我在 Java 中没有得到正确的数量（使用 HashMap）

Question

I am studying programming and I have a trouble to get the correct amount of products when I try to add 3 same type products. For example:

basket.add("milk", 3);
basket.add("milk", 3);
basket.add("milk", 3);
System.out.println("basket price: " + basket.price() +"\n"); 

I get the wrong price because 'amount' doesn't increase to the value 3 since I do:

Purchase purchase = new Purchase(product, 1, price);

I am not allowed to add additional object instances. Can you help me to fix my 'add' method to be able to have the correct amount of same type items?

public class ShoppingBasket {
    private Map<String, Purchase> basket;    
    public ShoppingBasket() { basket = new HashMap<String, Purchase>(); }    
    public void add(String product, int price) {        
        Purchase purchase = new Purchase(product, 1, price);
        if (basket.containsKey(product)) {
            purchase.increaseAmount();   
            basket.put(product, purchase);               
        } else
            basket.put(product, purchase);        
    }    
    public int price() {
        int price = 0;
        for (Purchase item : basket.values())
            price += item.price();
        return price;
    }
public class Purchase {
    private String product;
    private int amount;
    private int unitPrice;       
    public Purchase(String product, int amount, int unitPrice) { 
        this.product = product;
        this.amount = amount;
        this.unitPrice = unitPrice;
    }            
    public int price() {
        return this.amount * this.unitPrice;
    }     
    public void increaseAmount() {
        this.amount++;
    }    

Answer 1

At most the 'amount' of the purchase will be 2, because you start with 1, increase it by 1.

The new amount needs to be the old amount plus 1, not 1 plus 1.

I suggest getting the existing purchase, and if it's not there, adding a new purchase for no items. Then in either case, add 1 for the new item. No need to keep creating a new purchase every time we want to increase the amount.

  purchase = basket.get(product);
  if (purchase == null) {
       purchase = new Purchase(product, 0, price);
       basket.put(product, purchase);
  }
  purchase.increaseAmount();