Count total price of items in HashMap via Streams

[Andrzej Zahorski]

Hi, I'm trying to remind myself streams and started small project with shopping cart. However I don't know how to get total price of items, which I want to count via Stream API.
How to get into collection, use getPrice() method (method of ShoppingItem, which is the key in HashMap), multiply it by quantity (Value of Hashmap) and get sum of this multiplications?
Here is  shrunk down code:

public class ShoppingItem
{
    private int price;
    private String name;

    public ShoppingItem(int price, String name)
    {
        this.price = price;
        this.name = name;
    }

    public int getPrice()
    {
        return price;
    }

    public String getName()
    {
        return name;
    }
}

public class ShoppingCart
{
    private  int wallet;
    private HashMap<ShoppingItem, Integer> shoppingMap;

    public ShoppingCart(int wallet)
    {
        this.wallet = wallet;
        shoppingMap = new HashMap<>();
    }

    public int getTotalItems()
    {
        return shoppingMap.values().stream().reduce(0, Integer::sum);
    }

    public int getTotal()
    {
//        return this.shoppingMap.entrySet().stream().reduce()
    }

    public void putItem(ShoppingItem item, int item_quantity)
    {
        if (!shoppingMap.containsKey(item))
        {
            shoppingMap.put(item, item_quantity);
        }
        else
        {
            shoppingMap.merge(item, item_quantity, Integer::sum);
        }
    }

}

[Campbell Ritchie]

You can't iterate a Map. only one of the three sets you can get out of it. You can get access to those three sets via three of the Map's methods. You can use the map() mathod to take the price and count and produce a number. If you have the price as a primitive you may be able to get a Stream of primitives with a method called mapToXYZ() and use its sum() method, otherwise you would have to write a Collector with its three parts yourself, or use the reduce() method which you will find examples of in the package documentation.
Show us what you have tried and tell us how it is going. This is one of the circumstances where it is much easier to use an IDE than the command line.

[Knute Snortum]

Not related to your question, but I believe that the method putItem() in ShoppingCart can be simplified to one line:

shoppingMap.merge(item, item_quantity, Integer::sum);

[Andrzej Zahorski]

Campbell Ritchie wrote:You can't iterate a Map. only one of the three sets you can get out of it. You can get access to those three sets via three of the Map's methods. You can use the map() mathod to take the price and count and produce a number. If you have the price as a primitive you may be able to get a Stream of primitives with a method called mapToXYZ() and use its sum() method, otherwise you would have to write a Collector with its three parts yourself, or use the reduce() method which you will find examples of in the package documentation.
Show us what you have tried and tell us how it is going. This is one of the circumstances where it is much easier to use an IDE than the command line.

I was thinking about something like this, however I couldn't put it in code

HashMap.entrySet -> stream -> iterate over entries -> f(entry): entry.key.getPrice()  * entry.value) -> reduce to sum 

[Andrzej Zahorski]

Okay, I wrote something like this:

   public int getTotal()
    {
        return this.shoppingMap.entrySet().stream()
                .map( e -> e.getKey().getPrice() * e.getValue())
                .reduce(0, Integer::sum);
    }

It seems to work.

[Piet Souris]

Yes, it works, well done!

Instead of using .map and .reduce you can use .mapToInt and .sum, as Campbell hinted.
But you can even go one line shorter:

return this.shoppingMap.entrySet().stream()
     .collect(summingInt( e -> e.getKey().getPrice() * e.getValue()));

[Campbell Ritchie]

Piet Souris wrote:. . . well done! . . .

Agree with Piet, that's good But Piet has made some good suggestions for improvements.