Java中的arraylist中的arraylist

Question

I have to create an arraylist of Shops objects which have a name, description and another arraylist of products objects. The addProduct method is left blank because this is where I face a problem. I need to select a specific shop name in the shop arraylist created with Shops objects and then add one or more products in the arraylist of products. I don't understand how I can manage to do that. If you guys could help me.

This is the code I have so far:

//class of shops but I removed the getters and setters to keep the code short
public class Shops {
    private  String name;
    private String desc;
    private ArrayList<Products> product;

    public Shops(String name, String desc, ArrayList<Products> product) {
        this.name = name;
        this.desc = desc;
        this.product = product;
    }

//another class called Shop assistant which adds products to a specific shop

 public class ShopAssistant {

   ArrayList<Shops> shop = new ArrayList<>();

public void addShops(Shops shop) {
    shop.add(shop);
}
public void addProduct(Products product ) {
    //add products to product arraylist which should be linked to shop arraylist
}

Answer 1

You have to know where to add the product. You have to add it to a Shop object inside the ArrayList, not to the ArrayList. You can get a shop by its index in the ArrayList, so you probably need another int parameter.

Answer 2

You first need the 'addProduct' method to be in the 'Shop' class, to add a product to the shop's arraylist.

Then, you can create a method such as 'addProductToShop' in the 'ShopAssistant' class that takes a shop and a product, and adds the product to the shop eg use a for-loop to find the shop, and add the product using the 'addProduct' method from the 'Shop' class.

I would also advise renaming 'shop' to 'shops' and 'product' to 'products' to make it more clear what they hold.

Answer 3

public class Shop {
    private String name; 
    private String description;
    private ArrayList<Products> products;

    public Shop(String name, String description, ArrayList<Products>products){
        this.name = name; 
        this.description = description;
        this.products = products;
    }

    //This is the main issue that you were missing. 
    //You need to have addProducts in both the shop and assistant. 
    public void addProducts(ArrayList<Products>newProducts){
        newProducts.forEach((product) -> {
            this.products.add(product);
        });
    }
}

public class ShopAssistant {
    ArrayList<Shop> shops = new ArrayList<>();
    public void addShop(Shop newShop){
        shops.add(newShop);
    }

    //Call AddProducts for each shop in the list of shops that need products updated.
    public void addProducts(ArrayList<Products>newProducts, ArrayList<Shop>shopsToAddTo){
        shopsToAddTo.forEach((shop) -> {
            shop.addProducts(newProducts);
        });
    }
}