Question

我正在学习本教程，以了解如何在NetBeans CRUD应用程序中使用Java DB： https://platform.netbeans.org/tutorials/nbm-crud.html

我已经达到了运行应用程序的程度。

问题是：为什么产出说：

warning: [options] bootstrap class path not set in conjunction with -source 1.7
warning: No processor claimed any of these annotations: javax.annotation.Generated
C:\Users\YonePC\Documents\NetBeansProjects\DBManager\CustomerViewer\src\org\shop\viewer\CustomerViewerTopComponent.java:51: warning: [unchecked] unchecked cast
        List<Customer> resultList = (List<Customer>) query.getResultList();
  required: List<Customer>
  found:    List
2 warnings

我们正在尝试的代码以及错误在哪里：

public CustomerViewerTopComponent() {
        initComponents();
        setName(Bundle.CTL_CustomerViewerTopComponent());
        setToolTipText(Bundle.HINT_CustomerViewerTopComponent());
        EntityManager entityManager = Persistence.createEntityManagerFactory("CustomerLibraryPU").createEntityManager();
        Query query = entityManager.createNamedQuery("Customer.findAll");
        List<Customer> resultList =  query.getResultList();
        for (Customer c : resultList) {
            jTextArea1.append(c.getName() + " (" + c.getCity() + ")" + "\n");
        }

    }

客户是实体类：

public class Customer implements Serializable {

    private static final long serialVersionUID = 1L;
    @Id
    @Basic(optional = false)
    @Column(name = "CUSTOMER_ID")
    private Integer customerId;
    @Column(name = "NAME")
    private String name;
    @Column(name = "ADDRESSLINE1")
    private String addressline1;
    @Column(name = "ADDRESSLINE2")
    private String addressline2;
    @Column(name = "CITY")
    private String city;
    @Column(name = "STATE")
    private String state;
    @Column(name = "PHONE")
    private String phone;
    @Column(name = "FAX")
    private String fax;
    @Column(name = "EMAIL")
    private String email;
    @Column(name = "CREDIT_LIMIT")
    private Integer creditLimit;
    @JoinColumn(name = "DISCOUNT_CODE", referencedColumnName = "DISCOUNT_CODE")
    @ManyToOne(optional = false)
    private DiscountCode discountCode;
    @JoinColumn(name = "ZIP", referencedColumnName = "ZIP_CODE")
    @ManyToOne(optional = false)
    private MicroMarket zip;

... getters and setters ...

我尝试了以下内容：

public final class CustomerViewerTopComponent extends TopComponent {

    public CustomerViewerTopComponent() {
        initComponents();
        setName(Bundle.CTL_CustomerViewerTopComponent());
        setToolTipText(Bundle.HINT_CustomerViewerTopComponent());
        EntityManager entityManager = Persistence.createEntityManagerFactory("CustomerLibraryPU").createEntityManager();
        Query query = entityManager.createNamedQuery("Customer.findAll");
        List<Customer> resultList = (List<Customer>) query.getResultList();
        for (Customer c : resultList) {
            jTextArea1.append(c.getName() + " (" + c.getCity() + ")" + "\n");
        }

    }

我尝试强制转换List，将给定的输入与List String匹配，并将其声明为：List String

你能帮我吗？

Answer 1

这不是一个错误。这是一个简单的编译器警告。它告诉你，在运行时，不会检查列表确实是List<Customer>。

禁止警告的正确方法是通过调用使用TypedQuery createNamedQuery("Customer.findAll", Customer.class)。这不会保证列表确实在运行时包含客户，但它会抑制警告。

编译器没有发现代码中的主要问题：不应该使用Swing组件中的EntityManager。这应该委托给另一个对象。每次需要执行查询时创建一个新的EntityManagerFactory实际上并不是一个好主意。 EntityManagerFactory是一个重量级，线程安全的对象。你应该只创建一次，并在你需要的地方使用它。

如何在不知道其类型的情况下迭代到List的元素？‽‽

1 个答案: