Why am I getting null pointer exception when I add an item to ArrayList

Question

I have my Prescription class as follows:

import java.util.ArrayList;
import java.util.List;

public class Prescription {

    private final ArrayList<Product> prescriptionItems = new     ArrayList<Product>();
    private long prescriptionId;

    public List<Product> getPrescriptionItems() {
        return prescriptionItems;
    }

    public void addPrescriptionItem(final Product product) {
        this.prescriptionItems.add(product);
    }
}

I have my Product class as follows

public class Product {

    public Product(final long productCode, final double productCost) {
        this.productCode = productCode;
        this.productCost = productCost;
    }


    private final long productCode;
    private final double productCost;

    public long getProductCode() {
        return productCode;
    }

      public double getProductCost() {
        return productCost;
       }
}

I have my test setup as follows:
I have my setUp method as follows:

        @BeforeEach
    void setUp() throws Exception 
        {
            medAvailControllerImpl = new MedAvailControllerImpl(medAvailDAO, dispenser, paymentHandlerFactory);
        prescriptionProduct = new Product(789, 44.0);
        customer = new Customer(45678, "Amofa Baffoe", "Claregalway", "kbaffoe@hotmail.com", "paypal", paypalStrategy);
        when(medAvailDAO.getCustomerForId(45678)).thenReturn(customer);
    }

I have my JUnit/Mockito test as follows:

    @Test
    public void testOneItemOnPrescriptionSuccess() throws Exception {
        //prescriptionProduct = new Product(789, 44.0);
        prescriptionItems.add(prescriptionProduct);
            when(prescription.getPrescriptionItems()).thenReturn(prescriptionItems);

    }

Am getting NullPointerException at where prescriptionItems is added to prescriptionProduct . Any help?

Answer 1

I tried to reproduce your code in my computer and it worked, added a product and returned it using List<Product> getPrescriptionItems() flawlessly.

import java.util.ArrayList; 
import java.util.List;

class Product {
    public Product(final long productCode, final double productCost) {
        this.productCode = productCode;
        this.productCost = productCost;
    }


    private final long productCode;
    private final double productCost;

    public long getProductCode() {
        return productCode;
    }

      public double getProductCost() {
        return productCost;
      }
}  
public class Prescription {

    private final ArrayList<Product> prescriptionItems = new ArrayList<Product>();
    private long prescriptionId;

    public List<Product> getPrescriptionItems() {
        return prescriptionItems;
    }

    public void addPrescriptionItem(final Product product) {
        this.prescriptionItems.add(product);
    }


    public static void main(String[] args) {

        // instantiate Prescription
        Prescription pres = new Prescription();

        // add a product to ArrayList using addPrescriptionItem()
        pres.addPrescriptionItem(new Product(50, 15.0));

        // get the product details from the ArrayList
        Product product = pres.getPrescriptionItems().get(0);

        System.out.println(product.getProductCode()+ " " + product.getProductCost()); // will print "name in sup Class"
    }
}

Here is the output:

Answer 2

try initializing prescriptionItems ArrayList :

private ArrayList <Product> prescriptionItems;

to

private ArrayList <Product> prescriptionItems = new ArrayList<Product>(); 

when you declared it in the test code.

By the way why you adding a new ArrayList for products if you want to test adding product through addPrescriptionItem method inside prescription class which adds the product to the ArrayList already defined inside Prescription class.