ShoppingBasket.java

public class ShoppingBasket {
	
	private final List<BasketItem> basket = new ArrayList<>();

	void addItem(BasketItem basketItem) {		
		basket.add(basketItem);
	}

	double total() {
		double total = basket.stream()
				.mapToDouble(i -> i.getPrice() * i.getQuantity())
				.sum();
		return total;
	}
}