	Calendar calendar = Calendar.getInstance();
    	calendar.add(Calendar.YEAR, -1);
    	Date date = calendar.getTime();
    	
    	for (Order order: ordersById) {
    		if (order.getDate().compareTo(date) > 0) {
    			for (OrderLine orderLine: order.getLines()) {
        			int qtdy = 0;
        			
        			if (subject.equalsIgnoreCase(orderLine.getBook().getSubject()))  {
        				if (map.get(orderLine.getBook()) != null) {
        					qtdy = map.get(orderLine.getBook());
        				}
        				
        				map.put(orderLine.getBook(), orderLine.getQty() + qtdy);
        			}
        		}
    		}
    	}
    	
        // now let's sort the map in decreasing order of value
		List<Book> arr = new ArrayList<Book>();
		
		if (map.size() == 0) {
			throw new BestSellersNotFoundException("Quantidade de livros é 0");
		} 
		
		Map<Book, Integer> sorted = map
	            .entrySet()
	            .stream()
	            .sorted(Collections.reverseOrder(Map.Entry.comparingByValue()))
	            .collect(
	               Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (e1, e2) -> e2,
	                    LinkedHashMap::new));
	    	
		arr = new ArrayList<Book>(map.keySet());
    	
    	return arr.subList(0, arr.size() >= 50 ? 50 : arr.size());
    }