java - 实体无法转换为 java.lang.Double

Question

  public static List<Transaction1> debitSourceAccBalance (Integer youraccinput, Integer toaccinput, String recname, Double amtsender) {
    EntityManager em = emf.createEntityManager();
    em.getTransaction().begin();
    Query q = em.createQuery("SELECT t from Transaction1 t where t.fromAccNo =:youraccinput AND t.toAccNo =:toaccinput AND t.name =:recname AND t.amount >=:amtsender");
    q.setParameter("youraccinput", youraccinput);
    q.setParameter("toaccinput", toaccinput);
    q.setParameter("recname", recname);
    q.setParameter("amtsender", amtsender);
    Double currbal = (Double)q.getSingleResult();

    amtsender = currbal - amtsender;
    Query u = em.createQuery("UPDATE Transaction1 t SET t.amount =:amtsender WHERE t.fromAccNo =:youraccinput AND t.toAccNo =:toaccinput");
    u.setParameter("youraccinput", youraccinput);
    u.setParameter("toaccinput", toaccinput);
    u.setParameter("amtsender", amtsender);

    u.executeUpdate();
    return q.getResultList();

Servlet:

              Integer youraccinput = Integer.parseInt(request.getParameter("fromAccNo"));
              Integer toaccinput = Integer.parseInt(request.getParameter("toAccNo"));
              String recname = request.getParameter("name");
              Double amtsender = Double.parseDouble(request.getParameter("amount"));


        List<Transaction1> results = Transaction1.debitSourceAccBalance(youraccinput, toaccinput, recname, amtsender);
                       results.listIterator();
                       request.getRequestDispatcher("ListSingleTransaction").forward(request, response);

为什么我会收到此错误消息？请帮忙修复!!

"Entities.Transaction1 cannot be cast to java.lang.Double"

Answer 1

转换错误位于

Double currbal = (Double)q.getSingleResult();

你连自己为什么要选它都不知道？

作为帮助您通过此问题的快速修复，解决方法非常简单。只需将其更改为:

Transaction1 transaction = (Transaction1) q.getSingleResult();
Double currbal = transaction.getCurrentBalance();  // method name assumed

您的 JPA 查询返回一个实体对象，而不是单个原始值，因此您无法将其转换为 double ，因为您的查询没有选择 double - 它选择了一个 Transaction1 对象，因此您需要将查询的单个结果转换为您选择的任何结果。

了解有关 JPA 的更多信息。