如何用相同的键java求和hashmap值

Question

So I am having a problem where I have to add up all values with the same key in my HashMap. The data(petshop and pet price) is retrieve from an ArrayList. at the moment, the program only gets the last value for each shop since there are multiple shops with the same name but different pet price. I would like to be able to sum the pet price for each shop. so if we have for example,
Law Pet shop: 7.00
and another Law Pet shop: 5.00,
I would like to output it like this:
Law Pet shop: 13.00.
Here is the code and output:

public class AverageCost {

    public void calc(ArrayList<Pet> pets){

        String name = "";
        double price = 0;
        HashMap hm = new HashMap();

        for (Pet i : pets) {
            name = i.getShop();
            price = i.getPrice();

            hm.put(name, price);
        }

        System.out.println("");
        // Get a set of the entries
        Set set = hm.entrySet();
        // Get an iterator
        Iterator i = set.iterator();
        // Display elements
        while(i.hasNext()) {

            Map.Entry me = (Map.Entry)i.next();
            System.out.print(me.getKey() + ": ");
            System.out.println(me.getValue());
        }
    }
}

At the moment this is the output:

Aquatic Acrobatics: 7.06
The Briar Patch Pet Store: 5.24
Preston Pets: 18.11
The Menagerie: 18.7
Galley Pets: 16.8
Anything Except Badgers: 8.53
Petsmart: 21.87
Morris Pets and Supplies: 7.12

Answer 1

First, please program to the interface (not the concrete collection type). Second, please don't use raw types . Next, your Map only needs to contain the name of the pet and the sum of the prices (so String, Double ). Something like,

public void calc(List<Pet> pets) {
    Map<String, Double> hm = new HashMap<>();
    for (Pet i : pets) {
        String name = i.getShop();
        // If the map already has the pet use the current value, otherwise 0.
        double price = hm.containsKey(name) ? hm.get(name) : 0;
        price += i.getPrice();
        hm.put(name, price);
    }
    System.out.println("");
    for (String key : hm.keySet()) {
        System.out.printf("%s: %.2f%n", key, hm.get(key));
    }
}

Answer 2

In Java 8 you could use streams api to do this:

Map<String, Double> map = 
        pets.stream().collect(
            Collectors.groupingBy(
                Pet::getShop,
                Collectors.summingDouble(Pet::getPrice)
            )
        );

Answer 3

There is a useful method V getOrDefault(Object key, V defaultValue) in the Map interface. It returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key. In our case it could be used like this:

HashMap<String,Double> hm = new HashMap<>();

        for (Pet i : pets) {
            name = i.getShop();
            price = i.getPrice();

            hm.put(name, getOrDefault(name, 0) + price);
        }

In addition, we could get more elegant solution using method reference in Java 8:

HashMap<String,Double> hm = new HashMap<>();

        for (Pet i : pets) {
            name = i.getShop();
            price = i.getPrice();

            hm.merge(name, price, Double::sum);
        }

Answer 4

If you the sum then you need add up the value you should be getting the value from HashMap and adding the price to that

double price = hm.get(name) == null ? 0 : hm.get(name) ;
hm.put(name,price + i.getPrice())

;