更改存在 object 的 arraylist 字段的值

Question

I need some help cause I got stuck. I have an ArrayList called orderList with type ItemOrdered(int quantity,Item item). I want to set a new value for item's quantity where the given item exists into ArrayList. I tried everything but didn't work.

//place new order from buyer class
    public void placeOrder(int quantity, Item item) {
        //ItemOrdered newitemordered = new ItemOrdered(quantity,item );
        ShoppingCart shoppingCart = new ShoppingCart();
        shoppingCart.addItemOrdered(quantity,item);

    }

//call placeOrder method
buyer.placeOrder(quantity,eshop.ItemListPen.get(x));

import java.util.ArrayList;

public class ShoppingCart {
    public ArrayList<ItemOrdered> orderList = new ArrayList<ItemOrdered>();


    public void addItemOrdered(int quantity,Item item) {
        if(item.getStock() >= quantity && (!item.equals(orderList))){
            ItemOrdered newitemordered = new ItemOrdered(quantity,item);
            orderList.add(new ItemOrdered(quantity,item));
        }else if(item.getStock() < quantity){
            System.out.println("Sorry,this quantity is not available in stock.");
        }else{

            orderList.get(item).setQuantity(quantity);
            System.out.println("nothing");
        }

    }

}

public class ItemOrdered {
    static  Item item;
    private  int quantity;
    public ItemOrdered(int quantity, Item item){
        this.quantity=quantity;
        this.item=item;
    }

    public void setQuantity(int x){
        quantity=quantity + x;
    }

}

Answer 1

It looks like you're always going to enter the if block of your if-else if-else when the stock is greater than or equal to quantity because the second condition, (.item.equals(orderList)) , will be true. This condition should check to see if the list contains the item, not that the list and the item are equal. The next piece of the puzzle is that the objects in orderList are ItemOrdered objects and not Item objects. That being the case, simply doing .orderList.contains(item) will also always return true.

You could solve this by having a list containing Item objects as part of your ShoppingCart , or adding a new method to loop through the orderList and check item against each Item in the list.

Option #1 - Adding a new List of Item objects to ShoppingCart :

public class ShoppingCart {
    public ArrayList<ItemOrdered> orderList = new ArrayList<ItemOrdered>();

    // This could also be public if you needed for some reason, but you could also achieve that by adding getter and setter methods
    // depending on version of Java, you may need to include Item in <> when creating the new list: new ArrayList<Item>()
    private ArrayList<Item> itemsInCart = new ArrayList<>(); 

    public void addItemOrdered(int quantity,Item item) {
        if(item.getStock() >= quantity && !itemsInCart.contains(item)){
            ItemOrdered newitemordered = new ItemOrdered(quantity,item); // this could be removed, and you just create the new ItemOrdered within .add() on the next line
            orderList.add(newitemordered);
            itemsInCart.add(item); // have to update the new list of items
        }else if(item.getStock() < quantity){
            System.out.println("Sorry,this quantity is not available in stock.");
        }else{
            orderList.get(item).setQuantity(quantity);
            System.out.println("nothing");
        }

    }

}

Option #2 - Looping through the orderList:

public class ShoppingCart {
    public ArrayList<ItemOrdered> orderList = new ArrayList<ItemOrdered>();

    public void addItemOrdered(int quantity,Item item) {
        if(item.getStock() >= quantity && !isItemInCart(item)){
            orderList.add(new ItemOrdered(quantity,item));
        }else if(item.getStock() < quantity){
            System.out.println("Sorry,this quantity is not available in stock.");
        }else{
            orderList.get(item).setQuantity(quantity);
            System.out.println("nothing");
        }

    }

    private boolean isItemInCart(Item item){
        for(ItemOrdered itemOrdered : orderList){
            if(itemOrdered.getItem().equals(item)){
                return true;
            }
        }
        // Item is not in cart
        return false;
    }

}

public class ItemOrdered {
    static  Item item;
    private  int quantity;
    public ItemOrdered(int quantity, Item item){
        this.quantity=quantity;
        this.item=item;
    }

    public void setQuantity(int x){
        quantity=quantity + x;
    }

    public Item getItem(){
        return this.item;
    }

}

Notes on the different options:

• Option 1:
  • Simple
  • Have to maintain a second list
• Option 2:
  • Requires more work up front
  • You don't have to maintain two lists containing similar data
  • Looping could be expensive if the orderList is exceedingly large (probably not a huge deal for most practical cases)
  • You may want to override the .equals() method of Item to define exactly what's being compared to determine if two items are equal