错误：未为类型List定义方法getId（） <Product>

Question

I have a method to create a list of objects of class

public List<Product> initProducts(){
    List<Product> product = new ArrayList<Product>();

        Product prod = new Product(product.getId(),product.getItemName(),product.getPrice(),product.getCount());
        product.add(prod);

    return product;
}

My Product class is:

public class Product {

int ItemCode;
String ItemName;
double UnitPrice;
int Count;

/**
 * Initialise the fields of the item.
 * @param Name The name of this member of product.
 * @param id The number of this member of product.
 * @param Price The price of this member of product.
 */

public Product(int id, String Name, double Price, int c)
{
    ItemCode=id;
    ItemName=Name;
    UnitPrice=Price;
    Count = c;
}

public int getId()
{
   return this.ItemCode;
}

public String getItemName()
{
   return this.ItemName;
}

public double getPrice()
{
   return this.UnitPrice;
}

public int getCount()
{
   return this.Count;
}



/**
 * Print details about this members of product class to the text terminal.
 */
public void print()
{
    System.out.println("ID: " + ItemCode);
    System.out.println("Name: " + ItemName);
    System.out.println("Staff Number: " +UnitPrice);
    System.out.println("Office: " + Count);

}
}

I am getting an error that the method getId() is undefined for the type List<Product> , Similarly for other methods. Please help me out with this error.

Is my statement correct??

Product prod = new Product(product.getId(),product.getItemName(), product.getPrice(),  
product.getCount());
product.add(prod);

Answer 1

product is a reference of List

List<Product> product = new ArrayList<Product>();

which doesn't have that method

Answer 2

product is reference to List object.

and List/ArrayList has no methosd named getId() .

You have written getId() method for Prodct class , so you can call this method using ref to Product class object.

If you want to get any product object form list use get(int index) method of ArrayList .

eg.

Product prod = product.get(0);
String id= prod.getId();

Answer 3

I believe, the reason you are facing this issue is more due not following the code conventions, that any other.

Whenever you make a collection of any objects, the convention is to use plurals for reference names of the collection. And singular reference name of the Object itself. You can find more details here.

Below is the re-written code with the code conventions being followed:

Method to create a list of objects of class Product:

public List<Product> initProducts(){
    List<Product> products = new ArrayList<Product>();
    Product product = new Product(products.getId(), products.getItemName(), products.getPrice(), products.getCount());
    products.add(prod);    
}

Product Class:

class Product {

int itemCode;
String itemName;
double unitPrice;
int count;

public Product(int itemCode, String itemName, double unitPrice, int count)
{
    this.itemCode = itemCode;
    this.itemName = itemName;
    this.unitPrice = unitPrice;
    this.count = count;
}

public int getId()
{
   return this.itemCode;
}

public String getItemName()
{
   return this.itemName;
}

public double getPrice()
{
   return this.unitPrice;
}

public int getCount()
{
   return this.count;
}
}

Now, it is easy to see, that the products Object (which is of the type List) will not have any methods name getId() or getCount(). Infact , these are methods of the Object contained in the List.

Following conventions will help you avoid, such hassles in futures.

Answer 4

Is my statement correct??

Product prod = new Product(product.getId(),product.getItemName(), product.getPrice(),  
product.getCount());
product.add(prod);

NO this is incorrect. product is not an instance of class Product ,rather it is an instance of List . List does not have any method called getId .

If you want to retrieve the elements from the list and use it to create another instance of you can do something like:

Product exisProd = product.get(0);
Product prod = new Product(exisProd .getId(),exisProd .getItemName(), exisProd .getPrice(),  
    exisProd .getCount());

But make sure that you have elements in the list, otherwise u may run into exception. product.add(prod);