计算列表中元素的出现次数

Question

我正在尝试列出一个列表,我将把项目添加到我的购物车中,然后在结账时它将通过使用HashMap显示有多少项目出现.

public class ShoppingCart 
{
    private ArrayList<Items> ShoppingCart;

    public ShoppingCart()
    {
        ShoppingCart = new ArrayList<Items>();
    }

    public void addItems(Items newItems)
    {
        ShoppingCart.add(newItems);
    }

    public ArrayList<Items> getShoppingCart()
    {
        return ShoppingCart;
    }

    public void CheckOut()
    {
        for(int i = 0; i < ShoppingCart.size(); i++)
        {
                HashMap<String, Integer> itemsMap = new HashMap<String, Integer>();
                int a = 1;

                if (itemsMap.containsKey(ShoppingCart.get(i).getName()))
                {
                    itemsMap.replace(ShoppingCart.get(i).getName(), a, a++);
                } 
                else 
                {
                    itemsMap.put(ShoppingCart.get(i).getName(), a);
                }
            System.out.println(a +"x "+ShoppingCart.get(i));
        }
    }

和我的项目是用

public class Items 
{
    private String name;

    public Items (String name)
    {
        this.name = name;
    }

    public String toString()
    {
        return name;
    }

    public String getName()
    {
        return name;
    }

}

在Main中,我将创建Items,并将它们添加到购物车,然后ShoppingCart.CheckOut(); 我的物品.

但是,如果我这样添加4个"白面包",

    Items bread = new Items("White Bread");

    ShoppingCart ShoppingCart = new ShoppingCart();

    ShoppingCart.addItems(bread);
    ShoppingCart.addItems(bread);
    ShoppingCart.addItems(bread);
    ShoppingCart.addItems(bread);

我明白了

1x White Bread
1x White Bread
1x White Bread
1x White Bread 

代替

4x White Bread

我究竟做错了什么？

Answer 1

更改

itemsMap.replace(ShoppingCart.get(i).getName(), a, a++);

至

a = itemsMap.get(ShoppingCart.get(i).getName()); // in order to increment the 
                                                 // current counter value
itemsMap.replace(ShoppingCart.get(i).getName(), a, ++a);

使用post decrement(a++)不会修改Map中的值,因为a++返回原始值a.

另外,HashMap应该在循环外初始化.否则它将始终只包含一个元素.