如何从另一个 class 传递 class 参考？

Question

I'm building a basic beginner Java program but I keep getting the NullPointerException error and I can't seem to solve the problem.

I have two classes - User and Cart.

The User class includes the Cart class with the following code:

package eCommerceApp;

import java.util.List;
import java.util.Collections;
import java.util.Comparator;

public class User extends Person {

    public User(String name, String surname, int budget) {
        super(name, surname, budget);
    }

    public Cart myCart;

    public String getReceipt(List<Product> products) {

        int sum = 0;

        for (Product product : products) {

            sum += product.getPrice();
        }
        return "Your total bill is " + sum + " rsd.";
    }

    public String getMostExpensiveItem(List<Product> products) {

        Product product = Collections.max(products, Comparator.comparing(s -> s.getPrice()));

        return "The most expensive item in your shopping cart is " + product.getName() + ".";

    }

    public String getCheapestItem(List<Product> products) {

        Product product = Collections.min(products, Comparator.comparing(s -> s.getPrice()));
        return "The cheapest item in your shopping cart is " + product.getName() + ".";

    }

}

And the Cart Class:

import java.util.List;

public class Cart {

    List<Product> products;

    public Cart(List products) {

        this.products = products;
    }

    public String getProducts() {
        return this.products.toString();
    }

    public void setProducts(List products) {
        this.products = products;
    }

}

Now, when I want to set a list of products to my User's Cart by typing User.myCart.setProducts(list) in the main class, I get the NullPointerException error. How can I solve this?

Answer 1

To use classes in other classes, you'll need to make an instance of them like so:

public Cart myCart = new Cart();

Unless your entire class is static, this is the way to go. But it is generally not good practice to make your classes static all the time.