Question

我在让这个程序工作时遇到了问题。我收到错误，我无法弄清楚原因。

这是我的主要代码

@Entity
@Table(name = "users")
@XmlRootElement
@NamedQueries({
@NamedQuery(name = "Users.findAll", query = "SELECT u FROM Users u"),
@NamedQuery(name = "Users.findByIdUser", query = "SELECT u FROM Users u WHERE u.idUser = :idUser"),
@NamedQuery(name = "Users.findByNome", query = "SELECT u FROM Users u WHERE u.nome = :nome"),
@NamedQuery(name = "Users.findByDataNasc", query = "SELECT u FROM Users u WHERE u.dataNasc = :dataNasc"),
@NamedQuery(name = "Users.findByEmail", query = "SELECT u FROM Users u WHERE u.email = :email")})
public class Users implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "id_user")
    private Integer idUser;
    @Basic(optional = false)
    @Column(name = "nome")
    private String nome;
    @Basic(optional = false)
    @Column(name = "data_nasc")
    private String dataNasc;
    @Basic(optional = false)
    @Column(name = "email")
    private String email;

    public Users() {
    }

    public Users(Integer idUser) {
        this.idUser = idUser;
    }

    public Users(Integer idUser, String nome, String dataNasc, String email) {
        this.idUser = idUser;
        this.nome = nome;
        this.dataNasc = dataNasc;
        this.email = email;
    }

    public Integer getIdUser() {
        return idUser;
    }

    public void setIdUser(Integer idUser) {
        this.idUser = idUser;
    }

    public String getNome() {
        return nome;
    }

    public void setNome(String nome) {
        this.nome = nome;
    }

    public String getDataNasc() {
        return dataNasc;
    }

    public void setDataNasc(String dataNasc) {
        this.dataNasc = dataNasc;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    @Override
    public int hashCode() {
        int hash = 0;
        hash += (idUser != null ? idUser.hashCode() : 0);
        return hash;
    }

    @Override
    public boolean equals(Object object) {
        // TODO: Warning - this method won't work in the case the id fields are not set
        if (!(object instanceof Users)) {
            return false;
        }
        Users other = (Users) object;
        if ((this.idUser == null && other.idUser != null) || (this.idUser != null && !this.idUser.equals(other.idUser))) {
            return false;
        }
        return true;
    }

    @Override
    public String toString() {
        return "database.Users[ idUser=" + idUser + " ]";
    }

}

这是发生错误的Java文件

public class TabelaUsers extends AbstractTableModel {
    EntityManagerFactory emf = Persistence.createEntityManagerFactory("EsmaiorPU");
    private final List listagem;
    private final ArrayList<Users>lista = new ArrayList();

    public TabelaUsers() {
        EntityManager em = emf.createEntityManager();
        em.getTransaction().begin();
        listagem = em.createQuery("Select t from Users t").getResultList();
        for (Iterator<String> it = listagem.iterator(); it.hasNext();) {
           String elem = it.next();
           lista.add((Users) em.createNamedQuery("Users.findByDataNasc").setParameter("dataNasc", elem).getSingleResult());         
        } 
        em.close();        
    }


    @Override
    public int getRowCount() {
      return lista.size();
    }

    @Override
    public int getColumnCount() {
       return 4;
    }

    @Override
    public Object getValueAt(int rowIndex, int columnIndex) {
        if(columnIndex==0){
            return lista.get(rowIndex).getNome();
        }else if(columnIndex==1){
            return lista.get(rowIndex).getDataNasc();
        }else if(columnIndex==2){
            return lista.get(rowIndex).getEmail();
        }
        return null;
    }
}

这一行发生了错误

 String elem = it.next();

抛出以下异常

线程中的异常＆＃34; AWT-EventQueue-0＆＃34; java.lang.ClassCastException：model.Users无法强制转换为java.lang.String 我失去了，有什么帮助吗？

谢谢

Answer 1

我最好的猜测是：em.createQuery（...）。getResultList（）in：

listagem = em.createQuery("Select t from Users t").getResultList();

正在返回List<Users>。因此，当您从此List获取迭代器时，迭代器包含Users。由于Users不是String的子类（因为没有任何内容可以是String的子类），因此您获得了ClassCastException。

Answer 2

您需要更改查询以获取dataNasc而不是完整对象。

listagem = em.createQuery("Select t.dataNasc  from Users t").getResultList()

或者更好地使用TypedQuery来避免类型转换。

List<String> listagem = em.createQuery("Select t.dataNasc  from Users t", String.class).getResultList()

java.lang.ClassCastException：model.Users无法强制转换为java.lang.String

2 个答案: