迭代ArrayList以按对象属性计数

Question

I have an ArrayList of products already initialized. The Product constructor is:

public Product(String number, String type, double rentalDeposit, double rentalPrice, double lateFee, double buyPrice, int maxDuration)

The type is determined by an enumeration:

protected enum productType {Basket, BabySeat, Helmet, Headlight, Bell};

I pass in the type using the toString method for the enumeration. I need to iterate through the ArrayList<Product> (given by shop.getInventory() ) that I have and count how many of each type there are, ie how many are of type Basket , BabySeat , Helmet , etc.

The Product class has a getType() method that returns a string.

for (Product.productType product : Product.productType.values()) {
    int occurences = Collections.frequency(shop.getInventory(), product.toString());
}

I have tried using Collections.frequency , but it keeps returning 0 and I'm not sure why.

Is there another way to iterate through and find this amount without using a ton of if statements?

Answer 1

shop.getInventory() I'll assume has the type Collection<Product> . You can either define product such that .equals(Product) will check equality against the Product's internal type, or even more simply, shop.getInventory().stream().filter(item -> product.toString().equals(item.getType())).count() . (replace item.getType() with however you extract the type field from Product, like maybe item.type etc).

Answer 2

A simple method of counting items in a list that correspond to some condition is to use Collectors.groupingBy and Collectors.counting . Something like the following:

Map<ProductType,Long> counts = products.stream()
    .collect(groupingBy(Product::getType, counting()));

If you're not familiar with streams, this statement can be read as 'turn the list into a stream of products, group the products by product type then count each of those groups creating a map from the type to the count.'