向 ArrayList 添加項目時，為什么會出現空指針異常

Question

我的處方課如下：

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);
    }
}

我的產品類如下

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;
       }
}

我的測試設置如下：
我的設置方法如下：

        @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);
    }

我的 JUnit/Mockito 測試如下：

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

    }

在將prescriptionItems添加到prescriptionProduct位置處收到NullPointerException 。 有什么幫助嗎？

Answer 1

我嘗試在我的計算機中重現您的代碼並且它工作正常，添加了一個產品並使用List<Product> getPrescriptionItems()完美地返回了它。

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"
    }
}

這是輸出：

Answer 2

嘗試初始化處方項 ArrayList ：

private ArrayList <Product> prescriptionItems;

到

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

當您在測試代碼中聲明它時。

順便說一下，如果您想通過處方類中的addPrescriptionItem方法測試添加產品，該方法將產品添加到已在 Prescription 類中定義的 ArrayList，為什么要為產品添加新的 ArrayList。